cpython_2/tests/private/eval_tests.patch

diff --git a/Lib/test/test_zoneinfo/__init__.py b/Lib/test/test_zoneinfo/__init__.py
new file mode 100644
index 00000000000..98cc4412ae1
--- /dev/null
+++ b/Lib/test/test_zoneinfo/__init__.py
@@ -0,0 +1 @@
+from .test_zoneinfo import *
diff --git a/Lib/test/test_zoneinfo/__main__.py b/Lib/test/test_zoneinfo/__main__.py
new file mode 100644
index 00000000000..5cc4e055d5e
--- /dev/null
+++ b/Lib/test/test_zoneinfo/__main__.py
@@ -0,0 +1,3 @@
+import unittest
+
+unittest.main('test.test_zoneinfo')
diff --git a/Lib/test/test_zoneinfo/_support.py b/Lib/test/test_zoneinfo/_support.py
new file mode 100644
index 00000000000..6bd8d8dc0fb
--- /dev/null
+++ b/Lib/test/test_zoneinfo/_support.py
@@ -0,0 +1,76 @@
+import contextlib
+import functools
+import sys
+import threading
+import unittest
+from test.support import import_fresh_module
+
+OS_ENV_LOCK = threading.Lock()
+TZPATH_LOCK = threading.Lock()
+TZPATH_TEST_LOCK = threading.Lock()
+
+
+def call_once(f):
+    """Decorator that ensures a function is only ever called once."""
+    lock = threading.Lock()
+    cached = functools.lru_cache(None)(f)
+
+    @functools.wraps(f)
+    def inner():
+        with lock:
+            return cached()
+
+    return inner
+
+
+@call_once
+def get_modules():
+    """Retrieve two copies of zoneinfo: pure Python and C accelerated.
+
+    Because this function manipulates the import system in a way that might
+    be fragile or do unexpected things if it is run many times, it uses a
+    `call_once` decorator to ensure that this is only ever called exactly
+    one time — in other words, when using this function you will only ever
+    get one copy of each module rather than a fresh import each time.
+    """
+    import zoneinfo as c_module
+
+    py_module = import_fresh_module("zoneinfo", blocked=["_zoneinfo"])
+
+    return py_module, c_module
+
+
+@contextlib.contextmanager
+def set_zoneinfo_module(module):
+    """Make sure sys.modules["zoneinfo"] refers to `module`.
+
+    This is necessary because `pickle` will refuse to serialize
+    an type calling itself `zoneinfo.ZoneInfo` unless `zoneinfo.ZoneInfo`
+    refers to the same object.
+    """
+
+    NOT_PRESENT = object()
+    old_zoneinfo = sys.modules.get("zoneinfo", NOT_PRESENT)
+    sys.modules["zoneinfo"] = module
+    yield
+    if old_zoneinfo is not NOT_PRESENT:
+        sys.modules["zoneinfo"] = old_zoneinfo
+    else:  # pragma: nocover
+        sys.modules.pop("zoneinfo")
+
+
+class ZoneInfoTestBase(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.klass = cls.module.ZoneInfo
+        super().setUpClass()
+
+    @contextlib.contextmanager
+    def tzpath_context(self, tzpath, lock=TZPATH_LOCK):
+        with lock:
+            old_path = self.module.TZPATH
+            try:
+                self.module.reset_tzpath(tzpath)
+                yield
+            finally:
+                self.module.reset_tzpath(old_path)
diff --git a/Lib/test/test_zoneinfo/data/update_test_data.py b/Lib/test/test_zoneinfo/data/update_test_data.py
new file mode 100644
index 00000000000..f531ab316a1
--- /dev/null
+++ b/Lib/test/test_zoneinfo/data/update_test_data.py
@@ -0,0 +1,122 @@
+"""
+Script to automatically generate a JSON file containing time zone information.
+
+This is done to allow "pinning" a small subset of the tzdata in the tests,
+since we are testing properties of a file that may be subject to change. For
+example, the behavior in the far future of any given zone is likely to change,
+but "does this give the right answer for this file in 2040" is still an
+important property to test.
+
+This must be run from a computer with zoneinfo data installed.
+"""
+from __future__ import annotations
+
+import base64
+import functools
+import json
+import lzma
+import pathlib
+import textwrap
+import typing
+
+import zoneinfo
+
+KEYS = [
+    "Africa/Abidjan",
+    "Africa/Casablanca",
+    "America/Los_Angeles",
+    "America/Santiago",
+    "Asia/Tokyo",
+    "Australia/Sydney",
+    "Europe/Dublin",
+    "Europe/Lisbon",
+    "Europe/London",
+    "Pacific/Kiritimati",
+    "UTC",
+]
+
+TEST_DATA_LOC = pathlib.Path(__file__).parent
+
+
+@functools.lru_cache(maxsize=None)
+def get_zoneinfo_path() -> pathlib.Path:
+    """Get the first zoneinfo directory on TZPATH containing the "UTC" zone."""
+    key = "UTC"
+    for path in map(pathlib.Path, zoneinfo.TZPATH):
+        if (path / key).exists():
+            return path
+    else:
+        raise OSError("Cannot find time zone data.")
+
+
+def get_zoneinfo_metadata() -> typing.Dict[str, str]:
+    path = get_zoneinfo_path()
+
+    tzdata_zi = path / "tzdata.zi"
+    if not tzdata_zi.exists():
+        # tzdata.zi is necessary to get the version information
+        raise OSError("Time zone data does not include tzdata.zi.")
+
+    with open(tzdata_zi, "r") as f:
+        version_line = next(f)
+
+    _, version = version_line.strip().rsplit(" ", 1)
+
+    if (
+        not version[0:4].isdigit()
+        or len(version) < 5
+        or not version[4:].isalpha()
+    ):
+        raise ValueError(
+            "Version string should be YYYYx, "
+            + "where YYYY is the year and x is a letter; "
+            + f"found: {version}"
+        )
+
+    return {"version": version}
+
+
+def get_zoneinfo(key: str) -> bytes:
+    path = get_zoneinfo_path()
+
+    with open(path / key, "rb") as f:
+        return f.read()
+
+
+def encode_compressed(data: bytes) -> typing.List[str]:
+    compressed_zone = lzma.compress(data)
+    raw = base64.b85encode(compressed_zone)
+
+    raw_data_str = raw.decode("utf-8")
+
+    data_str = textwrap.wrap(raw_data_str, width=70)
+    return data_str
+
+
+def load_compressed_keys() -> typing.Dict[str, typing.List[str]]:
+    output = {key: encode_compressed(get_zoneinfo(key)) for key in KEYS}
+
+    return output
+
+
+def update_test_data(fname: str = "zoneinfo_data.json") -> None:
+    TEST_DATA_LOC.mkdir(exist_ok=True, parents=True)
+
+    # Annotation required: https://github.com/python/mypy/issues/8772
+    json_kwargs: typing.Dict[str, typing.Any] = dict(
+        indent=2, sort_keys=True,
+    )
+
+    compressed_keys = load_compressed_keys()
+    metadata = get_zoneinfo_metadata()
+    output = {
+        "metadata": metadata,
+        "data": compressed_keys,
+    }
+
+    with open(TEST_DATA_LOC / fname, "w") as f:
+        json.dump(output, f, **json_kwargs)
+
+
+if __name__ == "__main__":
+    update_test_data()
diff --git a/Lib/test/test_zoneinfo/data/zoneinfo_data.json b/Lib/test/test_zoneinfo/data/zoneinfo_data.json
new file mode 100644
index 00000000000..ec4414a0cde
--- /dev/null
+++ b/Lib/test/test_zoneinfo/data/zoneinfo_data.json
@@ -0,0 +1,190 @@
+{
+  "data": {
+    "Africa/Abidjan": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~f{VGF<>F7KxBg5R*{Ksocg8-YYVul=v7vZzaHN",
+      "uC=da5UI2rH18c!OnjV{y4u(+A!!VBKmY&$ORw>7UO^(500B;v0RR91bXh%WvBYQl0ssI2",
+      "00dcD"
+    ],
+    "Africa/Casablanca": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0b&Kz+C_;7KxBg5R*{N&yjMUR~;C-fDaSOU;q-~",
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+      "{cR_IvBYQl0ssI200dcD"
+    ],
+    "America/Los_Angeles": [
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+    "Europe/Dublin": [
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+      "fJ?;_U;D|^>5by2ul@i4Zf()InfFN}00EQ=q#FPL>RM>svBYQl0ssI200dcD"
+    ],
+    "Europe/Lisbon": [
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+      "+gRRm6Wn)VWVNHeK*TEV=f#2KZqu%y?mTx#EfRiK0)TG7$$~=LGxx@0D|lS2up|oCON{YQ",
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+      ">zi+jmOBFrVpW;)@UsU#%$8BcV#h@}m$#!Fglo&bwb78aYqOG_W7h{eb(+39&-mk4EIXq_",
+      "_`30=8sfA3=!3TO_TyS5X22~?6nKngZ|bq=grdq=9X)3xAkA42L!~rmS)n3w-~;lgz%Fhn",
+      "(?rXdp2ho~9?wmVs2JwVt~?@FVD%`tN69{(i3oQa;O0<Hp#T5?$WIy3h`IlL00Hv}jT-;}",
+      "Z2tpNvBYQl0ssI200dcD"
+    ],
+    "Europe/London": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0`|pJ6!-O7KxBg5R*;$9DqzW!kQs3DZt(=0_!m1",
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+      "!GV{d*C!9KXv;4v<seWRpo=ZZxGf)-5Qsn$3dw`uhF)+6#mgUoNF-Y2jN73pVhdTs*p0`Z",
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+      "yApq26RxgqA|*tmsvtL#OhcF(C<0EGWHP)BF<g*iSWicU6k1<Ps?BQ$IWg-#s2uF-qXgJ_",
+      "!H_mZIMx*L%&a*_6;_trMCULk0ZYM<hfJlYBddHwRyYUDu3!C_lJZWTQ?c-R&@9054pj0k",
+      "kQ{Xi{A$&)&b#^G*}8w^qE5i<@aDxaJQs2E$W)AIqUXO{gQ;U8|FA%BD~sORzq44)AntUu",
+      "QHBO{{Pi<EpK!$x4(~7w)la!dN=M@L_j};6|5G&QfuO~2?Q7996z)78fqW<D#8tKNV(*qc",
+      "mfA>l?h)_*7!{LoJiv%RsOs!q->n+DcV%9~B@Rb<ISu%16c5H-7zQIq+SuS+s<lQOWK5+C",
+      "d*>C_1G_1g6`Yd~8|%-=2l~oGN!~TVv2Bnk>7wW8L@^?vX$f3AiT)(4nrCuTm9%(XC6Nai",
+      "E(;}7&=YZagjAN$O-cN;1u{dTkElmB0GT$|Wa)QMmKrx<|LCJ9qlUoFsUbD^H^6_8(w<0{",
+      "ftj&O1~p_%lh5z;zNV&sP<T$*OgK)_0B#JDtXOkhC;Bo7h)#RUy;vBiVLN-T$*7t*t9@ey",
+      "3Woa&24QZ_z38BQ@A(A<(9n@%R?}B`7%w2wowt~UU;bAlqCzr(H$M5t==jGIqMqCsE=Jwa",
+      "$3P+3^&|~i28@=d_u6Cgthe(Lq(wxKpdSDL|7X6Un<nrt00Gwuz#ISo`BbmvvBYQl0ssI2",
+      "00dcD"
+    ],
+    "Pacific/Kiritimati": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~jCaVO;<!7KxBg5R*{K!`A|q%C5j6({{dSEy5>+",
+      "NF2>iK{8KMUf+)<-)VxXbLxD(alL}N$AT-ogNbJSMMYeX+Z{jS)b8TK^PB=FxyBxzfmFto",
+      "eo0R`a(%NO?#aEH9|?Cv00000NIsFh6BW2800DjO0RR918Pu^`vBYQl0ssI200dcD"
+    ],
+    "UTC": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~e#|9bEt_7KxBg5R*|3h1|xhHLji!C57qW6L*|H",
+      "pEErm00000ygu;I+>V)?00B92fhY-(AGY&-0RR9100dcD"
+    ]
+  },
+  "metadata": {
+    "version": "2020a"
+  }
+}
\ No newline at end of file
diff --git a/Lib/test/test_zoneinfo/test_zoneinfo.py b/Lib/test/test_zoneinfo/test_zoneinfo.py
new file mode 100644
index 00000000000..05db03abf25
--- /dev/null
+++ b/Lib/test/test_zoneinfo/test_zoneinfo.py
