Context Variables Objects

Added in version 3.7.

Changed in version 3.7.1:

Note

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

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

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

See bpo-34762 for more details.

This section details the public C API for the contextvars module.

type PyContext

The C structure used to represent a contextvars.Context object.

type PyContextVar

The C structure used to represent a contextvars.ContextVar object.

type PyContextToken

The C structure used to represent a contextvars.Token object.

PyTypeObject PyContext_Type

The type object representing the context type.

PyTypeObject PyContextVar_Type

The type object representing the context variable type.

PyTypeObject PyContextToken_Type

The type object representing the context variable token type.

Type-check macros:

int PyContext_CheckExact(PyObject *o)

Return true if o is of type PyContext_Type. o must not be NULL. This function always succeeds.

int PyContextVar_CheckExact(PyObject *o)

Return true if o is of type PyContextVar_Type. o must not be NULL. This function always succeeds.

int PyContextToken_CheckExact(PyObject *o)

Return true if o is of type PyContextToken_Type. o must not be NULL. This function always succeeds.

Context object management functions:

PyObject *PyContext_New(void)
Return value: New reference.

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

PyObject *PyContext_Copy(PyObject *ctx)
Return value: New reference.

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

PyObject *PyContext_CopyCurrent(void)
Return value: New reference.

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

int PyContext_Enter(PyObject *ctx)

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

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.

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 PyContext_ClearWatcher(). In case of error (e.g. no more watcher IDs available), return -1 and set an exception.

Added in version 3.14.

int PyContext_ClearWatcher(int watcher_id)

Clear watcher identified by watcher_id previously returned from 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.)

Added in version 3.14.

type PyContextEvent

Enumeration of possible context object watcher events:

  • Py_CONTEXT_EVENT_ENTER: A context has been entered, causing the current context to switch to it. The object passed to the watch callback is the now-current contextvars.Context object. Each enter event will eventually have a corresponding exit event for the same context object after any subsequently entered contexts have themselves been exited.

  • Py_CONTEXT_EVENT_EXIT: A context is about to be exited, which will cause the current context to switch back to what it was before the context was entered. The object passed to the watch callback is the still-current contextvars.Context object.

Added in version 3.14.

typedef void (*PyContext_WatchCallback)(PyContextEvent event, PyObject *obj)

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

Any pending exception is cleared before the callback is called and restored after the callback returns.

If the callback raises an exception it will be printed as an unraisable exception using PyErr_FormatUnraisable() and discarded.

Added in version 3.14.

Context variable functions:

PyObject *PyContextVar_New(const char *name, PyObject *def)
Return value: New reference.

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.

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.

PyObject *PyContextVar_Set(PyObject *var, PyObject *value)
Return value: New reference.

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.

int PyContextVar_Reset(PyObject *var, PyObject *token)

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