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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>Flit Migration guide — map setup files to flit config</title>
<meta name="description" content="This free browser tool maps setup.py, setup.cfg, and older project configuration to replacement Flit settings and shows build blockers.">
<style>
:root{color-scheme:light}body{margin:0;font:15px/1.55 system-ui,-apple-system,"Segoe UI",sans-serif;background:#faf9f5;color:#1a1512}
.wrap{max-width:960px;margin:0 auto;padding:28px 18px 60px}h1{font-size:26px;margin:0 0 6px}.lede{color:#4a403a;margin:0 0 16px}
textarea{width:100%;min-height:260px;font:13px/1.45 ui-monospace,Menlo,Consolas,monospace;padding:12px;border:1px solid #d9d0c4;border-radius:8px;box-sizing:border-box}
.bar{display:flex;gap:10px;align-items:center;margin:12px 0}button{font:600 15px system-ui;padding:10px 18px;border:0;border-radius:8px;background:#1a1512;color:#fff;cursor:pointer}
input[type=date]{font:14px system-ui;padding:8px;border:1px solid #d9d0c4;border-radius:6px}
table{border-collapse:collapse;width:100%;font-size:14px;margin-top:10px}th,td{text-align:left;padding:8px 10px;border-bottom:1px solid #eee;vertical-align:top}th{font-size:12px;letter-spacing:.06em;text-transform:uppercase;color:#7a6e65}
.sev.error{color:#a8551f;font-weight:700}.sev.warn,.sev.warning{color:#96691f}.ok{color:#2a6446;font-weight:600}.sum{font-weight:600}
.note{font-size:13px;color:#7a6e65;margin-top:18px}#paid{margin-top:22px;padding:14px 16px;border:1px solid #e2d8cb;border-radius:10px;background:#fffdfa}
</style>
<script data-grow-initializer="">!(function(){window.growMe||((window.growMe=function(e){window.growMe._.push(e);}),(window.growMe._=[]));var e=document.createElement("script");(e.type="text/javascript"),(e.src="https://faves.grow.me/main.js"),(e.defer=!0),e.setAttribute("data-grow-faves-site-id","U2l0ZTphZjljMzgyMC0yMmU5LTRhYjgtYTIxNS03OTRhMGY1N2ExYzM=");var t=document.getElementsByTagName("script")[0];t.parentNode.insertBefore(e,t);})();</script>
<!--yj-canon-s101--><link rel="canonical" href="https://getreadystack.com/tools/flit-migration-check"><style data-yj-mfit="1">#out{max-width:100%}#out td,#out th{overflow-wrap:anywhere}#out pre,#out code{white-space:pre-wrap;overflow-wrap:anywhere}@media (max-width:640px){table{display:block;max-width:100%;overflow-x:auto}pre{max-width:100%;overflow-x:auto}code,a{overflow-wrap:anywhere}img,svg,video,canvas,iframe{max-width:100%}textarea,input,select{max-width:100%;box-sizing:border-box}.yj-lgbar{position:static!important;justify-content:flex-end;margin:0 0 10px}h1,h2,h3,h4,p,li,dd,dt,blockquote,figcaption,summary,label,.res,.lead{overflow-wrap:anywhere}label{max-width:100%;box-sizing:border-box}#out table:not(:has(tr>:nth-child(4))){display:table;table-layout:fixed;width:100%}#out table:has(tr>:nth-child(4)),#out table:has(tr>:nth-child(4)) tbody,#out table:has(tr>:nth-child(4)) tr,#out table:has(tr>:nth-child(4)) td{display:block;width:auto;overflow:visible}#out table:has(tr>:nth-child(4)) thead{display:none}#out table:has(tr>:nth-child(4)) tr{padding:10px 0;border-bottom:1px solid rgba(128,128,128,.28)}#out table:has(tr>:nth-child(4)) td{border:0;padding:1px 0}#out table:has(tr>:nth-child(4)) td:nth-child(-n+3){display:inline-block;margin-right:10px;font-size:12px;opacity:.78}}</style>
</head>
<body>
<div class="wrap">
<h1>Flit Migration guide — map setup files to flit config</h1>
<p class="lede">Flit migration map for Python packages: finds every setup.py, setup.cfg and setuptools/old-flit pyproject.toml line and prints the exact flit_core 4 [project] line that replaces it, plus the blockers Flit cannot build. Runs entirely in your browser — nothing is uploaded.</p>
<textarea id="src" spellcheck="false"># setup.py for "tidyframe" -- the file this project still builds from
import os
from setuptools import setup, find_packages

here = os.getcwd()
with open(os.path.join(here, "README.md")) as f:
    readme = f.read()

setup(
    name="tidyframe",
    version="2.4.1",
    description="Tidy helpers for pandas frames",
    long_description=readme,
    long_description_content_type="text/markdown",
    author="Dana Example",
    author_email="dana@example.org",
    url="https://github.com/example/tidyframe",
    license="MIT",
    packages=find_packages(exclude=["tests"]),
    package_dir={"": "src"},
    include_package_data=True,
    package_data={"tidyframe": ["schemas/*.json"]},
    data_files=[("share/man/man1", ["docs/tidyframe.1"])],