@@ -0,0 +1,1994 @@
+from __future__ import annotations
+
+import base64
+import contextlib
+import dataclasses
+import importlib.metadata
+import io
+import json
+import lzma
+import os
+import pathlib
+import pickle
+import re
+import shutil
+import struct
+import tempfile
+import unittest
+from datetime import date, datetime, time, timedelta, timezone
+
+from . import _support as test_support
+from ._support import (
+    OS_ENV_LOCK,
+    TZPATH_LOCK,
+    TZPATH_TEST_LOCK,
+    ZoneInfoTestBase,
+)
+
+py_zoneinfo, c_zoneinfo = test_support.get_modules()
+
+try:
+    importlib.metadata.metadata("tzdata")
+    HAS_TZDATA_PKG = True
+except importlib.metadata.PackageNotFoundError:
+    HAS_TZDATA_PKG = False
+
+ZONEINFO_DATA = None
+ZONEINFO_DATA_V1 = None
+TEMP_DIR = None
+DATA_DIR = pathlib.Path(__file__).parent / "data"
+ZONEINFO_JSON = DATA_DIR / "zoneinfo_data.json"
+
+# Useful constants
+ZERO = timedelta(0)
+ONE_H = timedelta(hours=1)
+
+
+def setUpModule():
+    global TEMP_DIR
+    global ZONEINFO_DATA
+    global ZONEINFO_DATA_V1
+
+    TEMP_DIR = pathlib.Path(tempfile.mkdtemp(prefix="zoneinfo"))
+    ZONEINFO_DATA = ZoneInfoData(ZONEINFO_JSON, TEMP_DIR / "v2")
+    ZONEINFO_DATA_V1 = ZoneInfoData(ZONEINFO_JSON, TEMP_DIR / "v1", v1=True)
+
+
+def tearDownModule():
+    shutil.rmtree(TEMP_DIR)
+
+
+class TzPathUserMixin:
+    """
+    Adds a setUp() and tearDown() to make TZPATH manipulations thread-safe.
+
+    Any tests that require manipulation of the TZPATH global are necessarily
+    thread unsafe, so we will acquire a lock and reset the TZPATH variable
+    to the default state before each test and release the lock after the test
+    is through.
+    """
+
+    @property
+    def tzpath(self):  # pragma: nocover
+        return None
+
+    def setUp(self):
+        with contextlib.ExitStack() as stack:
+            stack.enter_context(
+                self.tzpath_context(self.tzpath, lock=TZPATH_TEST_LOCK)
+            )
+            self.addCleanup(stack.pop_all().close)
+
+        super().setUp()
+
+
+class DatetimeSubclassMixin:
+    """
+    Replaces all ZoneTransition transition dates with a datetime subclass.
+    """
+
+    class DatetimeSubclass(datetime):
+        @classmethod
+        def from_datetime(cls, dt):
+            return cls(
+                dt.year,
+                dt.month,
+                dt.day,
+                dt.hour,
+                dt.minute,
+                dt.second,
+                dt.microsecond,
+                tzinfo=dt.tzinfo,
+                fold=dt.fold,
+            )
+
+    def load_transition_examples(self, key):
+        transition_examples = super().load_transition_examples(key)
+        for zt in transition_examples:
+            dt = zt.transition
+            new_dt = self.DatetimeSubclass.from_datetime(dt)
+            new_zt = dataclasses.replace(zt, transition=new_dt)
+            yield new_zt
+
+
+class ZoneInfoTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+    class_name = "ZoneInfo"
+
+    def setUp(self):
+        super().setUp()
+
+        # This is necessary because various subclasses pull from different
+        # data sources (e.g. tzdata, V1 files, etc).
+        self.klass.clear_cache()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def zone_from_key(self, key):
+        return self.klass(key)
+
+    def zones(self):
+        return ZoneDumpData.transition_keys()
+
+    def fixed_offset_zones(self):
+        return ZoneDumpData.fixed_offset_zones()
+
+    def load_transition_examples(self, key):
+        return ZoneDumpData.load_transition_examples(key)
+
+    def test_str(self):
+        # Zones constructed with a key must have str(zone) == key
+        for key in self.zones():
+            with self.subTest(key):
+                zi = self.zone_from_key(key)
+
+                self.assertEqual(str(zi), key)
+
+        # Zones with no key constructed should have str(zone) == repr(zone)
+        file_key = self.zoneinfo_data.keys[0]
+        file_path = self.zoneinfo_data.path_from_key(file_key)
+
+        with open(file_path, "rb") as f:
+            with self.subTest(test_name="Repr test", path=file_path):
+                zi_ff = self.klass.from_file(f)
+                self.assertEqual(str(zi_ff), repr(zi_ff))
+
+    def test_repr(self):
+        # The repr is not guaranteed, but I think we can insist that it at
+        # least contain the name of the class.
+        key = next(iter(self.zones()))
+
+        zi = self.klass(key)
+        class_name = self.class_name
+        with self.subTest(name="from key"):
+            self.assertRegex(repr(zi), class_name)
+
+        file_key = self.zoneinfo_data.keys[0]
+        file_path = self.zoneinfo_data.path_from_key(file_key)
+        with open(file_path, "rb") as f:
+            zi_ff = self.klass.from_file(f, key=file_key)
+
+        with self.subTest(name="from file with key"):
+            self.assertRegex(repr(zi_ff), class_name)
+
+        with open(file_path, "rb") as f:
+            zi_ff_nk = self.klass.from_file(f)
+
+        with self.subTest(name="from file without key"):
+            self.assertRegex(repr(zi_ff_nk), class_name)
+
+    def test_key_attribute(self):
+        key = next(iter(self.zones()))
+
+        def from_file_nokey(key):
+            with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+                return self.klass.from_file(f)
+
+        constructors = (
+            ("Primary constructor", self.klass, key),
+            ("no_cache", self.klass.no_cache, key),
+            ("from_file", from_file_nokey, None),
+        )
+
+        for msg, constructor, expected in constructors:
+            zi = constructor(key)
+
+            # Ensure that the key attribute is set to the input to ``key``
+            with self.subTest(msg):
+                self.assertEqual(zi.key, expected)
+
+            # Ensure that the key attribute is read-only
+            with self.subTest(f"{msg}: readonly"):
+                with self.assertRaises(AttributeError):
+                    zi.key = "Some/Value"
+
+    def test_bad_keys(self):
+        bad_keys = [
+            "Eurasia/Badzone",  # Plausible but does not exist
+            "BZQ",
+            "America.Los_Angeles",
+            "🇨🇦",  # Non-ascii
+            "America/New\ud800York",  # Contains surrogate character
+        ]
+
+        for bad_key in bad_keys:
+            with self.assertRaises(self.module.ZoneInfoNotFoundError):
+                self.klass(bad_key)
+
+    def test_bad_keys_paths(self):
+        bad_keys = [
+            "/America/Los_Angeles",  # Absolute path
+            "America/Los_Angeles/",  # Trailing slash - not normalized
+            "../zoneinfo/America/Los_Angeles",  # Traverses above TZPATH
+            "America/../America/Los_Angeles",  # Not normalized
+            "America/./Los_Angeles",
+        ]
+
+        for bad_key in bad_keys:
+            with self.assertRaises(ValueError):
+                self.klass(bad_key)
+
+    def test_bad_zones(self):
+        bad_zones = [
+            b"",  # Empty file
+            b"AAAA3" + b" " * 15,  # Bad magic
+        ]
+
+        for bad_zone in bad_zones:
+            fobj = io.BytesIO(bad_zone)
+            with self.assertRaises(ValueError):
+                self.klass.from_file(fobj)
+
+    def test_fromutc_errors(self):
+        key = next(iter(self.zones()))
+        zone = self.zone_from_key(key)
+
+        bad_values = [
+            (datetime(2019, 1, 1, tzinfo=timezone.utc), ValueError),
+            (datetime(2019, 1, 1), ValueError),
+            (date(2019, 1, 1), TypeError),
+            (time(0), TypeError),
+            (0, TypeError),
+            ("2019-01-01", TypeError),
+        ]
+
+        for val, exc_type in bad_values:
+            with self.subTest(val=val):
+                with self.assertRaises(exc_type):
+                    zone.fromutc(val)
+
+    def test_utc(self):
+        zi = self.klass("UTC")
+        dt = datetime(2020, 1, 1, tzinfo=zi)
+
+        self.assertEqual(dt.utcoffset(), ZERO)
+        self.assertEqual(dt.dst(), ZERO)
+        self.assertEqual(dt.tzname(), "UTC")
+
+    def test_unambiguous(self):
+        test_cases = []
+        for key in self.zones():
+            for zone_transition in self.load_transition_examples(key):
+                test_cases.append(
+                    (
+                        key,
+                        zone_transition.transition - timedelta(days=2),
+                        zone_transition.offset_before,
+                    )
+                )
+
+                test_cases.append(
+                    (
+                        key,
+                        zone_transition.transition + timedelta(days=2),
+                        zone_transition.offset_after,
+                    )
+                )
+
+        for key, dt, offset in test_cases:
+            with self.subTest(key=key, dt=dt, offset=offset):
+                tzi = self.zone_from_key(key)
+                dt = dt.replace(tzinfo=tzi)
+
+                self.assertEqual(dt.tzname(), offset.tzname, dt)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset, dt)
+                self.assertEqual(dt.dst(), offset.dst, dt)
+
+    def test_folds_and_gaps(self):
+        test_cases = []
+        for key in self.zones():
+            tests = {"folds": [], "gaps": []}
+            for zt in self.load_transition_examples(key):
+                if zt.fold:
+                    test_group = tests["folds"]
+                elif zt.gap:
+                    test_group = tests["gaps"]
+                else:
+                    # Assign a random variable here to disable the peephole
+                    # optimizer so that coverage can see this line.
