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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Expose `_PyCriticalSection_BeginMutex` as a public API","articleBody":"# Feature or enhancement\n\n### Proposal:\n\nIt would be helpful to expose `_PyCriticalSection_BeginMutex` as a public API.\n\nThere are times when one does not have really have a `PyObject`, but still might want to use a critical section. For example, we wanted to use a critical section to protect some initialization code in a C extension module, where we wanted to write something like this:\n\n```c\nstatic PyMutex mu;\nstatic PyObject* mymodule = nullptr;\n\n// It is important that we use a critical section here rather than a regular\n// PyMutex, say 'M', because we need to avoid the following deadlock:\n// * thread T1 holds M and calls ImportModule, which attempts to acquire a\n//   critical section C.\n// * thread T2 holds critical section C and attempts to call this code and\n//   acquire M.\n// In general, if you're going to hold a lock while calling into Python code, it had better be a critical section.\n\nPyCriticalSection cs;\n_PyCriticalSection_BeginMutex(\u0026cs, \u0026mu);\nif (!mymodule) {\n    PyObject *m = PyImport_ImportModule(\"mymodule\");\n\n    // Importing may have released the critical section, so it is possible\n    // another thread might have populated mymodule. Make sure we clean up after\n    // ourselves in that case.\n    std::swap(mymodule, m);\n    Py_XDECREF(m);\n}\nPyCriticalSection_End(\u0026cs);\n```\n\nCurrently one would have to write something like the following instead:\n\n```c\nstatic PyObject object_for_mu;  // Not really an object, we just want its mutex.\nstatic PyObject* mymodule = nullptr;\n\nPyCriticalSection cs;\nPyCriticalSection_Begin(\u0026cs, \u0026object_for_mu);\nif (!mymodule) {\n    PyObject *m = PyImport_ImportModule(\"mymodule\");\n\n    std::swap(mymodule, m);\n    Py_XDECREF(m);\n}\nPyCriticalSection_End(\u0026cs);\n```\n\n@colesbury \n\n### Has this already been discussed elsewhere?\n\nThis is a minor feature, which does not need previous discussion elsewhere\n\n### Links to previous discussion of this feature:\n\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-135899\n* gh-136969\n* gh-137024\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/hawkinsp","@type":"Person","name":"hawkinsp"},"datePublished":"2025-05-02T12:47:31.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/133296/cpython/issues/133296"}