    python_requires="&gt;=3.8",
    setup_requires=["wheel"],
    install_requires=["pandas&gt;=2.0", "click&gt;=8"],
    extras_require={"test": ["pytest&gt;=7"], "docs": ["sphinx"]},
    entry_points={
        "console_scripts": ["tidyframe=tidyframe.cli:main"],
        "pandas_plugins": ["tidy=tidyframe.plugin:register"],
    },
    classifiers=["License :: OSI Approved :: MIT License"],
)</textarea>
<div class="bar"><button id="go">Check</button><label>as of <input type="date" id="today"></label></div>
<div id="out" aria-live="polite"></div>
<div id="paid"><strong>Free here:</strong> Check the open setup.py, setup.cfg or pyproject.toml: every setuptools or old-flit line with its line number and the exact flit_core 4 replacement line, plus the blockers Flit cannot build. <strong>In the editor:</strong> Whole workspace: one dated migration plan mapping every item in every package to its flit_core 4 replacement, and a team CI gate that fails a build adding new setuptools-only use. — that part asks for a licence key. <a href="https://getreadystack.com/api/buy/cl/polar_cl_VI4ER6LWKbRiLZHzzqUpAK77hPReaZmifZSkj19xgaN">Workspace migration plan + team CI gate — $29 once</a></div>
<p class="note">Same engine as the VS Code extension, byte for byte.</p>
</div>
<script>var FLIT_RULES = [
 {
  "id": "SP01",
  "check": "setuptools_setup_py",
  "files": [
   "setup.py"
  ],
  "re": "^\\s*(from\\s+(setuptools|distutils)[\\w.]*\\s+import|import\\s+(setuptools|distutils))",
  "sev": "warn",
  "msg": "setup.py drives this build; flit_core reads no setup.py",
  "fix": "delete setup.py; in pyproject.toml write [build-system] requires = [\"flit_core \u003e=3.11,\u003c5\"] and build-backend = \"flit_core.buildapi\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP02",
  "check": "compiled_extension",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bext_modules\\s*=|\\bExtension\\s*\\(|\\bcythonize\\s*\\(|\\bPybind11Extension\\b",
  "sev": "error",
  "msg": "BLOCKER: compiled extension; Flit builds pure Python packages only (\"If your package needs a build step, you won't be able to use Flit\")",
  "fix": "keep setuptools for this package, or split the pure-Python part into its own Flit package",
  "source": "https://flit.pypa.io/en/stable/rationale.html"
 },
 {
  "id": "SP03",
  "check": "custom_cmdclass",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bcmdclass\\s*=",
  "sev": "error",
  "msg": "BLOCKER: custom build command (cmdclass); flit_core runs no build hooks",
  "fix": "move the generated file into the source tree before the build, or keep setuptools",
  "source": "https://flit.pypa.io/en/stable/rationale.html"
 },
 {
  "id": "SP04",
  "check": "setup_requires",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bsetup_requires\\s*=",
  "sev": "warn",
  "msg": "setup_requires has no Flit equivalent",
  "fix": "list build tools in [build-system] requires; runtime needs go in [project] dependencies",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP05",
  "check": "install_requires",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\binstall_requires\\s*=",
  "sev": "warn",
  "msg": "install_requires",
  "fix": "[project] dependencies = [\"requests \u003e=2.6\", ...] (same specifiers and ; markers)",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP06",
  "check": "extras_require",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bextras_require\\s*=|^\\s*\\[options\\.extras_require\\]",
  "sev": "warn",
  "msg": "extras_require",
  "fix": "[project.optional-dependencies] test = [\"pytest \u003e=2.7.3\"]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP07",
  "check": "console_scripts",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "console_scripts",
  "sev": "warn",
  "msg": "console_scripts entry point",
  "fix": "[project.scripts] mycmd = \"mypkg:main\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP08",
  "check": "gui_scripts",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "gui_scripts",
  "sev": "warn",
  "msg": "gui_scripts entry point",
  "fix": "[project.gui-scripts] mygui = \"mypkg.gui:main\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP09",
  "check": "entry_points",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bentry_points\\s*=|^\\s*\\[options\\.entry_points\\]",
  "sev": "info",
  "msg": "entry_points block",
  "fix": "other groups become [project.entry-points.\"group.name\"] (quote a dotted group); console_scripts/gui_scripts go to [project.scripts]/[project.gui-scripts]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP10",
