python_doc_md/Advanced/c-api/dict.md

Dictionary objects {#dictobjects}

::: index pair: object; dictionary :::

This subtype of PyObject{.interpreted-text role="c:type"} represents a Python dictionary object.

This instance of PyTypeObject{.interpreted-text role="c:type"} represents the Python dictionary type. This is the same object as dict{.interpreted-text role="class"} in the Python layer.

Return true if p is a dict object or an instance of a subtype of the dict type. This function always succeeds.

Return true if p is a dict object, but not an instance of a subtype of the dict type. This function always succeeds.

Return a new empty dictionary, or NULL on failure.

Return a types.MappingProxyType{.interpreted-text role="class"} object for a mapping which enforces read-only behavior. This is normally used to create a view to prevent modification of the dictionary for non-dynamic class types.

The type object for mapping proxy objects created by PyDictProxy_New{.interpreted-text role="c:func"} and for the read-only __dict__ attribute of many built-in types. A PyDictProxy_Type{.interpreted-text role="c:type"} instance provides a dynamic, read-only view of an underlying dictionary: changes to the underlying dictionary are reflected in the proxy, but the proxy itself does not support mutation operations. This corresponds to types.MappingProxyType{.interpreted-text role="class"} in Python.

Empty an existing dictionary of all key-value pairs.

Do nothing if the argument is not a dict{.interpreted-text role="class"} or a !dict{.interpreted-text role="class"} subclass.

Determine if dictionary p contains key. If an item in p matches key, return 1, otherwise return 0. On error, return -1. This is equivalent to the Python expression key in p.

This is the same as PyDict_Contains{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

::: versionadded 3.13 :::

Return a new dictionary that contains the same key-value pairs as p.

Insert val into the dictionary p with a key of key. key must be hashable{.interpreted-text role="term"}; if it isn't, TypeError{.interpreted-text role="exc"} will be raised. Return 0 on success or -1 on failure. This function does not steal a reference to val.

This is the same as PyDict_SetItem{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

Remove the entry in dictionary p with key key. key must be hashable{.interpreted-text role="term"}; if it isn't, TypeError{.interpreted-text role="exc"} is raised. If key is not in the dictionary, KeyError{.interpreted-text role="exc"} is raised. Return 0 on success or -1 on failure.

This is the same as PyDict_DelItem{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

Return a new strong reference{.interpreted-text role="term"} to the object from dictionary p which has a key key:

• If the key is present, set *result to a new strong reference{.interpreted-text role="term"} to the value and return 1.
• If the key is missing, set *result to NULL and return 0.
• On error, raise an exception and return -1.

::: versionadded 3.13 :::

See also the PyObject_GetItem{.interpreted-text role="c:func"} function.

Return a borrowed reference{.interpreted-text role="term"} to the object from dictionary p which has a key key. Return NULL if the key key is missing without setting an exception.

:::: note ::: title Note :::

Exceptions that occur while this calls ~object.__hash__{.interpreted-text role="meth"} and ~object.__eq__{.interpreted-text role="meth"} methods are silently ignored. Prefer the PyDict_GetItemWithError{.interpreted-text role="c:func"} function instead. ::::

::: versionchanged 3.10 Calling this API without an attached thread state{.interpreted-text role="term"} had been allowed for historical reason. It is no longer allowed. :::

Variant of PyDict_GetItem{.interpreted-text role="c:func"} that does not suppress exceptions. Return NULL with an exception set if an exception occurred. Return NULL without an exception set if the key wasn't present.

This is the same as PyDict_GetItem{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

:::: note ::: title Note :::

Exceptions that occur while this calls ~object.__hash__{.interpreted-text role="meth"} and ~object.__eq__{.interpreted-text role="meth"} methods or while creating the temporary str{.interpreted-text role="class"} object are silently ignored. Prefer using the PyDict_GetItemWithError{.interpreted-text role="c:func"} function with your own PyUnicode_FromString{.interpreted-text role="c:func"} key instead. ::::

Similar to PyDict_GetItemRef{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

::: versionadded 3.13 :::

This is the same as the Python-level dict.setdefault{.interpreted-text role="meth"}. If present, it returns the value corresponding to key from the dictionary p. If the key is not in the dict, it is inserted with value defaultobj and defaultobj is returned. This function evaluates the hash function of key only once, instead of evaluating it independently for the lookup and the insertion.