+                    # See bpo-2506 for more information.
+                    no_peephole_opt = None
+                    continue
+
+                # Cases are of the form key, dt, fold, offset
+                dt = zt.anomaly_start - timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_before))
+
+                dt = zt.anomaly_start
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_start + timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end - timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end
+                test_group.append((dt, 0, zt.offset_after))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end + timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_after))
+                test_group.append((dt, 1, zt.offset_after))
+
+            for grp, test_group in tests.items():
+                test_cases.append(((key, grp), test_group))
+
+        for (key, grp), tests in test_cases:
+            with self.subTest(key=key, grp=grp):
+                tzi = self.zone_from_key(key)
+
+                for dt, fold, offset in tests:
+                    dt = dt.replace(fold=fold, tzinfo=tzi)
+
+                    self.assertEqual(dt.tzname(), offset.tzname, dt)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset, dt)
+                    self.assertEqual(dt.dst(), offset.dst, dt)
+
+    def test_folds_from_utc(self):
+        tests = []
+        for key in self.zones():
+            zi = self.zone_from_key(key)
+            with self.subTest(key=key):
+                for zt in self.load_transition_examples(key):
+                    if not zt.fold:
+                        continue
+
+                    dt_utc = zt.transition_utc
+                    dt_before_utc = dt_utc - timedelta(seconds=1)
+                    dt_after_utc = dt_utc + timedelta(seconds=1)
+
+                    dt_before = dt_before_utc.astimezone(zi)
+                    self.assertEqual(dt_before.fold, 0, (dt_before, dt_utc))
+
+                    dt_after = dt_after_utc.astimezone(zi)
+                    self.assertEqual(dt_after.fold, 1, (dt_after, dt_utc))
+
+    def test_time_variable_offset(self):
+        # self.zones() only ever returns variable-offset zones
+        for key in self.zones():
+            zi = self.zone_from_key(key)
+            t = time(11, 15, 1, 34471, tzinfo=zi)
+
+            with self.subTest(key=key):
+                self.assertIs(t.tzname(), None)
+                self.assertIs(t.utcoffset(), None)
+                self.assertIs(t.dst(), None)
+
+    def test_time_fixed_offset(self):
+        for key, offset in self.fixed_offset_zones():
+            zi = self.zone_from_key(key)
+
+            t = time(11, 15, 1, 34471, tzinfo=zi)
+
+            with self.subTest(key=key):
+                self.assertEqual(t.tzname(), offset.tzname)
+                self.assertEqual(t.utcoffset(), offset.utcoffset)
+                self.assertEqual(t.dst(), offset.dst)
+
+
+class CZoneInfoTest(ZoneInfoTest):
+    module = c_zoneinfo
+
+    def test_fold_mutate(self):
+        """Test that fold isn't mutated when no change is necessary.
+
+        The underlying C API is capable of mutating datetime objects, and
+        may rely on the fact that addition of a datetime object returns a
+        new datetime; this test ensures that the input datetime to fromutc
+        is not mutated.
+        """
+
+        def to_subclass(dt):
+            class SameAddSubclass(type(dt)):
+                def __add__(self, other):
+                    if other == timedelta(0):
+                        return self
+
+                    return super().__add__(other)  # pragma: nocover
+
+            return SameAddSubclass(
+                dt.year,
+                dt.month,
+                dt.day,
+                dt.hour,
+                dt.minute,
+                dt.second,
+                dt.microsecond,
+                fold=dt.fold,
+                tzinfo=dt.tzinfo,
+            )
+
+        subclass = [False, True]
+
+        key = "Europe/London"
+        zi = self.zone_from_key(key)
+        for zt in self.load_transition_examples(key):
+            if zt.fold and zt.offset_after.utcoffset == ZERO:
+                example = zt.transition_utc.replace(tzinfo=zi)
+                break
+
+        for subclass in [False, True]:
+            if subclass:
+                dt = to_subclass(example)
+            else:
+                dt = example
+
+            with self.subTest(subclass=subclass):
+                dt_fromutc = zi.fromutc(dt)
+
+                self.assertEqual(dt_fromutc.fold, 1)
+                self.assertEqual(dt.fold, 0)
+
+
+class ZoneInfoDatetimeSubclassTest(DatetimeSubclassMixin, ZoneInfoTest):
+    pass
+
+
+class CZoneInfoDatetimeSubclassTest(DatetimeSubclassMixin, CZoneInfoTest):
+    pass
+
+
+class ZoneInfoTestSubclass(ZoneInfoTest):
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+        class ZISubclass(cls.klass):
+            pass
+
+        cls.class_name = "ZISubclass"
+        cls.parent_klass = cls.klass
+        cls.klass = ZISubclass
+
+    def test_subclass_own_cache(self):
+        base_obj = self.parent_klass("Europe/London")
+        sub_obj = self.klass("Europe/London")
+
+        self.assertIsNot(base_obj, sub_obj)
+        self.assertIsInstance(base_obj, self.parent_klass)
+        self.assertIsInstance(sub_obj, self.klass)
+
+
+class CZoneInfoTestSubclass(ZoneInfoTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoV1Test(ZoneInfoTest):
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA_V1
+
+    def load_transition_examples(self, key):
+        # We will discard zdump examples outside the range epoch +/- 2**31,
+        # because they are not well-supported in Version 1 files.
+        epoch = datetime(1970, 1, 1)
+        max_offset_32 = timedelta(seconds=2 ** 31)
+        min_dt = epoch - max_offset_32
+        max_dt = epoch + max_offset_32
+
+        for zt in ZoneDumpData.load_transition_examples(key):
+            if min_dt <= zt.transition <= max_dt:
+                yield zt
+
+
+class CZoneInfoV1Test(ZoneInfoV1Test):
+    module = c_zoneinfo
+
+
+@unittest.skipIf(
+    not HAS_TZDATA_PKG, "Skipping tzdata-specific tests: tzdata not installed"
+)
+class TZDataTests(ZoneInfoTest):
+    """
+    Runs all the ZoneInfoTest tests, but against the tzdata package
+
+    NOTE: The ZoneDumpData has frozen test data, but tzdata will update, so
+    some of the tests (particularly those related to the far future) may break
+    in the event that the time zone policies in the relevant time zones change.
+    """
+
+    @property
+    def tzpath(self):
+        return []
+
+    def zone_from_key(self, key):
+        return self.klass(key=key)
+
+
+@unittest.skipIf(
+    not HAS_TZDATA_PKG, "Skipping tzdata-specific tests: tzdata not installed"
+)
+class CTZDataTests(TZDataTests):
+    module = c_zoneinfo
+
+
+class WeirdZoneTest(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def test_one_transition(self):
+        LMT = ZoneOffset("LMT", -timedelta(hours=6, minutes=31, seconds=2))
+        STD = ZoneOffset("STD", -timedelta(hours=6))
+
+        transitions = [
+            ZoneTransition(datetime(1883, 6, 9, 14), LMT, STD),
+        ]
+
+        after = "STD6"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf)
+
+        dt0 = datetime(1883, 6, 9, 1, tzinfo=zi)
+        dt1 = datetime(1883, 6, 10, 1, tzinfo=zi)
+
+        for dt, offset in [(dt0, LMT), (dt1, STD)]:
+            with self.subTest(name="local", dt=dt):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        dts = [
+            (
+                datetime(1883, 6, 9, 1, tzinfo=zi),
+                datetime(1883, 6, 9, 7, 31, 2, tzinfo=timezone.utc),
+            ),
+            (
+                datetime(2010, 4, 1, 12, tzinfo=zi),
+                datetime(2010, 4, 1, 18, tzinfo=timezone.utc),
+            ),
+        ]
+
+        for dt_local, dt_utc in dts:
+            with self.subTest(name="fromutc", dt=dt_local):
+                dt_actual = dt_utc.astimezone(zi)
+                self.assertEqual(dt_actual, dt_local)
+
+                dt_utc_actual = dt_local.astimezone(timezone.utc)
+                self.assertEqual(dt_utc_actual, dt_utc)
+
+    def test_one_zone_dst(self):
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+        transitions = [
+            ZoneTransition(datetime(1970, 1, 1), DST, DST),
+        ]
+
+        after = "STD0DST-1,0/0,J365/25"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf)
+
+        dts = [
+            datetime(1900, 3, 1),
+            datetime(1965, 9, 12),
+            datetime(1970, 1, 1),
+            datetime(2010, 11, 3),
+            datetime(2040, 1, 1),
+        ]
+
+        for dt in dts:
+            dt = dt.replace(tzinfo=zi)
+            with self.subTest(dt=dt):
+                self.assertEqual(dt.tzname(), DST.tzname)
+                self.assertEqual(dt.utcoffset(), DST.utcoffset)
+                self.assertEqual(dt.dst(), DST.dst)
+
+    def test_no_tz_str(self):
+        STD = ZoneOffset("STD", ONE_H, ZERO)
+        DST = ZoneOffset("DST", 2 * ONE_H, ONE_H)
+
+        transitions = []
+        for year in range(1996, 2000):
+            transitions.append(
+                ZoneTransition(datetime(year, 3, 1, 2), STD, DST)
+            )
+            transitions.append(
+                ZoneTransition(datetime(year, 11, 1, 2), DST, STD)
+            )
+
+        after = ""
+
+        zf = self.construct_zone(transitions, after)
+
+        # According to RFC 8536, local times after the last transition time
+        # with an empty TZ string are unspecified. We will go with "hold the
+        # last transition", but the most we should promise is "doesn't crash."