  "check": "data_files",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bdata_files\\s*=",
  "sev": "warn",
  "msg": "data_files installs files outside the package",
  "fix": "put them under data/ and add [tool.flit.external-data] directory = \"data\" (flit_core \u003e=3.7)",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP11",
  "check": "package_data",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\b(package_data|include_package_data|exclude_package_data)\\s*=|^\\s*\\[options\\.package_data\\]",
  "sev": "info",
  "msg": "package_data / include_package_data",
  "fix": "delete it: Flit includes data files inside the package folder automatically",
  "source": "https://flit.pypa.io/en/stable/rationale.html"
 },
 {
  "id": "SP12",
  "check": "package_dir",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bpackage_dir\\s*=",
  "sev": "info",
  "msg": "package_dir",
  "fix": "delete it: Flit finds the package next to pyproject.toml or in src/; if the import name differs from the PyPI name add [tool.flit.module] name = \"importname\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP13",
  "check": "find_packages",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bfind_(namespace_)?packages\\s*\\(|^\\s*packages\\s*=",
  "sev": "info",
  "msg": "packages / find_packages()",
  "fix": "delete it: name only the top-level import package; Flit includes its subpackages automatically",
  "source": "https://flit.pypa.io/en/stable/rationale.html"
 },
 {
  "id": "SP14",
  "check": "static_version",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "^\\s*version\\s*=\\s*(['\\\"]|[\\d])",
  "sev": "info",
  "msg": "version set in setup metadata",
  "fix": "[project] version = \"1.2.3\", or drop it and add dynamic = [\"version\"] so Flit reads {DV} from the package",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP15",
  "check": "attr_version",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "version\\s*=\\s*attr\\s*:|version\\s*=\\s*(get_version|read_version)\\s*\\(",
  "sev": "info",
  "msg": "version read from code at build time",
  "fix": "dynamic = [\"version\"]; Flit reads {DV} from the top-level module itself",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP16",
  "check": "scm_version",
  "files": [
   "setup.py",
   "setup.cfg",
   "pyproject.toml"
  ],
  "re": "use_scm_version|setuptools_scm|setuptools-scm|versioneer",
  "sev": "error",
  "msg": "BLOCKER: version from git tags (setuptools_scm/versioneer); Flit dynamic accepts only version, description, import-names, import-namespaces",
  "fix": "write the version into {DV} in the package and add dynamic = [\"version\"]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP17",
  "check": "python_requires",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\bpython_requires\\s*=",
  "sev": "warn",
  "msg": "python_requires",
  "fix": "[project] requires-python = \"\u003e=3.8\" (flit_core 4 itself needs Python 3.8+ to build)",
  "source": "https://flit.pypa.io/en/stable/history.html"
 },
 {
  "id": "SP18",
  "check": "long_description",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "\\blong_description\\s*=|\\blong_description_content_type\\s*=",
  "sev": "warn",
  "msg": "long_description",
  "fix": "[project] readme = \"README.md\" (.md, .rst or .txt picks the content type)",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP19",
  "check": "legacy_license",
  "files": [
   "setup.py",
   "setup.cfg"
  ],
  "re": "^\\s*license(_files?)?\\s*=",
  "sev": "warn",
  "msg": "license set in setup metadata",
  "fix": "[project] license = \"MIT\" (a valid SPDX expression, flit_core \u003e=3.11) and license-files = [\"LICENSE*\"]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
 {
  "id": "SP20",
  "check": "script_files",
  "files": [
   "setup.py",
   "setup.cfg"
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  "re": "^\\s*scripts\\s*=",
  "sev": "warn",
  "msg": "scripts= copies raw script files; Flit has no script-files field",
  "fix": "turn each script into a function and add [project.scripts] name = \"mypkg.cli:main\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "SP21",
  "check": "author_fields",
  "files": [
   "setup.py",
   "setup.cfg"
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  "re": "\\b(author|author_email|maintainer|maintainer_email)\\s*=",
  "sev": "info",
  "msg": "author / author_email",
  "fix": "[project] authors = [{ name = \"Jane Doe\", email = \"jane@example.org\" }]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "SP22",