::: versionadded 3.4 :::

Inserts default_value into the dictionary p with a key of key if the key is not already present in the dictionary. If result is not NULL, then *result is set to a strong reference{.interpreted-text role="term"} to either default_value, if the key was not present, or the existing value, if key was already present in the dictionary. Returns 1 if the key was present and default_value was not inserted, or 0 if the key was not present and default_value was inserted. On failure, returns -1, sets an exception, and sets *result to NULL.

For clarity: if you have a strong reference to default_value before calling this function, then after it returns, you hold a strong reference to both default_value and *result (if it's not NULL). These may refer to the same object: in that case you hold two separate references to it.

::: versionadded 3.13 :::

Remove key from dictionary p and optionally return the removed value. Do not raise KeyError{.interpreted-text role="exc"} if the key is missing.

• If the key is present, set *result to a new reference to the removed value if result is not NULL, and return 1.
• If the key is missing, set *result to NULL if result is not NULL, and return 0.
• On error, raise an exception and return -1.

Similar to dict.pop{.interpreted-text role="meth"}, but without the default value and not raising KeyError{.interpreted-text role="exc"} if the key is missing.

::: versionadded 3.13 :::

Similar to PyDict_Pop{.interpreted-text role="c:func"}, but key is specified as a const char*{.interpreted-text role="c:expr"} UTF-8 encoded bytes string, rather than a PyObject*{.interpreted-text role="c:expr"}.

::: versionadded 3.13 :::

Return a PyListObject{.interpreted-text role="c:type"} containing all the items from the dictionary.

Return a PyListObject{.interpreted-text role="c:type"} containing all the keys from the dictionary.

Return a PyListObject{.interpreted-text role="c:type"} containing all the values from the dictionary p.

::: index pair: built-in function; len :::

Return the number of items in the dictionary. This is equivalent to len(p) on a dictionary.

Similar to PyDict_Size{.interpreted-text role="c:func"}, but without error checking.

Iterate over all key-value pairs in the dictionary p. The Py_ssize_t{.interpreted-text role="c:type"} referred to by ppos must be initialized to 0 prior to the first call to this function to start the iteration; the function returns true for each pair in the dictionary, and false once all pairs have been reported. The parameters pkey and pvalue should either point to PyObject*{.interpreted-text role="c:expr"} variables that will be filled in with each key and value, respectively, or may be NULL. Any references returned through them are borrowed. ppos should not be altered during iteration. Its value represents offsets within the internal dictionary structure, and since the structure is sparse, the offsets are not consecutive.

For example:

PyObject *key, *value;
Py_ssize_t pos = 0;

while (PyDict_Next(self->dict, &pos, &key, &value)) {
    /* do something interesting with the values... */
    ...
}

The dictionary p should not be mutated during iteration. It is safe to modify the values of the keys as you iterate over the dictionary, but only so long as the set of keys does not change. For example:

PyObject *key, *value;
Py_ssize_t pos = 0;

while (PyDict_Next(self->dict, &pos, &key, &value)) {
    long i = PyLong_AsLong(value);
    if (i == -1 && PyErr_Occurred()) {
        return -1;
    }
    PyObject *o = PyLong_FromLong(i + 1);
    if (o == NULL)
        return -1;
    if (PyDict_SetItem(self->dict, key, o) < 0) {
        Py_DECREF(o);
        return -1;
    }
    Py_DECREF(o);
}

The function is not thread-safe in the free-threaded <free threading>{.interpreted-text role="term"} build without external synchronization. You can use Py_BEGIN_CRITICAL_SECTION{.interpreted-text role="c:macro"} to lock the dictionary while iterating over it:

Py_BEGIN_CRITICAL_SECTION(self->dict);
while (PyDict_Next(self->dict, &pos, &key, &value)) {
    ...
}
Py_END_CRITICAL_SECTION();

:::: note ::: title Note :::

