_contextvarsmodule.c — contextvars module C wrapper

_contextvarsmodule.c is the thin C wrapper that registers the contextvars module and exposes its three public types: Context, ContextVar, and Token. All real logic lives in Python/context.c, which implements the HAMT-backed immutable snapshot model. This file only wires up the module.

Map

Lines	Symbol	Role
1–20	Includes	context.h, moduleobject.h
21–60	_contextvars_copy_context	Python-callable wrapper for PyContext_CopyCurrent
61–90	_contextvarsmodule_methods	Method table: only copy_context is a free function
91–130	_contextvarsmodule_exec	Adds Context, ContextVar, Token types to module dict
131–160	_contextvarsmodule_slots	Py_mod_exec slot pointing to _exec
161–200	PyModuleDef + PyInit__contextvars	Multi-phase init entry point

Reading

Module exec: type registration

The three public types are defined in Python/context.c and exposed here via PyContext_Type, PyContextVar_Type, and PyContextToken_Type. The exec function adds them to the module dict under their public names.

// CPython: Modules/_contextvarsmodule.c:91 _contextvarsmodule_exec
static int
_contextvarsmodule_exec(PyObject *m)
{
    if (PyModule_AddType(m, &PyContext_Type) < 0)    return -1;
    if (PyModule_AddType(m, &PyContextVar_Type) < 0) return -1;
    if (PyModule_AddType(m, &PyContextToken_Type) < 0) return -1;
    return 0;
}

copy_context: the only free function

The module exposes exactly one free function: copy_context(), which snapshots the current context by calling PyContext_CopyCurrent. All other operations (ContextVar.get, ContextVar.set, ContextVar.reset, Context.run) are methods on the types defined in Python/context.c.

// CPython: Modules/_contextvarsmodule.c:21 _contextvars_copy_context
static PyObject *
_contextvars_copy_context(PyObject *self, PyObject *args)
{
    if (!_PyArg_NoKeywords("copy_context", args)) return NULL;
    return PyContext_CopyCurrent();
}

Multi-phase init

The module uses the PEP 451 multi-phase init pattern (Py_mod_exec slot) rather than a single-phase PyInit_ function. This allows the module to be re-imported cleanly in sub-interpreters.

// CPython: Modules/_contextvarsmodule.c:161 PyInit__contextvars
PyMODINIT_FUNC
PyInit__contextvars(void)
{
    return PyModuleDef_Init(&_contextvarsmodule);
}

gopy notes

In gopy the contextvars port spans two packages:

• module/contextvars/ holds the module registration, matching this file.
• The HAMT-backed Context implementation lives in objects/context.go (ported from Python/context.c), alongside the HAMT code in objects/hamt.go.

The public API surface is:

• PyContextVar_New, PyContextVar_Get, PyContextVar_Set, PyContextVar_Reset
• PyContext_New, PyContext_Copy, PyContext_CopyCurrent, PyContext_Enter, PyContext_Exit, PyContext_Run

module/contextvars/module.go calls into objects.ContextVar and objects.Context via those function names, keeping the layer separation clean.

CPython 3.14 changes

• The module was migrated from single-phase to multi-phase init in 3.12; no further structural changes in 3.14.
• PyContextVar_Type gained a __class_getitem__ implementation so that ContextVar[int] is valid as a generic alias in type annotations.
• Token.old_value now uses a sentinel type (Token.MISSING) that is its own distinct object, replacing the previous approach of returning None for unset-before-first-set cases.