  "check": "url_fields",
  "files": [
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  "re": "\\b(url|project_urls|download_url)\\s*=",
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  "fix": "[project.urls] Source = \"https://github.com/you/pkg\"",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
 },
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  "id": "SP23",
  "check": "classifiers",
  "files": [
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  "re": "\\bclassifiers\\s*=",
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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  "files": [
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  "re": "^\\s*(name|description)\\s*=",
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  "msg": "name / description",
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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  "re": "^\\s*\\[(metadata|options)\\]",
  "sev": "warn",
  "msg": "setup.cfg [metadata]/[options] section",
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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  "sev": "error",
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  "source": "https://flit.pypa.io/en/stable/history.html"
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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  "source": "https://flit.pypa.io/en/stable/history.html"
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "PP08",
  "check": "spdx_with_needs_41",
  "files": [
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  "re": null,
  "sev": "error",
  "msg": "license expression with WITH needs flit_core \u003e=4.1 (released 2026-09-16)",
  "fix": "requires = [\"flit_core \u003e=4.1,\u003c5\"]",
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  "id": "PP09",
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  "files": [
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  "re": null,
  "sev": "error",
  "msg": "import-names / import-namespaces need flit_core \u003e=4",
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "PP10",
  "check": "bad_dynamic",
  "files": [
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  "re": null,
  "sev": "error",
  "msg": "dynamic lists a field Flit cannot fill (only version, description, import-names, import-namespaces)",
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  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "PP11",
  "check": "console_scripts_group",
  "files": [
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  "re": "^\\s*\\[project\\.entry-points\\.\\\"?(console_scripts|gui_scripts)\\\"?\\]",
  "sev": "error",
  "msg": "[project.entry-points.console_scripts] is not allowed",
  "fix": "[project.scripts] (or [project.gui-scripts])",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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  "re": "^\\s*\\[tool\\.setuptools[\\w.\\\"-]*\\]",
  "sev": "info",
  "msg": "[tool.setuptools] table has no effect under flit_core",
  "fix": "package-data: delete (automatic); packages.find/package-dir: [tool.flit.module] name = \"importname\"; data-files: [tool.flit.external-data]",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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 {
  "id": "PP13",
  "check": "tool_hatch_poetry",
  "files": [
   "pyproject.toml"
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  "re": "^\\s*\\[tool\\.(hatch|poetry)[\\w.\\\"-]*\\]",
  "sev": "info",
  "msg": "[tool.hatch]/[tool.poetry] table is ignored by flit_core",
  "fix": "dependencies/scripts go into [project]; build.targets.sdist include/exclude becomes [tool.flit.sdist] include/exclude",
  "source": "https://flit.pypa.io/en/stable/pyproject_toml.html"
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];</script>
<script>/* Flit Migration Check engine: setup.py / setup.cfg / pyproject.toml -> flit_core 4 map. Runs in Node and in the browser. */
(function () {
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  function fmt(rule) {
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      if (kind !== 'pyproject.toml') return;
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      if (/^tool\.flit\.metadata/.test(section)) st.hasFlitMeta = true;
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  if (typeof window !== 'undefined') window.FLITENGINE = api;
})();
</script>
<script>// ⛔뼈대 — kit/scaffold.py 가 찍는다 (s143). findings [{check,sev,msg,line?}] → 글/HTML. 같은 파일이 Node 와 브라우저에서 산다.