+        zi = self.klass.from_file(zf)
+
+        cases = [
+            (datetime(1995, 1, 1), STD),
+            (datetime(1996, 4, 1), DST),
+            (datetime(1996, 11, 2), STD),
+            # (datetime(2001, 1, 1), STD),  # RFC 8536: offset after last transition is unspecified
+        ]
+
+        for dt, offset in cases:
+            dt = dt.replace(tzinfo=zi)
+            with self.subTest(dt=dt):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        # Test that offsets return None when using a datetime.time
+        t = time(0, tzinfo=zi)
+        with self.subTest("Testing datetime.time"):
+            self.assertIs(t.tzname(), None)
+            self.assertIs(t.utcoffset(), None)
+            self.assertIs(t.dst(), None)
+
+    def test_tz_before_only(self):
+        # From RFC 8536 Section 3.2:
+        #
+        #   If there are no transitions, local time for all timestamps is
+        #   specified by the TZ string in the footer if present and nonempty;
+        #   otherwise, it is specified by time type 0.
+
+        offsets = [
+            ZoneOffset("STD", ZERO, ZERO),
+            ZoneOffset("DST", ONE_H, ONE_H),
+        ]
+
+        for offset in offsets:
+            # Phantom transition to set time type 0.
+            transitions = [
+                ZoneTransition(None, offset, offset),
+            ]
+
+            after = ""
+
+            zf = self.construct_zone(transitions, after)
+            zi = self.klass.from_file(zf)
+
+            dts = [
+                datetime(1900, 1, 1),
+                datetime(1970, 1, 1),
+                datetime(2000, 1, 1),
+            ]
+
+            for dt in dts:
+                dt = dt.replace(tzinfo=zi)
+                with self.subTest(offset=offset, dt=dt):
+                    self.assertEqual(dt.tzname(), offset.tzname)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                    self.assertEqual(dt.dst(), offset.dst)
+
+    def test_empty_zone(self):
+        zf = self.construct_zone([], "")
+
+        with self.assertRaises(ValueError):
+            self.klass.from_file(zf)
+
+    def test_zone_very_large_timestamp(self):
+        """Test when a transition is in the far past or future.
+
+        Particularly, this is a concern if something:
+
+            1. Attempts to call ``datetime.timestamp`` for a datetime outside
+               of ``[datetime.min, datetime.max]``.
+            2. Attempts to construct a timedelta outside of
+               ``[timedelta.min, timedelta.max]``.
+
+        This actually occurs "in the wild", as some time zones on Ubuntu (at
+        least as of 2020) have an initial transition added at ``-2**58``.
+        """
+
+        LMT = ZoneOffset("LMT", timedelta(seconds=-968))
+        GMT = ZoneOffset("GMT", ZERO)
+
+        transitions = [
+            (-(1 << 62), LMT, LMT),
+            ZoneTransition(datetime(1912, 1, 1), LMT, GMT),
+            ((1 << 62), GMT, GMT),
+        ]
+
+        after = "GMT0"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf, key="Africa/Abidjan")
+
+        offset_cases = [
+            (datetime.min, LMT),
+            (datetime.max, GMT),
+            (datetime(1911, 12, 31), LMT),
+            (datetime(1912, 1, 2), GMT),
+        ]
+
+        for dt_naive, offset in offset_cases:
+            dt = dt_naive.replace(tzinfo=zi)
+            with self.subTest(name="offset", dt=dt, offset=offset):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        utc_cases = [
+            (datetime.min, datetime.min + timedelta(seconds=968)),
+            (datetime(1898, 12, 31, 23, 43, 52), datetime(1899, 1, 1)),
+            (
+                datetime(1911, 12, 31, 23, 59, 59, 999999),
+                datetime(1912, 1, 1, 0, 16, 7, 999999),
+            ),
+            (datetime(1912, 1, 1, 0, 16, 8), datetime(1912, 1, 1, 0, 16, 8)),
+            (datetime(1970, 1, 1), datetime(1970, 1, 1)),
+            (datetime.max, datetime.max),
+        ]
+
+        for naive_dt, naive_dt_utc in utc_cases:
+            dt = naive_dt.replace(tzinfo=zi)
+            dt_utc = naive_dt_utc.replace(tzinfo=timezone.utc)
+
+            self.assertEqual(dt_utc.astimezone(zi), dt)
+            self.assertEqual(dt, dt_utc)
+
+    def test_fixed_offset_phantom_transition(self):
+        UTC = ZoneOffset("UTC", ZERO, ZERO)
+
+        transitions = [ZoneTransition(datetime(1970, 1, 1), UTC, UTC)]
+
+        after = "UTC0"
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf, key="UTC")
+
+        dt = datetime(2020, 1, 1, tzinfo=zi)
+        with self.subTest("datetime.datetime"):
+            self.assertEqual(dt.tzname(), UTC.tzname)
+            self.assertEqual(dt.utcoffset(), UTC.utcoffset)
+            self.assertEqual(dt.dst(), UTC.dst)
+
+        t = time(0, tzinfo=zi)
+        with self.subTest("datetime.time"):
+            self.assertEqual(t.tzname(), UTC.tzname)
+            self.assertEqual(t.utcoffset(), UTC.utcoffset)
+            self.assertEqual(t.dst(), UTC.dst)
+
+    def construct_zone(self, transitions, after=None, version=3):
+        # These are not used for anything, so we're not going to include
+        # them for now.
+        isutc = []
+        isstd = []
+        leap_seconds = []
+
+        offset_lists = [[], []]
+        trans_times_lists = [[], []]
+        trans_idx_lists = [[], []]
+
+        v1_range = (-(2 ** 31), 2 ** 31)
+        v2_range = (-(2 ** 63), 2 ** 63)
+        ranges = [v1_range, v2_range]
+
+        def zt_as_tuple(zt):
+            # zt may be a tuple (timestamp, offset_before, offset_after) or
+            # a ZoneTransition object — this is to allow the timestamp to be
+            # values that are outside the valid range for datetimes but still
+            # valid 64-bit timestamps.
+            if isinstance(zt, tuple):
+                return zt
+
+            if zt.transition:
+                trans_time = int(zt.transition_utc.timestamp())
+            else:
+                trans_time = None
+
+            return (trans_time, zt.offset_before, zt.offset_after)
+
+        transitions = sorted(map(zt_as_tuple, transitions), key=lambda x: x[0])
+
+        for zt in transitions:
+            trans_time, offset_before, offset_after = zt
+
+            for v, (dt_min, dt_max) in enumerate(ranges):
+                offsets = offset_lists[v]
+                trans_times = trans_times_lists[v]
+                trans_idx = trans_idx_lists[v]
+
+                if trans_time is not None and not (
+                    dt_min <= trans_time <= dt_max
+                ):
+                    continue
+
+                if offset_before not in offsets:
+                    offsets.append(offset_before)
+
+                if offset_after not in offsets:
+                    offsets.append(offset_after)
+
+                if trans_time is not None:
+                    trans_times.append(trans_time)
+                    trans_idx.append(offsets.index(offset_after))
+
+        isutcnt = len(isutc)
+        isstdcnt = len(isstd)
+        leapcnt = len(leap_seconds)
+
+        zonefile = io.BytesIO()
+
+        time_types = ("l", "q")
+        for v in range(min((version, 2))):
+            offsets = offset_lists[v]
+            trans_times = trans_times_lists[v]
+            trans_idx = trans_idx_lists[v]
+            time_type = time_types[v]
+
+            # Translate the offsets into something closer to the C values
+            abbrstr = bytearray()
+            ttinfos = []
+
+            for offset in offsets:
+                utcoff = int(offset.utcoffset.total_seconds())
+                isdst = bool(offset.dst)
+                abbrind = len(abbrstr)
+
+                ttinfos.append((utcoff, isdst, abbrind))
+                abbrstr += offset.tzname.encode("ascii") + b"\x00"
+            abbrstr = bytes(abbrstr)
+
+            typecnt = len(offsets)
+            timecnt = len(trans_times)
+            charcnt = len(abbrstr)
+
+            # Write the header
+            zonefile.write(b"TZif")
+            zonefile.write(b"%d" % version)
+            zonefile.write(b" " * 15)
+            zonefile.write(
+                struct.pack(
+                    ">6l", isutcnt, isstdcnt, leapcnt, timecnt, typecnt, charcnt
+                )
+            )
+
+            # Now the transition data
+            zonefile.write(struct.pack(f">{timecnt}{time_type}", *trans_times))
+            zonefile.write(struct.pack(f">{timecnt}B", *trans_idx))
+
+            for ttinfo in ttinfos:
+                zonefile.write(struct.pack(">lbb", *ttinfo))
+
+            zonefile.write(bytes(abbrstr))
+
+            # Now the metadata and leap seconds
+            zonefile.write(struct.pack(f"{isutcnt}b", *isutc))
+            zonefile.write(struct.pack(f"{isstdcnt}b", *isstd))
+            zonefile.write(struct.pack(f">{leapcnt}l", *leap_seconds))
+
+            # Finally we write the TZ string if we're writing a Version 2+ file
+            if v > 0:
+                zonefile.write(b"\x0A")
+                zonefile.write(after.encode("ascii"))
+                zonefile.write(b"\x0A")
+
+        zonefile.seek(0)
+        return zonefile
+
+
+class CWeirdZoneTest(WeirdZoneTest):
+    module = c_zoneinfo
+
+
+class TZStrTest(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    NORMAL = 0
+    FOLD = 1
+    GAP = 2
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+        cls._populate_test_cases()
+        cls.populate_tzstr_header()
+
+    @classmethod
+    def populate_tzstr_header(cls):
+        out = bytearray()
+        # The TZif format always starts with a Version 1 file followed by
+        # the Version 2+ file. In this case, we have no transitions, just
+        # the tzstr in the footer, so up to the footer, the files are
+        # identical and we can just write the same file twice in a row.
+        for i in range(2):
+            out += b"TZif"  # Magic value
+            out += b"3"  # Version
+            out += b" " * 15  # Reserved
+
+            # We will not write any of the manual transition parts
+            out += struct.pack(">6l", 0, 0, 0, 0, 0, 0)
+
+        cls._tzif_header = bytes(out)
+
+    def zone_from_tzstr(self, tzstr):
+        """Creates a zoneinfo file following a POSIX rule."""
