cpython/Doc/c-api/contextvars.rst

.. highlight:: c

.. _contextvarsobjects:

Context Variables Objects
-------------------------

.. _contextvarsobjects_pointertype_change:
.. versionadded:: 3.7

.. versionchanged:: 3.7.1

   .. note::

      In Python 3.7.1 the signatures of all context variables
      C APIs were **changed** to use :c:type:`PyObject` pointers instead
      of :c:type:`PyContext`, :c:type:`PyContextVar`, and
      :c:type:`PyContextToken`, e.g.::

         // in 3.7.0:
         PyContext *PyContext_New(void);

         // in 3.7.1+:
         PyObject *PyContext_New(void);

      See :issue:`34762` for more details.


This section details the public C API for the :mod:`contextvars` module.

.. c:type:: PyContext

   The C structure used to represent a :class:`contextvars.Context`
   object.

.. c:type:: PyContextVar

   The C structure used to represent a :class:`contextvars.ContextVar`
   object.

.. c:type:: PyContextToken

   The C structure used to represent a :class:`contextvars.Token` object.

.. c:var:: PyTypeObject PyContext_Type

   The type object representing the *context* type.

.. c:var:: PyTypeObject PyContextVar_Type

   The type object representing the *context variable* type.

.. c:var:: PyTypeObject PyContextToken_Type

   The type object representing the *context variable token* type.


Type-check macros:

.. c:function:: int PyContext_CheckExact(PyObject *o)

   Return true if *o* is of type :c:data:`PyContext_Type`. *o* must not be
   ``NULL``.  This function always succeeds.

.. c:function:: int PyContextVar_CheckExact(PyObject *o)

   Return true if *o* is of type :c:data:`PyContextVar_Type`. *o* must not be
   ``NULL``.  This function always succeeds.

.. c:function:: int PyContextToken_CheckExact(PyObject *o)

   Return true if *o* is of type :c:data:`PyContextToken_Type`.
   *o* must not be ``NULL``.  This function always succeeds.


Context object management functions:

.. c:function:: PyObject *PyContext_New(void)

   Create a new empty context object.  Returns ``NULL`` if an error
   has occurred.

.. c:function:: PyObject *PyContext_Copy(PyObject *ctx)

   Create a shallow copy of the passed *ctx* context object.
   Returns ``NULL`` if an error has occurred.

.. c:function:: PyObject *PyContext_CopyCurrent(void)

   Create a shallow copy of the current thread context.
   Returns ``NULL`` if an error has occurred.

.. c:function:: int PyContext_Enter(PyObject *ctx)

   Set *ctx* as the current context for the current thread.
   Returns ``0`` on success, and ``-1`` on error.

.. c:function:: int PyContext_Exit(PyObject *ctx)

   Deactivate the *ctx* context and restore the previous context as the
   current context for the current thread.  Returns ``0`` on success,
   and ``-1`` on error.

.. c:function:: int PyContext_AddWatcher(PyContext_WatchCallback callback)

   Register *callback* as a context object watcher for the current interpreter.
   Return an ID which may be passed to :c:func:`PyContext_ClearWatcher`.
   In case of error (e.g. no more watcher IDs available),
   return ``-1`` and set an exception.

   .. versionadded:: 3.14

.. c:function:: int PyContext_ClearWatcher(int watcher_id)

   Clear watcher identified by *watcher_id* previously returned from
   :c:func:`PyContext_AddWatcher` 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:: 3.14

.. c:type:: PyContextEvent

   Enumeration of possible context object watcher events:

   - ``Py_CONTEXT_SWITCHED``: The :term:`current context` has switched to a
     different context.  The object passed to the watch callback is the
     now-current :class:`contextvars.Context` object, or None if no context is
     current.

   .. versionadded:: 3.14

.. c:type:: int (*PyContext_WatchCallback)(PyContextEvent event, PyObject *obj)

   Context object watcher callback function.  The object passed to the callback
   is event-specific; see :c:type:`PyContextEvent` for details.

   If the callback returns with an exception set, it must return ``-1``; this
   exception will be printed as an unraisable exception using
   :c:func:`PyErr_FormatUnraisable`. 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:: 3.14


Context variable functions:

.. c:function:: PyObject *PyContextVar_New(const char *name, PyObject *def)

   Create a new ``ContextVar`` object.  The *name* parameter is used
   for introspection and debug purposes.  The *def* parameter specifies
   a default value for the context variable, or ``NULL`` for no default.
   If an error has occurred, this function returns ``NULL``.

.. c:function:: int PyContextVar_Get(PyObject *var, PyObject *default_value, PyObject **value)

   Get the value of a context variable.  Returns ``-1`` if an error has
   occurred during lookup, and ``0`` if no error occurred, whether or not
   a value was found.

   If the context variable was found, *value* will be a pointer to it.
   If the context variable was *not* found, *value* will point to:

   - *default_value*, if not ``NULL``;
   - the default value of *var*, if not ``NULL``;
   - ``NULL``

   Except for ``NULL``, the function returns a new reference.

.. c:function:: PyObject *PyContextVar_Set(PyObject *var, PyObject *value)

   Set the value of *var* to *value* in the current context.  Returns
   a new token object for this change, or ``NULL`` if an error has occurred.

.. c:function:: int PyContextVar_Reset(PyObject *var, PyObject *token)

   Reset the state of the *var* context variable to that it was in before
   :c:func:`PyContextVar_Set` that returned the *token* was called.
   This function returns ``0`` on success and ``-1`` on error.