'use strict';
function escapeHtml(s) { return String(s == null ? '' : s).replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;'); }
function summaryLine(res) {
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function toText(res, label) {
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function toHtml(res) {
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}
var FLITREPORT = { escapeHtml: escapeHtml, summaryLine: summaryLine, toText: toText, toHtml: toHtml };
if (typeof module !== 'undefined' && module.exports) module.exports = FLITREPORT;
else window.FLITREPORT = FLITREPORT;
</script>
<script>
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<section data-yj-install="free" style="margin:2rem auto 1rem;max-width:46rem;padding:1.25rem 1.25rem;border:1px solid #cfe3d6;border-radius:12px;background:#f6fbf8;font:16px/1.6 system-ui,-apple-system,Segoe UI,sans-serif"><h2 style="margin:0 0 .4rem;font-size:1.2rem">Install free</h2><p style="margin:0 0 .8rem;color:#41505f">Free for the file open in your editor - no key, no limit. The workspace sweep and the report ask for a key.</p><a href="https://open-vsx.org/extension/readystack/flit-migration-check" rel="noopener" data-yj-install-door="ovsx" onclick="try{navigator.sendBeacon('/api/ev',new Blob([JSON.stringify({t:'install',slug:'flit-migration-check',src:'ovsx',path:location.pathname})],{type:'application/json'}))}catch(e){}" style="display:inline-block;margin:0 .5rem .5rem 0;padding:.6rem 1rem;border-radius:8px;background:#0f7b3f;color:#fff;text-decoration:none;font-weight:600">Open VSX (Cursor, VSCodium)</a><p style="margin:.6rem 0 0;font-size:.92rem;color:#41505f"><a href="https://getreadystack.com/api/buy/team" rel="nofollow" style="color:#0f7b3f;text-decoration:underline">Team? One key for every ReadyStack linter, 5 seats, $149 once</a></p></section>
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<p>Six setup.py lines from a sample Python package show what a Flit migration really is: install_requires becomes [project] dependencies, extras_require becomes [project.optional-dependencies], a console_scripts entry becomes [project.scripts], data_files becomes [tool.flit.external-data], python_requires becomes requires-python, and license=&quot;MIT&quot; becomes an SPDX license string that needs flit_core &gt;=3.11. If you maintain a pure-Python library on PyPI and you are moving from setuptools, Hatch or the old Flit 3 layout to flit_core 4, this is the list you would otherwise write by hand.</p>
<h3>Migrate from setuptools to Flit: what changed in August and September 2026</h3>
<p>flit_core 4.0 (on PyPI 2026-08-04) no longer reads [tool.flit.metadata]. The Flit release history says it plainly: the old table &quot;is no longer used for project metadata&quot;; use the [project] table, or build with flit_core &lt;4. A package whose [build-system] asks for &quot;flit_core &gt;=3.2&quot; with no upper bound now receives flit_core 4 when pip builds it from the sdist, and the build stops.</p>
<p>flit_core 4.1 (on PyPI 2026-09-16) accepts license expressions with WITH, such as &quot;Apache-2.0 WITH LLVM-exception&quot;. That line needs &quot;flit_core &gt;=4.1,&lt;5&quot; in requires. A plain SPDX expression like &quot;MIT&quot; needs flit_core &gt;=3.11, and import-names needs flit_core &gt;=4. The Flit docs also ask every package to pin &quot;&lt;5&quot;.</p>
<h3>What the checker does</h3>
<p>Flit Migration Check is a VS Code extension with a free web version that runs the same engine. Open setup.py, setup.cfg or pyproject.toml and it prints one finding per line: the line number, what the line is, and the flit_core 4 line that replaces it. It has 38 rules, and each one names its source page on flit.pypa.io.</p>
<p>The bundled sample setup.py gives 22 findings on 22 lines (10 warnings, 12 notes, 0 blockers). A finished flit_core 4 pyproject.toml gives 0 findings, which is the point where you delete setup.py.</p>
<p>Three pyproject.toml cases show how the answer changes with the input:</p>
<ul><li>An old Flit 3 file with [tool.flit.metadata], [tool.flit.scripts] and &quot;flit_core &gt;=3.2&quot;: 3 findings. The metadata table, the scripts table, and the missing &lt;5 bound.</li><li>A Hatch file with hatchling, dynamic = [&quot;version&quot;, &quot;dependencies&quot;] and [project.entry-points.console_scripts]: 5 findings. Flit fills only version, description, import-names and import-namespaces from dynamic, and console_scripts belongs in [project.scripts].</li><li>A Flit 3.x file pinned &quot;flit_core &gt;=3.2,&lt;4&quot; with license = &quot;Apache-2.0 WITH LLVM-exception&quot;: 3 findings. The &lt;4 pin can become &lt;5, and the WITH license needs flit_core &gt;=4.1.</li></ul>