+        zonefile = io.BytesIO(self._tzif_header)
+        zonefile.seek(0, 2)
+
+        # Write the footer
+        zonefile.write(b"\x0A")
+        zonefile.write(tzstr.encode("ascii"))
+        zonefile.write(b"\x0A")
+
+        zonefile.seek(0)
+
+        return self.klass.from_file(zonefile, key=tzstr)
+
+    def test_tzstr_localized(self):
+        i = 0
+        for tzstr, cases in self.test_cases.items():
+            with self.subTest(tzstr=tzstr):
+                zi = self.zone_from_tzstr(tzstr)
+
+            for dt_naive, offset, _ in cases:
+                dt = dt_naive.replace(tzinfo=zi)
+
+                with self.subTest(tzstr=tzstr, dt=dt, offset=offset):
+                    self.assertEqual(dt.tzname(), offset.tzname)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                    self.assertEqual(dt.dst(), offset.dst)
+
+    def test_tzstr_from_utc(self):
+        for tzstr, cases in self.test_cases.items():
+            with self.subTest(tzstr=tzstr):
+                zi = self.zone_from_tzstr(tzstr)
+
+            for dt_naive, offset, dt_type in cases:
+                if dt_type == self.GAP:
+                    continue  # Cannot create a gap from UTC
+
+                dt_utc = (dt_naive - offset.utcoffset).replace(
+                    tzinfo=timezone.utc
+                )
+
+                # Check that we can go UTC -> Our zone
+                dt_act = dt_utc.astimezone(zi)
+                dt_exp = dt_naive.replace(tzinfo=zi)
+
+                self.assertEqual(dt_act, dt_exp)
+
+                if dt_type == self.FOLD:
+                    self.assertEqual(dt_act.fold, dt_naive.fold, dt_naive)
+                else:
+                    self.assertEqual(dt_act.fold, 0)
+
+                # Now check that we can go our zone -> UTC
+                dt_act = dt_exp.astimezone(timezone.utc)
+
+                self.assertEqual(dt_act, dt_utc)
+
+    def test_invalid_tzstr(self):
+        invalid_tzstrs = [
+            "PST8PDT",  # DST but no transition specified
+            "+11",  # Unquoted alphanumeric
+            "GMT,M3.2.0/2,M11.1.0/3",  # Transition rule but no DST
+            "GMT0+11,M3.2.0/2,M11.1.0/3",  # Unquoted alphanumeric in DST
+            "PST8PDT,M3.2.0/2",  # Only one transition rule
+            # Invalid offsets
+            "STD+25",
+            "STD-25",
+            "STD+374",
+            "STD+374DST,M3.2.0/2,M11.1.0/3",
+            "STD+23DST+25,M3.2.0/2,M11.1.0/3",
+            "STD-23DST-25,M3.2.0/2,M11.1.0/3",
+            # Completely invalid dates
+            "AAA4BBB,M1443339,M11.1.0/3",
+            "AAA4BBB,M3.2.0/2,0349309483959c",
+            # Invalid months
+            "AAA4BBB,M13.1.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M13.1.1/2",
+            "AAA4BBB,M0.1.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M0.1.1/2",
+            # Invalid weeks
+            "AAA4BBB,M1.6.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M1.6.1/2",
+            # Invalid weekday
+            "AAA4BBB,M1.1.7/2,M2.1.1/2",
+            "AAA4BBB,M1.1.1/2,M2.1.7/2",
+            # Invalid numeric offset
+            "AAA4BBB,-1/2,20/2",
+            "AAA4BBB,1/2,-1/2",
+            "AAA4BBB,367,20/2",
+            "AAA4BBB,1/2,367/2",
+            # Invalid julian offset
+            "AAA4BBB,J0/2,J20/2",
+            "AAA4BBB,J20/2,J366/2",
+        ]
+
+        for invalid_tzstr in invalid_tzstrs:
+            with self.subTest(tzstr=invalid_tzstr):
+                # RFC 8536 does not guarantee the error message echoes the TZ
+                # string, so we assert only that a ValueError is raised rather
+                # than pinning the exact message text.
+                with self.assertRaises(ValueError):
+                    self.zone_from_tzstr(invalid_tzstr)
+
+    @classmethod
+    def _populate_test_cases(cls):
+        # This method uses a somewhat unusual style in that it populates the
+        # test cases for each tzstr by using a decorator to automatically call
+        # a function that mutates the current dictionary of test cases.
+        #
+        # The population of the test cases is done in individual functions to
+        # give each set of test cases its own namespace in which to define
+        # its offsets (this way we don't have to worry about variable reuse
+        # causing problems if someone makes a typo).
+        #
+        # The decorator for calling is used to make it more obvious that each
+        # function is actually called (if it's not decorated, it's not called).
+        def call(f):
+            """Decorator to call the addition methods.
+
+            This will call a function which adds at least one new entry into
+            the `cases` dictionary. The decorator will also assert that
+            something was added to the dictionary.
+            """
+            prev_len = len(cases)
+            f()
+            assert len(cases) > prev_len, "Function did not add a test case!"
+
+        NORMAL = cls.NORMAL
+        FOLD = cls.FOLD
+        GAP = cls.GAP
+
+        cases = {}
+
+        @call
+        def _add():
+            # Transition to EDT on the 2nd Sunday in March at 4 AM, and
+            # transition back on the first Sunday in November at 3AM
+            tzstr = "EST5EDT,M3.2.0/4:00,M11.1.0/3:00"
+
+            EST = ZoneOffset("EST", timedelta(hours=-5), ZERO)
+            EDT = ZoneOffset("EDT", timedelta(hours=-4), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 9), EST, NORMAL),
+                (datetime(2019, 3, 10, 3, 59), EST, NORMAL),
+                (datetime(2019, 3, 10, 4, 0, fold=0), EST, GAP),
+                (datetime(2019, 3, 10, 4, 0, fold=1), EDT, GAP),
+                (datetime(2019, 3, 10, 4, 1, fold=0), EST, GAP),
+                (datetime(2019, 3, 10, 4, 1, fold=1), EDT, GAP),
+                (datetime(2019, 11, 2), EDT, NORMAL),
+                (datetime(2019, 11, 3, 1, 59, fold=1), EDT, NORMAL),
+                (datetime(2019, 11, 3, 2, 0, fold=0), EDT, FOLD),
+                (datetime(2019, 11, 3, 2, 0, fold=1), EST, FOLD),
+                (datetime(2020, 3, 8, 3, 59), EST, NORMAL),
+                (datetime(2020, 3, 8, 4, 0, fold=0), EST, GAP),
+                (datetime(2020, 3, 8, 4, 0, fold=1), EDT, GAP),
+                (datetime(2020, 11, 1, 1, 59, fold=1), EDT, NORMAL),
+                (datetime(2020, 11, 1, 2, 0, fold=0), EDT, FOLD),
+                (datetime(2020, 11, 1, 2, 0, fold=1), EST, FOLD),
+            )
+
+        @call
+        def _add():
+            # Transition to BST happens on the last Sunday in March at 1 AM GMT
+            # and the transition back happens the last Sunday in October at 2AM BST
+            tzstr = "GMT0BST-1,M3.5.0/1:00,M10.5.0/2:00"
+
+            GMT = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 30), GMT, NORMAL),
+                (datetime(2019, 3, 31, 0, 59), GMT, NORMAL),
+                (datetime(2019, 3, 31, 2, 0), BST, NORMAL),
+                (datetime(2019, 10, 26), BST, NORMAL),
+                (datetime(2019, 10, 27, 0, 59, fold=1), BST, NORMAL),
+                (datetime(2019, 10, 27, 1, 0, fold=0), BST, GAP),
+                (datetime(2019, 10, 27, 2, 0, fold=1), GMT, GAP),
+                (datetime(2020, 3, 29, 0, 59), GMT, NORMAL),
+                (datetime(2020, 3, 29, 2, 0), BST, NORMAL),
+                (datetime(2020, 10, 25, 0, 59, fold=1), BST, NORMAL),
+                (datetime(2020, 10, 25, 1, 0, fold=0), BST, FOLD),
+                (datetime(2020, 10, 25, 2, 0, fold=1), GMT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Austrialian time zone - DST start is chronologically first
+            tzstr = "AEST-10AEDT,M10.1.0/2,M4.1.0/3"
+
+            AEST = ZoneOffset("AEST", timedelta(hours=10), ZERO)
+            AEDT = ZoneOffset("AEDT", timedelta(hours=11), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 4, 6), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 1, 59), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 1, 59, fold=1), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 2, 0, fold=0), AEDT, FOLD),
+                (datetime(2019, 4, 7, 2, 1, fold=0), AEDT, FOLD),
+                (datetime(2019, 4, 7, 2, 0, fold=1), AEST, FOLD),
+                (datetime(2019, 4, 7, 2, 1, fold=1), AEST, FOLD),
+                (datetime(2019, 4, 7, 3, 0, fold=0), AEST, NORMAL),
+                (datetime(2019, 4, 7, 3, 0, fold=1), AEST, NORMAL),
+                (datetime(2019, 10, 5, 0), AEST, NORMAL),
+                (datetime(2019, 10, 6, 1, 59), AEST, NORMAL),
+                (datetime(2019, 10, 6, 2, 0, fold=0), AEST, GAP),
+                (datetime(2019, 10, 6, 2, 0, fold=1), AEDT, GAP),
+                (datetime(2019, 10, 6, 3, 0), AEDT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Irish time zone - negative DST
+            tzstr = "IST-1GMT0,M10.5.0,M3.5.0/1"
+
+            GMT = ZoneOffset("GMT", ZERO, -ONE_H)
+            IST = ZoneOffset("IST", ONE_H, ZERO)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 30), GMT, NORMAL),
+                (datetime(2019, 3, 31, 0, 59), GMT, NORMAL),
+                (datetime(2019, 3, 31, 2, 0), IST, NORMAL),
+                (datetime(2019, 10, 26), IST, NORMAL),
+                (datetime(2019, 10, 27, 0, 59, fold=1), IST, NORMAL),
+                (datetime(2019, 10, 27, 1, 0, fold=0), IST, FOLD),
+                (datetime(2019, 10, 27, 1, 0, fold=1), GMT, FOLD),
+                (datetime(2019, 10, 27, 2, 0, fold=1), GMT, NORMAL),
+                (datetime(2020, 3, 29, 0, 59), GMT, NORMAL),
+                (datetime(2020, 3, 29, 2, 0), IST, NORMAL),
+                (datetime(2020, 10, 25, 0, 59, fold=1), IST, NORMAL),
+                (datetime(2020, 10, 25, 1, 0, fold=0), IST, FOLD),
+                (datetime(2020, 10, 25, 2, 0, fold=1), GMT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Pacific/Kosrae: Fixed offset zone with a quoted numerical tzname
+            tzstr = "<+11>-11"
+
+            cases[tzstr] = (
+                (
