.. 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(PyObject *callback)
Registers the callable object *callback* as a context object watcher for the
current interpreter. When a context event occurs, *callback* is called with
two arguments:
#. An event type ID from :c:type:`PyContextEvent`.
#. An object containing event-specific supplemental data; see
:c:type:`PyContextEvent` for details.
Any exception raised by *callback* will be printed as an unraisable
exception as if by a call to :c:func:`PyErr_FormatUnraisable`, then
discarded.
On success, this function returns a non-negative ID which may be passed to
:c:func:`PyContext_ClearWatcher` to unregister the callback and remove the
reference this function adds to *callback*. Sets an exception and returns
``-1`` on error (e.g., no more watcher IDs available).
Example using a C function as the callback::
static PyObject *
my_callback(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
{
if (PyVectorcall_NARGS(nargs) != 2) {
PyErr_Format(PyExc_TypeError, "want 2 args, got %zd", nargs);
return NULL;
}
int event = PyLong_AsInt(args[0]);
if (event == -1 && PyErr_Occurred()) {
return NULL;
}
if (event != Py_CONTEXT_SWITCHED) {
Py_RETURN_NONE;
}
PyObject *ctx = args[1];
// Do something interesting with self and ctx here.
Py_RETURN_NONE;
}
PyMethodDef my_callback_md = {
.ml_name = "my_callback",
.ml_meth = (PyCFunction)(void *)&my_callback,
.ml_flags = METH_FASTCALL,
.ml_doc = NULL,
};
int
register_my_callback(PyObject *callback_state)
{
PyObject *cb = PyCFunction_New(&my_callback_md, callback_state);
if (cb == NULL) {
return -1;
}
int id = PyContext_AddWatcher(cb);
Py_CLEAR(cb);
return id;
}
.. versionadded:: 3.14.. c:function:: int PyContext_ClearWatcher(int watcher_id)
Clears the watcher identified by *watcher_id* previously returned from
:c:func:`PyContext_AddWatcher` for the current interpreter, and removes the
reference created for the registered callback object. Returns ``0`` on
success, or sets an exception and returns ``-1`` 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
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.