python_doc_md/Advanced/c-api/contextvars.md

Context Variables Objects {#contextvarsobjects}

::::: versionchanged 3.7.1

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

In Python 3.7.1 the signatures of all context variables C APIs were changed to use PyObject{.interpreted-text role="c:type"} pointers instead of PyContext{.interpreted-text role="c:type"}, PyContextVar{.interpreted-text role="c:type"}, and PyContextToken{.interpreted-text role="c:type"}, e.g.:

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

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

See 34762{.interpreted-text role="issue"} for more details. :::: :::::

This section details the public C API for the contextvars{.interpreted-text role="mod"} module.

The C structure used to represent a contextvars.Context{.interpreted-text role="class"} object.

The C structure used to represent a contextvars.ContextVar{.interpreted-text role="class"} object.

The C structure used to represent a contextvars.Token{.interpreted-text role="class"} object.

The type object representing the context type.

The type object representing the context variable type.

The type object representing the context variable token type.

Type-check macros:

Return true if o is of type PyContext_Type{.interpreted-text role="c:data"}. o must not be NULL. This function always succeeds.

Return true if o is of type PyContextVar_Type{.interpreted-text role="c:data"}. o must not be NULL. This function always succeeds.

Return true if o is of type PyContextToken_Type{.interpreted-text role="c:data"}. o must not be NULL. This function always succeeds.

Context object management functions:

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

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

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

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

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.

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

::: versionadded 3.14 :::

Clear watcher identified by watcher_id previously returned from PyContext_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 3.14 :::

Enumeration of possible context object watcher events:

• Py_CONTEXT_SWITCHED: The current context{.interpreted-text role="term"} has switched to a different context. The object passed to the watch callback is the now-current contextvars.Context{.interpreted-text role="class"} object, or None if no context is current.

::: versionadded 3.14 :::

Context object watcher callback function. The object passed to the callback is event-specific; see PyContextEvent{.interpreted-text role="c:type"} for details.

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

Context variable functions:

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.

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.

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.

Reset the state of the var context variable to that it was in before PyContextVar_Set{.interpreted-text role="c:func"} that returned the token was called. This function returns 0 on success and -1 on error.