+                    datetime(2020, 1, 1),
+                    ZoneOffset("+11", timedelta(hours=11)),
+                    NORMAL,
+                ),
+            )
+
+        @call
+        def _add():
+            # Quoted STD and DST, transitions at 24:00
+            tzstr = "<-04>4<-03>,M9.1.6/24,M4.1.6/24"
+
+            M04 = ZoneOffset("-04", timedelta(hours=-4))
+            M03 = ZoneOffset("-03", timedelta(hours=-3), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2020, 5, 1), M04, NORMAL),
+                (datetime(2020, 11, 1), M03, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Permanent daylight saving time is modeled with transitions at 0/0
+            # and J365/25, as mentioned in RFC 8536 Section 3.3.1
+            tzstr = "EST5EDT,0/0,J365/25"
+
+            EDT = ZoneOffset("EDT", timedelta(hours=-4), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 1, 1), EDT, NORMAL),
+                (datetime(2019, 6, 1), EDT, NORMAL),
+                (datetime(2019, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+                (datetime(2020, 1, 1), EDT, NORMAL),
+                (datetime(2020, 3, 1), EDT, NORMAL),
+                (datetime(2020, 6, 1), EDT, NORMAL),
+                (datetime(2020, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+                (datetime(2400, 1, 1), EDT, NORMAL),
+                (datetime(2400, 3, 1), EDT, NORMAL),
+                (datetime(2400, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Transitions on March 1st and November 1st of each year
+            tzstr = "AAA3BBB,J60/12,J305/12"
+
+            AAA = ZoneOffset("AAA", timedelta(hours=-3))
+            BBB = ZoneOffset("BBB", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 1, 1), AAA, NORMAL),
+                (datetime(2019, 2, 28), AAA, NORMAL),
+                (datetime(2019, 3, 1, 11, 59), AAA, NORMAL),
+                (datetime(2019, 3, 1, 12, fold=0), AAA, GAP),
+                (datetime(2019, 3, 1, 12, fold=1), BBB, GAP),
+                (datetime(2019, 3, 1, 13), BBB, NORMAL),
+                (datetime(2019, 11, 1, 10, 59), BBB, NORMAL),
+                (datetime(2019, 11, 1, 11, fold=0), BBB, FOLD),
+                (datetime(2019, 11, 1, 11, fold=1), AAA, FOLD),
+                (datetime(2019, 11, 1, 12), AAA, NORMAL),
+                (datetime(2019, 12, 31, 23, 59, 59, 999999), AAA, NORMAL),
+                (datetime(2020, 1, 1), AAA, NORMAL),
+                (datetime(2020, 2, 29), AAA, NORMAL),
+                (datetime(2020, 3, 1, 11, 59), AAA, NORMAL),
+                (datetime(2020, 3, 1, 12, fold=0), AAA, GAP),
+                (datetime(2020, 3, 1, 12, fold=1), BBB, GAP),
+                (datetime(2020, 3, 1, 13), BBB, NORMAL),
+                (datetime(2020, 11, 1, 10, 59), BBB, NORMAL),
+                (datetime(2020, 11, 1, 11, fold=0), BBB, FOLD),
+                (datetime(2020, 11, 1, 11, fold=1), AAA, FOLD),
+                (datetime(2020, 11, 1, 12), AAA, NORMAL),
+                (datetime(2020, 12, 31, 23, 59, 59, 999999), AAA, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Taken from America/Godthab, this rule has a transition on the
+            # Saturday before the last Sunday of March and October, at 22:00
+            # and 23:00, respectively. This is encoded with negative start
+            # and end transition times.
+            tzstr = "<-03>3<-02>,M3.5.0/-2,M10.5.0/-1"
+
+            N03 = ZoneOffset("-03", timedelta(hours=-3))
+            N02 = ZoneOffset("-02", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2020, 3, 27), N03, NORMAL),
+                (datetime(2020, 3, 28, 21, 59, 59), N03, NORMAL),
+                (datetime(2020, 3, 28, 22, fold=0), N03, GAP),
+                (datetime(2020, 3, 28, 22, fold=1), N02, GAP),
+                (datetime(2020, 3, 28, 23), N02, NORMAL),
+                (datetime(2020, 10, 24, 21), N02, NORMAL),
+                (datetime(2020, 10, 24, 22, fold=0), N02, FOLD),
+                (datetime(2020, 10, 24, 22, fold=1), N03, FOLD),
+                (datetime(2020, 10, 24, 23), N03, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Transition times with minutes and seconds
+            tzstr = "AAA3BBB,M3.2.0/01:30,M11.1.0/02:15:45"
+
+            AAA = ZoneOffset("AAA", timedelta(hours=-3))
+            BBB = ZoneOffset("BBB", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2012, 3, 11, 1, 0), AAA, NORMAL),
+                (datetime(2012, 3, 11, 1, 30, fold=0), AAA, GAP),
+                (datetime(2012, 3, 11, 1, 30, fold=1), BBB, GAP),
+                (datetime(2012, 3, 11, 2, 30), BBB, NORMAL),
+                (datetime(2012, 11, 4, 1, 15, 44, 999999), BBB, NORMAL),
+                (datetime(2012, 11, 4, 1, 15, 45, fold=0), BBB, FOLD),
+                (datetime(2012, 11, 4, 1, 15, 45, fold=1), AAA, FOLD),
+                (datetime(2012, 11, 4, 2, 15, 45), AAA, NORMAL),
+            )
+
+        cls.test_cases = cases
+
+
+class CTZStrTest(TZStrTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoCacheTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def setUp(self):
+        self.klass.clear_cache()
+        super().setUp()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def test_ephemeral_zones(self):
+        self.assertIs(
+            self.klass("America/Los_Angeles"), self.klass("America/Los_Angeles")
+        )
+
+    def test_strong_refs(self):
+        tz0 = self.klass("Australia/Sydney")
+        tz1 = self.klass("Australia/Sydney")
+
+        self.assertIs(tz0, tz1)
+
+    def test_no_cache(self):
+
+        tz0 = self.klass("Europe/Lisbon")
+        tz1 = self.klass.no_cache("Europe/Lisbon")
+
+        self.assertIsNot(tz0, tz1)
+
+    def test_cache_reset_tzpath(self):
+        """Test that the cache persists when tzpath has been changed.
+
+        The PEP specifies that as long as a reference exists to one zone
+        with a given key, the primary constructor must continue to return
+        the same object.
+        """
+        zi0 = self.klass("America/Los_Angeles")
+        with self.tzpath_context([]):
+            zi1 = self.klass("America/Los_Angeles")
+
+        self.assertIs(zi0, zi1)
+
+    def test_clear_cache_explicit_none(self):
+        la0 = self.klass("America/Los_Angeles")
+        self.klass.clear_cache(only_keys=None)
+        la1 = self.klass("America/Los_Angeles")
+
+        self.assertIsNot(la0, la1)
+
+    def test_clear_cache_one_key(self):
+        """Tests that you can clear a single key from the cache."""
+        la0 = self.klass("America/Los_Angeles")
+        dub0 = self.klass("Europe/Dublin")
+
+        self.klass.clear_cache(only_keys=["America/Los_Angeles"])
+
+        la1 = self.klass("America/Los_Angeles")
+        dub1 = self.klass("Europe/Dublin")
+
+        self.assertIsNot(la0, la1)
+        self.assertIs(dub0, dub1)
+
+    def test_clear_cache_two_keys(self):
+        la0 = self.klass("America/Los_Angeles")
+        dub0 = self.klass("Europe/Dublin")
+        tok0 = self.klass("Asia/Tokyo")
+
+        self.klass.clear_cache(
+            only_keys=["America/Los_Angeles", "Europe/Dublin"]
+        )
+
+        la1 = self.klass("America/Los_Angeles")
+        dub1 = self.klass("Europe/Dublin")
+        tok1 = self.klass("Asia/Tokyo")
+
+        self.assertIsNot(la0, la1)
+        self.assertIsNot(dub0, dub1)
+        self.assertIs(tok0, tok1)
+
+
+class CZoneInfoCacheTest(ZoneInfoCacheTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoPickleTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def setUp(self):
+        self.klass.clear_cache()
+
+        with contextlib.ExitStack() as stack:
+            stack.enter_context(test_support.set_zoneinfo_module(self.module))
+            self.addCleanup(stack.pop_all().close)
+
+        super().setUp()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def test_cache_hit(self):
+        zi_in = self.klass("Europe/Dublin")
+        pkl = pickle.dumps(zi_in)
+        zi_rt = pickle.loads(pkl)
+
+        with self.subTest(test="Is non-pickled ZoneInfo"):
+            self.assertIs(zi_in, zi_rt)
+
+        zi_rt2 = pickle.loads(pkl)
+        with self.subTest(test="Is unpickled ZoneInfo"):
+            self.assertIs(zi_rt, zi_rt2)
+
+    def test_cache_miss(self):
+        zi_in = self.klass("Europe/Dublin")
+        pkl = pickle.dumps(zi_in)
+
+        del zi_in
+        self.klass.clear_cache()  # Induce a cache miss
+        zi_rt = pickle.loads(pkl)
+        zi_rt2 = pickle.loads(pkl)
+
+        self.assertIs(zi_rt, zi_rt2)
+
+    def test_no_cache(self):
+        zi_no_cache = self.klass.no_cache("Europe/Dublin")
+
+        pkl = pickle.dumps(zi_no_cache)
+        zi_rt = pickle.loads(pkl)
+
+        with self.subTest(test="Not the pickled object"):
+            self.assertIsNot(zi_rt, zi_no_cache)
+
+        zi_rt2 = pickle.loads(pkl)
+        with self.subTest(test="Not a second unpickled object"):
+            self.assertIsNot(zi_rt, zi_rt2)
+
+        zi_cache = self.klass("Europe/Dublin")
+        with self.subTest(test="Not a cached object"):
+            self.assertIsNot(zi_rt, zi_cache)
+
+    def test_from_file(self):
+        key = "Europe/Dublin"
+        with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+            zi_nokey = self.klass.from_file(f)
+
+            f.seek(0)
+            zi_key = self.klass.from_file(f, key=key)
+
+        test_cases = [
+            (zi_key, "ZoneInfo with key"),
+            (zi_nokey, "ZoneInfo without key"),
+        ]
+
+        for zi, test_name in test_cases:
+            with self.subTest(test_name=test_name):
+                with self.assertRaises(Exception):
+                    pickle.dumps(zi)
+
+    def test_pickle_after_from_file(self):
+        # This may be a bit of paranoia, but this test is to ensure that no
+        # global state is maintained in order to handle the pickle cache and
+        # from_file behavior, and that it is possible to interweave the
+        # constructors of each of these and pickling/unpickling without issues.