<h3>Blockers: when not to use Flit</h3>
<p>Flit builds pure Python only: &quot;If your package needs a build step, you won&#x27;t be able to use Flit&quot;. The checker marks ext_modules, Extension(, cythonize( and custom cmdclass as BLOCKER lines. Version from git tags (setuptools_scm, versioneer) is also a blocker: write the version into the package and add dynamic = [&quot;version&quot;].</p>
<h3>Before twine upload</h3>
<p>PyPI rejects an upload file over 100.0 MiB by default, and it refuses invalid Trove classifiers and a name too close to an existing project. The classifiers rule reminds you that &quot;Private :: Do Not Upload&quot; blocks an accidental upload.</p>
<h3>Free and paid</h3>
<p>The single-file map is free with no limit. A workspace key turns every setup.py, setup.cfg and pyproject.toml in the workspace into one dated migration plan. A team key adds an exposure report across projects and a CI gate that fails a build adding new setuptools-only use.</p></div><h2 style="font:600 19px/1.3 system-ui,sans-serif;margin:30px 0 10px;color:#1b1f24">15 seconds — what it actually does</h2><video controls muted playsinline preload="none" poster="/img/promo/sku428290_result_card.jpg" style="width:100%;max-width:360px;border-radius:10px;display:block;margin:0 auto"><source src="/media/sku428290_short.mp4" type="video/mp4"></video><h2 style="font:600 21px/1.3 system-ui,sans-serif;margin:34px 0 6px;color:#1b1f24">Questions people ask</h2><details style="border-bottom:1px solid #e6e9ed;padding:10px 0"><summary style="cursor:pointer;font:600 15px/1.4 system-ui,sans-serif;color:#1b1f24">What does Flit Migration Check do?</summary><p style="margin:8px 0 2px;font:400 14.5px/1.6 system-ui,sans-serif;color:#33404b">It reads the open setup.py, setup.cfg or pyproject.toml in VS Code and lists every line that still belongs to setuptools, Hatch, Poetry or the old Flit 3 layout, with its line number and the exact flit_core 4 line that replaces it. Compiled extensions and custom cmdclass are marked BLOCKER because Flit builds pure Python only. It has 38 rules.</p></details><details style="border-bottom:1px solid #e6e9ed;padding:10px 0"><summary style="cursor:pointer;font:600 15px/1.4 system-ui,sans-serif;color:#1b1f24">Who is Flit Migration Check for?</summary><p style="margin:8px 0 2px;font:400 14.5px/1.6 system-ui,sans-serif;color:#33404b">Python library maintainers moving a pure-Python package from setup.py or setuptools to Flit, teams whose Flit 3 packages still use [tool.flit.metadata], and anyone switching from Hatch or Poetry to flit_core. flit_core 4.0 (on PyPI 2026-08-04) no longer reads [tool.flit.metadata], so an unpinned old-layout package no longer builds from its sdist.</p></details><details style="border-bottom:1px solid #e6e9ed;padding:10px 0"><summary style="cursor:pointer;font:600 15px/1.4 system-ui,sans-serif;color:#1b1f24">Why not just ask a chatbot or read the Flit docs?</summary><p style="margin:8px 0 2px;font:400 14.5px/1.6 system-ui,sans-serif;color:#33404b">The docs are free and correct, but you must map each setup() argument yourself. A chatbot&#x27;s packaging advice from before August 2026 cannot know that flit_core 4.0 (on PyPI 2026-08-04) no longer reads [tool.flit.metadata], or that flit_core 4.1 (on PyPI 2026-09-16) accepts license expressions with WITH. The checker reads your actual file line by line, offline, and quotes the flit docs for each replacement.</p></details><details style="border-bottom:1px solid #e6e9ed;padding:10px 0"><summary style="cursor:pointer;font:600 15px/1.4 system-ui,sans-serif;color:#1b1f24">What is free and what needs a key?</summary><p style="margin:8px 0 2px;font:400 14.5px/1.6 system-ui,sans-serif;color:#33404b">The single-file map is free with no limit: every finding, line number and replacement line; the bundled sample setup.py gives 22 findings on 22 lines (10 warnings, 12 notes, 0 blockers). A $29 workspace key adds one dated migration plan across every setup.py, setup.cfg and pyproject.toml. A $149 team key adds the cross-project exposure report and a CI gate that fails a build adding new setuptools-only use.</p></details><details style="border-bottom:1px solid #e6e9ed;padding:10px 0"><summary style="cursor:pointer;font:600 15px/1.4 system-ui,sans-serif;color:#1b1f24">What does it cost compared with doing it by hand?</summary><p style="margin:8px 0 2px;font:400 14.5px/1.6 system-ui,sans-serif;color:#33404b">By hand you read three flit.pypa.io pages and translate each setup() argument, then learn about a wrong flit_core bound when the sdist build fails. The free map does the translation per file. 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