python_doc_md/Advanced/c-api/function.md

# Function Objects

::: index
pair: object; function
:::

There are a few functions specific to Python functions.

> The C structure used for functions.

> ::: index
> single: MethodType (in module types)
> :::
>
> This is an instance of `PyTypeObject`{.interpreted-text role="c:type"} and represents the Python function type. It is exposed to Python programmers as `types.FunctionType`.

> Return true if *o* is a function object (has type `PyFunction_Type`{.interpreted-text role="c:data"}). The parameter must not be `NULL`. This function always succeeds.

> Return a new function object associated with the code object *code*. *globals* must be a dictionary with the global variables accessible to the function.
>
> The function\'s docstring and name are retrieved from the code object. `~function.__module__`{.interpreted-text role="attr"} is retrieved from *globals*. The argument defaults, annotations and closure are set to `NULL`. `~function.__qualname__`{.interpreted-text role="attr"} is set to the same value as the code object\'s `~codeobject.co_qualname`{.interpreted-text role="attr"} field.

> As `PyFunction_New`{.interpreted-text role="c:func"}, but also allows setting the function object\'s `~function.__qualname__`{.interpreted-text role="attr"} attribute. *qualname* should be a unicode object or `NULL`; if `NULL`, the `!__qualname__`{.interpreted-text role="attr"} attribute is set to the same value as the code object\'s `~codeobject.co_qualname`{.interpreted-text role="attr"} field.
>
> ::: versionadded
> 3.3
> :::

> Return the code object associated with the function object *op*.

> Return the globals dictionary associated with the function object *op*.

> Return a `borrowed reference`{.interpreted-text role="term"} to the `~function.__module__`{.interpreted-text role="attr"} attribute of the `function object <user-defined-funcs>`{.interpreted-text role="ref"} *op*. It can be *NULL*.
>
> This is normally a `string <str>`{.interpreted-text role="class"} containing the module name, but can be set to any other object by Python code.

> Return the argument default values of the function object *op*. This can be a tuple of arguments or `NULL`.

> Set the argument default values for the function object *op*. *defaults* must be `Py_None` or a tuple.
>
> Raises `SystemError`{.interpreted-text role="exc"} and returns `-1` on failure.

> Set the vectorcall field of a given function object *func*.
>
> Warning: extensions using this API must preserve the behavior of the unaltered (default) vectorcall function!
>
> ::: versionadded
> 3.12
> :::

> Return the keyword-only argument default values of the function object *op*. This can be a dictionary of arguments or `NULL`.

> Set the keyword-only argument default values of the function object *op*. *defaults* must be a dictionary of keyword-only arguments or `Py_None`.
>
> This function returns `0` on success, and returns `-1` with an exception set on failure.

> Return the closure associated with the function object *op*. This can be `NULL` or a tuple of cell objects.

> Set the closure associated with the function object *op*. *closure* must be `Py_None` or a tuple of cell objects.
>
> Raises `SystemError`{.interpreted-text role="exc"} and returns `-1` on failure.

> Return the annotations of the function object *op*. This can be a mutable dictionary or `NULL`.

> Set the annotations for the function object *op*. *annotations* must be a dictionary or `Py_None`.
>
> Raises `SystemError`{.interpreted-text role="exc"} and returns `-1` on failure.

> Register *callback* as a function watcher for the current interpreter. Return an ID which may be passed to `PyFunction_ClearWatcher`{.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 `PyFunction_AddWatcher`{.interpreted-text role="c:func"} for the current interpreter. Return `0` on success, or `-1` and set an exception on error (e.g. if the given *watcher_id* was never registered.)
>
> ::: versionadded
> 3.12
> :::

> Enumeration of possible function watcher events:
>
> - `PyFunction_EVENT_CREATE`
> - `PyFunction_EVENT_DESTROY`
> - `PyFunction_EVENT_MODIFY_CODE`
> - `PyFunction_EVENT_MODIFY_DEFAULTS`
> - `PyFunction_EVENT_MODIFY_KWDEFAULTS`
>
> ::: versionadded
> 3.12
> :::
>
> - `PyFunction_PYFUNC_EVENT_MODIFY_QUALNAME`
>
> ::: versionadded
> 3.15
> :::

> Type of a function watcher callback function.
>
> If *event* is `PyFunction_EVENT_CREATE` or `PyFunction_EVENT_DESTROY` then *new_value* will be `NULL`. Otherwise, *new_value* will hold a `borrowed reference`{.interpreted-text role="term"} to the new value that is about to be stored in *func* for the attribute that is being modified.
>
> The callback may inspect but must not modify *func*; doing so could have unpredictable effects, including infinite recursion.
>
> If *event* is `PyFunction_EVENT_CREATE`, then the callback is invoked after *func* has been fully initialized. Otherwise, the callback is invoked before the modification to *func* takes place, so the prior state of *func* can be inspected. The runtime is permitted to optimize away the creation of function objects when possible. In such cases no event will be emitted. Although this creates the possibility of an observable difference of runtime behavior depending on optimization decisions, it does not change the semantics of the Python code being executed.
>
> If *event* is `PyFunction_EVENT_DESTROY`, taking a reference in the callback to the about-to-be-destroyed function will resurrect it, preventing 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.
>
> 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
> :::