+        key = "Europe/Dublin"
+        zi = self.klass(key)
+
+        pkl_0 = pickle.dumps(zi)
+        zi_rt_0 = pickle.loads(pkl_0)
+        self.assertIs(zi, zi_rt_0)
+
+        with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+            zi_ff = self.klass.from_file(f, key=key)
+
+        pkl_1 = pickle.dumps(zi)
+        zi_rt_1 = pickle.loads(pkl_1)
+        self.assertIs(zi, zi_rt_1)
+
+        with self.assertRaises(Exception):
+            pickle.dumps(zi_ff)
+
+        pkl_2 = pickle.dumps(zi)
+        zi_rt_2 = pickle.loads(pkl_2)
+        self.assertIs(zi, zi_rt_2)
+
+
+class CZoneInfoPickleTest(ZoneInfoPickleTest):
+    module = c_zoneinfo
+
+
+class CallingConventionTest(ZoneInfoTestBase):
+    """Tests for functions with restricted calling conventions."""
+
+    module = py_zoneinfo
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    def test_from_file(self):
+        with open(self.zoneinfo_data.path_from_key("UTC"), "rb") as f:
+            with self.assertRaises(TypeError):
+                self.klass.from_file(fobj=f)
+
+    def test_clear_cache(self):
+        with self.assertRaises(TypeError):
+            self.klass.clear_cache(["UTC"])
+
+
+class CCallingConventionTest(CallingConventionTest):
+    module = c_zoneinfo
+
+
+class TzPathTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    @staticmethod
+    @contextlib.contextmanager
+    def python_tzpath_context(value):
+        path_var = "PYTHONTZPATH"
+        try:
+            with OS_ENV_LOCK:
+                old_env = os.environ.get(path_var, None)
+                os.environ[path_var] = value
+                yield
+        finally:
+            if old_env is None:
+                del os.environ[path_var]
+            else:
+                os.environ[path_var] = old_env  # pragma: nocover
+
+    def test_env_variable(self):
+        """Tests that the environment variable works with reset_tzpath."""
+        new_paths = [
+            ("", []),
+            ("/etc/zoneinfo", ["/etc/zoneinfo"]),
+            (f"/a/b/c{os.pathsep}/d/e/f", ["/a/b/c", "/d/e/f"]),
+        ]
+
+        for new_path_var, expected_result in new_paths:
+            with self.python_tzpath_context(new_path_var):
+                with self.subTest(tzpath=new_path_var):
+                    self.module.reset_tzpath()
+                    tzpath = self.module.TZPATH
+                    self.assertSequenceEqual(tzpath, expected_result)
+
+    def test_env_variable_relative_paths(self):
+        test_cases = [
+            [("path/to/somewhere",), ()],
+            [
+                ("/usr/share/zoneinfo", "path/to/somewhere",),
+                ("/usr/share/zoneinfo",),
+            ],
+            [("../relative/path",), ()],
+            [
+                ("/usr/share/zoneinfo", "../relative/path",),
+                ("/usr/share/zoneinfo",),
+            ],
+            [("path/to/somewhere", "../relative/path",), ()],
+            [
+                (
+                    "/usr/share/zoneinfo",
+                    "path/to/somewhere",
+                    "../relative/path",
+                ),
+                ("/usr/share/zoneinfo",),
+            ],
+        ]
+
+        for input_paths, expected_paths in test_cases:
+            path_var = os.pathsep.join(input_paths)
+            with self.python_tzpath_context(path_var):
+                with self.subTest("warning", path_var=path_var):
+                    # Note: Per PEP 615 the warning is implementation-defined
+                    # behavior, other implementations need not warn.
+                    with self.assertWarns(self.module.InvalidTZPathWarning):
+                        self.module.reset_tzpath()
+
+                tzpath = self.module.TZPATH
+                with self.subTest("filtered", path_var=path_var):
+                    self.assertSequenceEqual(tzpath, expected_paths)
+
+    def test_reset_tzpath_kwarg(self):
+        self.module.reset_tzpath(to=["/a/b/c"])
+
+        self.assertSequenceEqual(self.module.TZPATH, ("/a/b/c",))
+
+    def test_reset_tzpath_relative_paths(self):
+        bad_values = [
+            ("path/to/somewhere",),
+            ("/usr/share/zoneinfo", "path/to/somewhere",),
+            ("../relative/path",),
+            ("/usr/share/zoneinfo", "../relative/path",),
+            ("path/to/somewhere", "../relative/path",),
+            ("/usr/share/zoneinfo", "path/to/somewhere", "../relative/path",),
+        ]
+        for input_paths in bad_values:
+            with self.subTest(input_paths=input_paths):
+                with self.assertRaises(ValueError):
+                    self.module.reset_tzpath(to=input_paths)
+
+    def test_tzpath_type_error(self):
+        bad_values = [
+            "/etc/zoneinfo:/usr/share/zoneinfo",
+            b"/etc/zoneinfo:/usr/share/zoneinfo",
+            0,
+        ]
+
+        for bad_value in bad_values:
+            with self.subTest(value=bad_value):
+                with self.assertRaises(TypeError):
+                    self.module.reset_tzpath(bad_value)
+
+    def test_tzpath_attribute(self):
+        tzpath_0 = ["/one", "/two"]
+        tzpath_1 = ["/three"]
+
+        with self.tzpath_context(tzpath_0):
+            query_0 = self.module.TZPATH
+
+        with self.tzpath_context(tzpath_1):
+            query_1 = self.module.TZPATH
+
+        self.assertSequenceEqual(tzpath_0, query_0)
+        self.assertSequenceEqual(tzpath_1, query_1)
+
+
+class CTzPathTest(TzPathTest):
+    module = c_zoneinfo
+
+
+class TestModule(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def test_getattr_error(self):
+        with self.assertRaises(AttributeError):
+            self.module.NOATTRIBUTE
+
+    def test_dir_contains_all(self):
+        """dir(self.module) should at least contain everything in __all__."""
+        module_all_set = set(self.module.__all__)
+        module_dir_set = set(dir(self.module))
+
+        difference = module_all_set - module_dir_set
+
+        self.assertFalse(difference)
+
+    def test_dir_unique(self):
+        """Test that there are no duplicates in dir(self.module)"""
+        module_dir = dir(self.module)
+        module_unique = set(module_dir)
+
+        self.assertCountEqual(module_dir, module_unique)
+
+
+class CTestModule(TestModule):
+    module = c_zoneinfo
+
+
+class ExtensionBuiltTest(unittest.TestCase):
+    """Smoke test to ensure that the C and Python extensions are both tested.
+
+    Because the intention is for the Python and C versions of ZoneInfo to
+    behave identically, these tests necessarily rely on implementation details,
+    so the tests may need to be adjusted if the implementations change. Do not
+    rely on these tests as an indication of stable properties of these classes.
+    """
+
+    def test_cache_location(self):
+        # The pure Python version stores caches on attributes, but the C
+        # extension stores them in C globals (at least for now)
+        self.assertFalse(hasattr(c_zoneinfo.ZoneInfo, "_weak_cache"))
+        self.assertTrue(hasattr(py_zoneinfo.ZoneInfo, "_weak_cache"))
+
+    def test_gc_tracked(self):
+        # The pure Python version is tracked by the GC but (for now) the C
+        # version is not.
+        import gc
+
+        self.assertTrue(gc.is_tracked(py_zoneinfo.ZoneInfo))
+        self.assertFalse(gc.is_tracked(c_zoneinfo.ZoneInfo))
+
+
+@dataclasses.dataclass(frozen=True)
+class ZoneOffset:
+    tzname: str
+    utcoffset: timedelta
+    dst: timedelta = ZERO
+
+
+@dataclasses.dataclass(frozen=True)
+class ZoneTransition:
+    transition: datetime
+    offset_before: ZoneOffset
+    offset_after: ZoneOffset
+
+    @property
+    def transition_utc(self):
+        return (self.transition - self.offset_before.utcoffset).replace(
+            tzinfo=timezone.utc
+        )
+
+    @property
+    def fold(self):
+        """Whether this introduces a fold"""
+        return self.offset_before.utcoffset > self.offset_after.utcoffset
+
+    @property
+    def gap(self):
+        """Whether this introduces a gap"""
+        return self.offset_before.utcoffset < self.offset_after.utcoffset
+
+    @property
+    def delta(self):
+        return self.offset_after.utcoffset - self.offset_before.utcoffset
+
+    @property
+    def anomaly_start(self):
+        if self.fold:
+            return self.transition + self.delta
+        else:
+            return self.transition
+
+    @property
+    def anomaly_end(self):
+        if not self.fold:
+            return self.transition + self.delta
+        else:
+            return self.transition
+
+
+class ZoneInfoData:
+    def __init__(self, source_json, tzpath, v1=False):
+        self.tzpath = pathlib.Path(tzpath)
+        self.keys = []
+        self.v1 = v1
+        self._populate_tzpath(source_json)
+
+    def path_from_key(self, key):
+        return self.tzpath / key
+
+    def _populate_tzpath(self, source_json):
+        with open(source_json, "rb") as f:
+            zoneinfo_dict = json.load(f)
+
+        zoneinfo_data = zoneinfo_dict["data"]
+
+        for key, value in zoneinfo_data.items():
+            self.keys.append(key)
+            raw_data = self._decode_text(value)
+
+            if self.v1:
+                data = self._convert_to_v1(raw_data)
+            else:
+                data = raw_data
+
+            destination = self.path_from_key(key)
+            destination.parent.mkdir(exist_ok=True, parents=True)
+            with open(destination, "wb") as f:
+                f.write(data)
+
+    def _decode_text(self, contents):
+        raw_data = b"".join(map(str.encode, contents))
+        decoded = base64.b85decode(raw_data)
+
+        return lzma.decompress(decoded)
+
+    def _convert_to_v1(self, contents):
+        assert contents[0:4] == b"TZif", "Invalid TZif data found!"
