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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Add Py_HOLD_REF() macro to hold a strong reference to a Python object while executing code","articleBody":"Reference counting in Python is complicated. It's never obvious if we are holding a strong reference or a borrowed reference to a Python object.\r\n\r\nIf we have a **borrowed reference** to a Python object, to make sure that it cannot be deleted while we execute **arbitrary Python code**, it's needed to creating a temporary **strong reference** by increasing temporarily its reference counter. Example from ``Modules/_abc.c``:\r\n\r\n~~~\r\n    subclasses = PyObject_CallMethod(self, \"__subclasses__\", NULL);\r\n    (...)\r\n    PyObject *scls = PyList_GET_ITEM(subclasses, pos);\r\n\r\n    Py_INCREF(scls);  // create a temporary strong reference\r\n    int r = PyObject_IsSubclass(subclass, scls);\r\n    Py_DECREF(scls);  // delete the temporary strong reference\r\n~~~\r\n\r\nThe **scls** variable is assigned to a **borrowed reference** to a list item. The list is the result of the ``__subclasses__()`` method. The code creates a temporarily **strong reference** to call ``PyObject_IsSubclass()``.\r\n\r\nIMO in terms of semantics, such code is hard to understand if read aloud in English, since INCREF and DECREF modify the object **in-place**. Py_INCREF() and Py_DECREF() are like the low-level implementation, I would prefer to have an abstraction on top of it.\r\n\r\nThat's also why I added ``Py_NewRef()`` in Python 3.10: Py_NewRef() semantics is simpler: it **creates a new strong reference**. In terms of semantics, it doesn't modify the object in-place, even if it's possible to write something like ``var = Py_NewRef(var)`` (I discourage to write such code). See issue #99300 for the usage of the Py_NewRef() function.\r\n\r\nIt would be nice to have a macro to clarify the scope of the temporary strong reference rather than having to INCREF/DECREF temporarily. Something like:\r\n\r\n```c\r\n#define Py_HOLD_REF(var, code) \\\r\n    Py_INCREF(var); \\\r\n    code; \\\r\n    Py_DECREF(var)\r\n```\r\n\r\nExample of usage:\r\n\r\n```c\r\nPy_HOLD_REF(scls,\r\n    int r = PyObject_IsSubclass(subclass, scls);\r\n);\r\n```\r\n\r\nExample holding two (different) strong references:\r\n\r\n```c\r\nPy_HOLD_REF(keys,\r\n    Py_HOLD_REF(values,\r\n        PyObject *res = PyTuple_Pack(2, keys, values);\r\n    )    // note: there is no \";\" here\r\n);\r\n```\r\n\r\nAlternative compact syntax:\r\n\r\n```c\r\nPy_HOLD_REF(keys, Py_HOLD_REF(values,\r\n    PyObject *res = PyTuple_Pack(2, keys, values);\r\n));\r\n```\r\n\r\nObviously, the macro name is open for bikeshedding :-)","author":{"url":"https://github.com/vstinner","@type":"Person","name":"vstinner"},"datePublished":"2022-11-14T16:24:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/99481/cpython/issues/99481"}