On the free-threaded build, this function can be used safely inside a critical section. However, the references returned for pkey and pvalue are borrowed <borrowed reference>{.interpreted-text role="term"} and are only valid while the critical section is held. If you need to use these objects outside the critical section or when the critical section can be suspended, create a strong reference <strong reference>{.interpreted-text role="term"} (for example, using Py_NewRef{.interpreted-text role="c:func"}). ::::

Iterate over mapping object b adding key-value pairs to dictionary a. b may be a dictionary, or any object supporting PyMapping_Keys{.interpreted-text role="c:func"} and PyObject_GetItem{.interpreted-text role="c:func"}. If override is true, existing pairs in a will be replaced if a matching key is found in b, otherwise pairs will only be added if there is not a matching key in a. Return 0 on success or -1 if an exception was raised.

This is the same as PyDict_Merge(a, b, 1) in C, and is similar to a.update(b) in Python except that PyDict_Update{.interpreted-text role="c:func"} doesn't fall back to the iterating over a sequence of key value pairs if the second argument has no "keys" attribute. Return 0 on success or -1 if an exception was raised.

Update or merge into dictionary a, from the key-value pairs in seq2. seq2 must be an iterable object producing iterable objects of length 2, viewed as key-value pairs. In case of duplicate keys, the last wins if override is true, else the first wins. Return 0 on success or -1 if an exception was raised. Equivalent Python (except for the return value):

def PyDict_MergeFromSeq2(a, seq2, override):
    for key, value in seq2:
        if override or key not in a:
            a[key] = value

Register callback as a dictionary watcher. Return a non-negative integer id which must be passed to future calls to PyDict_Watch{.interpreted-text role="c:func"}. In case of error (e.g. no more watcher IDs available), return -1 and set an exception.

::: versionadded 3.12 :::

Clear watcher identified by watcher_id previously returned from PyDict_AddWatcher{.interpreted-text role="c:func"}. Return 0 on success, -1 on error (e.g. if the given watcher_id was never registered.)

::: versionadded 3.12 :::

Mark dictionary dict as watched. The callback granted watcher_id by PyDict_AddWatcher{.interpreted-text role="c:func"} will be called when dict is modified or deallocated. Return 0 on success or -1 on error.

::: versionadded 3.12 :::

Mark dictionary dict as no longer watched. The callback granted watcher_id by PyDict_AddWatcher{.interpreted-text role="c:func"} will no longer be called when dict is modified or deallocated. The dict must previously have been watched by this watcher. Return 0 on success or -1 on error.

::: versionadded 3.12 :::

Enumeration of possible dictionary watcher events: PyDict_EVENT_ADDED, PyDict_EVENT_MODIFIED, PyDict_EVENT_DELETED, PyDict_EVENT_CLONED, PyDict_EVENT_CLEARED, or PyDict_EVENT_DEALLOCATED.

::: versionadded 3.12 :::

Type of a dict watcher callback function.

If event is PyDict_EVENT_CLEARED or PyDict_EVENT_DEALLOCATED, both key and new_value will be NULL. If event is PyDict_EVENT_ADDED or PyDict_EVENT_MODIFIED, new_value will be the new value for key. If event is PyDict_EVENT_DELETED, key is being deleted from the dictionary and new_value will be NULL.

PyDict_EVENT_CLONED occurs when dict was previously empty and another dict is merged into it. To maintain efficiency of this operation, per-key PyDict_EVENT_ADDED events are not issued in this case; instead a single PyDict_EVENT_CLONED is issued, and key will be the source dictionary.

The callback may inspect but must not modify dict; doing so could have unpredictable effects, including infinite recursion. Do not trigger Python code execution in the callback, as it could modify the dict as a side effect.

If event is PyDict_EVENT_DEALLOCATED, taking a new reference in the callback to the about-to-be-destroyed dictionary will resurrect it and prevent it from being freed at this time. When the resurrected object is destroyed later, any watcher callbacks active at that time will be called again.

Callbacks occur before the notified modification to dict takes place, so the prior state of dict can be inspected.

If the callback sets an exception, it must return -1; this exception will be printed as an unraisable exception using PyErr_WriteUnraisable{.interpreted-text role="c:func"}. Otherwise it should return 0.