+        version = int(contents[4:5])
+
+        header_start = 4 + 16
+        header_end = header_start + 24  # 6l == 24 bytes
+        assert version >= 2, "Version 1 file found: no conversion necessary"
+        isutcnt, isstdcnt, leapcnt, timecnt, typecnt, charcnt = struct.unpack(
+            ">6l", contents[header_start:header_end]
+        )
+
+        file_size = (
+            timecnt * 5
+            + typecnt * 6
+            + charcnt
+            + leapcnt * 8
+            + isstdcnt
+            + isutcnt
+        )
+        file_size += header_end
+        out = b"TZif" + b"\x00" + contents[5:file_size]
+
+        assert (
+            contents[file_size : (file_size + 4)] == b"TZif"
+        ), "Version 2 file not truncated at Version 2 header"
+
+        return out
+
+
+class ZoneDumpData:
+    @classmethod
+    def transition_keys(cls):
+        return cls._get_zonedump().keys()
+
+    @classmethod
+    def load_transition_examples(cls, key):
+        return cls._get_zonedump()[key]
+
+    @classmethod
+    def fixed_offset_zones(cls):
+        if not cls._FIXED_OFFSET_ZONES:
+            cls._populate_fixed_offsets()
+
+        return cls._FIXED_OFFSET_ZONES.items()
+
+    @classmethod
+    def _get_zonedump(cls):
+        if not cls._ZONEDUMP_DATA:
+            cls._populate_zonedump_data()
+        return cls._ZONEDUMP_DATA
+
+    @classmethod
+    def _populate_fixed_offsets(cls):
+        cls._FIXED_OFFSET_ZONES = {
+            "UTC": ZoneOffset("UTC", ZERO, ZERO),
+        }
+
+    @classmethod
+    def _populate_zonedump_data(cls):
+        def _Africa_Abidjan():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-968))
+            GMT = ZoneOffset("GMT", ZERO)
+
+            return [
+                ZoneTransition(datetime(1912, 1, 1), LMT, GMT),
+            ]
+
+        def _Africa_Casablanca():
+            P00_s = ZoneOffset("+00", ZERO, ZERO)
+            P01_d = ZoneOffset("+01", ONE_H, ONE_H)
+            P00_d = ZoneOffset("+00", ZERO, -ONE_H)
+            P01_s = ZoneOffset("+01", ONE_H, ZERO)
+
+            return [
+                # Morocco sometimes pauses DST during Ramadan
+                ZoneTransition(datetime(2018, 3, 25, 2), P00_s, P01_d),
+                ZoneTransition(datetime(2018, 5, 13, 3), P01_d, P00_s),
+                ZoneTransition(datetime(2018, 6, 17, 2), P00_s, P01_d),
+                # On October 28th Morocco set standard time to +01,
+                # with negative DST only during Ramadan
+                ZoneTransition(datetime(2018, 10, 28, 3), P01_d, P01_s),
+                ZoneTransition(datetime(2019, 5, 5, 3), P01_s, P00_d),
+                ZoneTransition(datetime(2019, 6, 9, 2), P00_d, P01_s),
+            ]
+
+        def _America_Los_Angeles():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-28378), ZERO)
+            PST = ZoneOffset("PST", timedelta(hours=-8), ZERO)
+            PDT = ZoneOffset("PDT", timedelta(hours=-7), ONE_H)
+            PWT = ZoneOffset("PWT", timedelta(hours=-7), ONE_H)
+            PPT = ZoneOffset("PPT", timedelta(hours=-7), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1883, 11, 18, 12, 7, 2), LMT, PST),
+                ZoneTransition(datetime(1918, 3, 31, 2), PST, PDT),
+                ZoneTransition(datetime(1918, 3, 31, 2), PST, PDT),
+                ZoneTransition(datetime(1918, 10, 27, 2), PDT, PST),
+                # Transition to Pacific War Time
+                ZoneTransition(datetime(1942, 2, 9, 2), PST, PWT),
+                # Transition from Pacific War Time to Pacific Peace Time
+                ZoneTransition(datetime(1945, 8, 14, 16), PWT, PPT),
+                ZoneTransition(datetime(1945, 9, 30, 2), PPT, PST),
+                ZoneTransition(datetime(2015, 3, 8, 2), PST, PDT),
+                ZoneTransition(datetime(2015, 11, 1, 2), PDT, PST),
+                # After 2038: Rules continue indefinitely
+                ZoneTransition(datetime(2450, 3, 13, 2), PST, PDT),
+                ZoneTransition(datetime(2450, 11, 6, 2), PDT, PST),
+            ]
+
+        def _America_Santiago():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-16966), ZERO)
+            SMT = ZoneOffset("SMT", timedelta(seconds=-16966), ZERO)
+            N05 = ZoneOffset("-05", timedelta(seconds=-18000), ZERO)
+            N04 = ZoneOffset("-04", timedelta(seconds=-14400), ZERO)
+            N03 = ZoneOffset("-03", timedelta(seconds=-10800), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1890, 1, 1), LMT, SMT),
+                ZoneTransition(datetime(1910, 1, 10), SMT, N05),
+                ZoneTransition(datetime(1916, 7, 1), N05, SMT),
+                ZoneTransition(datetime(2008, 3, 30), N03, N04),
+                ZoneTransition(datetime(2008, 10, 12), N04, N03),
+                ZoneTransition(datetime(2040, 4, 8), N03, N04),
+                ZoneTransition(datetime(2040, 9, 2), N04, N03),
+            ]
+
+        def _Asia_Tokyo():
+            JST = ZoneOffset("JST", timedelta(seconds=32400), ZERO)
+            JDT = ZoneOffset("JDT", timedelta(seconds=36000), ONE_H)
+
+            # Japan had DST from 1948 to 1951, and it was unusual in that
+            # the transition from DST to STD occurred at 25:00, and is
+            # denominated as such in the time zone database
+            return [
+                ZoneTransition(datetime(1948, 5, 2), JST, JDT),
+                ZoneTransition(datetime(1948, 9, 12, 1), JDT, JST),
+                ZoneTransition(datetime(1951, 9, 9, 1), JDT, JST),
+            ]
+
+        def _Australia_Sydney():
+            LMT = ZoneOffset("LMT", timedelta(seconds=36292), ZERO)
+            AEST = ZoneOffset("AEST", timedelta(seconds=36000), ZERO)
+            AEDT = ZoneOffset("AEDT", timedelta(seconds=39600), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1895, 2, 1), LMT, AEST),
+                ZoneTransition(datetime(1917, 1, 1, 0, 1), AEST, AEDT),
+                ZoneTransition(datetime(1917, 3, 25, 2), AEDT, AEST),
+                ZoneTransition(datetime(2012, 4, 1, 3), AEDT, AEST),
+                ZoneTransition(datetime(2012, 10, 7, 2), AEST, AEDT),
+                ZoneTransition(datetime(2040, 4, 1, 3), AEDT, AEST),
+                ZoneTransition(datetime(2040, 10, 7, 2), AEST, AEDT),
+            ]
+
+        def _Europe_Dublin():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-1500), ZERO)
+            DMT = ZoneOffset("DMT", timedelta(seconds=-1521), ZERO)
+            IST_0 = ZoneOffset("IST", timedelta(seconds=2079), ONE_H)
+            GMT_0 = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+            GMT_1 = ZoneOffset("GMT", ZERO, -ONE_H)
+            IST_1 = ZoneOffset("IST", ONE_H, ZERO)
+
+            return [
+                ZoneTransition(datetime(1880, 8, 2, 0), LMT, DMT),
+                ZoneTransition(datetime(1916, 5, 21, 2), DMT, IST_0),
+                ZoneTransition(datetime(1916, 10, 1, 3), IST_0, GMT_0),
+                ZoneTransition(datetime(1917, 4, 8, 2), GMT_0, BST),
+                ZoneTransition(datetime(2016, 3, 27, 1), GMT_1, IST_1),
+                ZoneTransition(datetime(2016, 10, 30, 2), IST_1, GMT_1),
+                ZoneTransition(datetime(2487, 3, 30, 1), GMT_1, IST_1),
+                ZoneTransition(datetime(2487, 10, 26, 2), IST_1, GMT_1),
+            ]
+
+        def _Europe_Lisbon():
+            WET = ZoneOffset("WET", ZERO, ZERO)
+            WEST = ZoneOffset("WEST", ONE_H, ONE_H)
+            CET = ZoneOffset("CET", ONE_H, ZERO)
+            CEST = ZoneOffset("CEST", timedelta(seconds=7200), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1992, 3, 29, 1), WET, WEST),
+                ZoneTransition(datetime(1992, 9, 27, 2), WEST, CET),
+                ZoneTransition(datetime(1993, 3, 28, 2), CET, CEST),
+                ZoneTransition(datetime(1993, 9, 26, 3), CEST, CET),
+                ZoneTransition(datetime(1996, 3, 31, 2), CET, WEST),
+                ZoneTransition(datetime(1996, 10, 27, 2), WEST, WET),
+            ]
+
+        def _Europe_London():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-75), ZERO)
+            GMT = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+
+            return [
+                ZoneTransition(datetime(1847, 12, 1), LMT, GMT),
+                ZoneTransition(datetime(2005, 3, 27, 1), GMT, BST),
+                ZoneTransition(datetime(2005, 10, 30, 2), BST, GMT),
+                ZoneTransition(datetime(2043, 3, 29, 1), GMT, BST),
+                ZoneTransition(datetime(2043, 10, 25, 2), BST, GMT),
+            ]
+
+        def _Pacific_Kiritimati():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-37760), ZERO)
+            N1040 = ZoneOffset("-1040", timedelta(seconds=-38400), ZERO)
+            N10 = ZoneOffset("-10", timedelta(seconds=-36000), ZERO)
+            P14 = ZoneOffset("+14", timedelta(seconds=50400), ZERO)
+
+            # This is literally every transition in Christmas Island history
+            return [
+                ZoneTransition(datetime(1901, 1, 1), LMT, N1040),
+                ZoneTransition(datetime(1979, 10, 1), N1040, N10),
+                # They skipped December 31, 1994
+                ZoneTransition(datetime(1994, 12, 31), N10, P14),
+            ]
+
+        cls._ZONEDUMP_DATA = {
+            "Africa/Abidjan": _Africa_Abidjan(),
+            "Africa/Casablanca": _Africa_Casablanca(),
+            "America/Los_Angeles": _America_Los_Angeles(),
+            "America/Santiago": _America_Santiago(),
+            "Australia/Sydney": _Australia_Sydney(),
+            "Asia/Tokyo": _Asia_Tokyo(),
+            "Europe/Dublin": _Europe_Dublin(),
+            "Europe/Lisbon": _Europe_Lisbon(),
+            "Europe/London": _Europe_London(),
+            "Pacific/Kiritimati": _Pacific_Kiritimati(),
+        }
+
+    _ZONEDUMP_DATA = None
+    _FIXED_OFFSET_ZONES = None