There may already be a pending exception set on entry to the callback. In this case, the callback should return 0 with the same exception still set. This means the callback may not call any other API that can set an exception unless it saves and clears the exception state first, and restores it before returning.

::: versionadded 3.12 :::

Dictionary view objects

Return true if op is a view of a set inside a dictionary. This is currently equivalent to PyDictKeys_Check(op) || PyDictItems_Check(op){.interpreted-text role="c:expr"}. This function always succeeds.

Type object for a view of dictionary keys. In Python, this is the type of the object returned by dict.keys{.interpreted-text role="meth"}.

Return true if op is an instance of a dictionary keys view. This function always succeeds.

Type object for a view of dictionary values. In Python, this is the type of the object returned by dict.values{.interpreted-text role="meth"}.

Return true if op is an instance of a dictionary values view. This function always succeeds.

Type object for a view of dictionary items. In Python, this is the type of the object returned by dict.items{.interpreted-text role="meth"}.

Return true if op is an instance of a dictionary items view. This function always succeeds.

Frozen dictionary objects

::: versionadded next :::

This instance of PyTypeObject{.interpreted-text role="c:type"} represents the Python frozen dictionary type. This is the same object as frozendict{.interpreted-text role="class"} in the Python layer.

Return true if p is a dict{.interpreted-text role="class"} object, a frozendict{.interpreted-text role="class"} object, or an instance of a subtype of the !dict{.interpreted-text role="class"} or !frozendict{.interpreted-text role="class"} type. This function always succeeds.

Return true if p is a dict{.interpreted-text role="class"} object or a frozendict{.interpreted-text role="class"} object, but not an instance of a subtype of the !dict{.interpreted-text role="class"} or !frozendict{.interpreted-text role="class"} type. This function always succeeds.

Return true if p is a frozendict{.interpreted-text role="class"} object or an instance of a subtype of the !frozendict{.interpreted-text role="class"} type. This function always succeeds.

Return true if p is a frozendict{.interpreted-text role="class"} object, but not an instance of a subtype of the !frozendict{.interpreted-text role="class"} type. This function always succeeds.

Return a new frozendict{.interpreted-text role="class"} from an iterable, or NULL on failure with an exception set.

Create an empty dictionary if iterable is NULL.

Ordered dictionaries

Python's C API provides interface for collections.OrderedDict{.interpreted-text role="class"} from C. Since Python 3.7, dictionaries are ordered by default, so there is usually little need for these functions; prefer PyDict* where possible.

Type object for ordered dictionaries. This is the same object as collections.OrderedDict{.interpreted-text role="class"} in the Python layer.

Return true if od is an ordered dictionary object or an instance of a subtype of the ~collections.OrderedDict{.interpreted-text role="class"} type. This function always succeeds.

Return true if od is an ordered dictionary object, but not an instance of a subtype of the ~collections.OrderedDict{.interpreted-text role="class"} type. This function always succeeds.

Analogous to PyDictKeys_Type{.interpreted-text role="c:type"} for ordered dictionaries.

Analogous to PyDictValues_Type{.interpreted-text role="c:type"} for ordered dictionaries.

Analogous to PyDictItems_Type{.interpreted-text role="c:type"} for ordered dictionaries.

Return a new empty ordered dictionary, or NULL on failure.

This is analogous to PyDict_New{.interpreted-text role="c:func"}.

Insert value into the ordered dictionary od with a key of key. Return 0 on success or -1 with an exception set on failure.

This is analogous to PyDict_SetItem{.interpreted-text role="c:func"}.

Remove the entry in the ordered dictionary od with key key. Return 0 on success or -1 with an exception set on failure.

This is analogous to PyDict_DelItem{.interpreted-text role="c:func"}.

These are soft deprecated{.interpreted-text role="term"} aliases to PyDict APIs:

PyODict PyDict

---

          `PyDict_GetItem`{.interpreted-text role="c:func"}
          `PyDict_GetItemWithError`{.interpreted-text role="c:func"}
          `PyDict_GetItemString`{.interpreted-text role="c:func"}
          `PyDict_Contains`{.interpreted-text role="c:func"}
          `PyDict_Size`{.interpreted-text role="c:func"}
          `PyDict_GET_SIZE`{.interpreted-text role="c:func"}