[New-bugs-announce] [issue42522] [C API] Add Py_Borrow() function: call Py_XDECREF() and return the object

STINNER Victor report at bugs.python.org
Tue Dec 1 06:21:37 EST 2020

New submission from STINNER Victor <vstinner at python.org>:

I'm working on a script to migrate old C extension modules to the latest flavor of the Python C API:

I would like to replace frame->f_code with PyFrame_GetCode(frame). The problem is that frame->f_code is a borrowed reference, whereas PyFrame_GetCode() returns a strong reference. Having a Py_Borrow() function would help to automate migration to PyFrame_GetCode():

static inline PyObject* Py_Borrow(PyObject *obj)
    return obj;

So frame->f_code can be replaced with Py_Borrow(PyFrame_GetCode(frame)).

Py_Borrow() is similar to Py_XDECREF() but can be used as an expression:

    PyObject *code = Py_Borrow(PyFrame_GetCode(frame));

Py_Borrow() is the opposite of Py_XNewRef(). For example, Py_Borrow(Py_XNewRef(obj)) leaves the reference count unchanged (+1 and then -1).

My pratical problem is that it's not easy not add Py_XDECREF() call when converting C code to the new C API.

Example 1:

    PyObject *code = frame->f_code;

This one is easy and can be written as:

    PyObject *code = PyFrame_GetCode(frame); Py_XDECREF(code);

Example 2:


This one is more tricky. For example, the following code using a macro is wrong:

    #define Py_BORROW(obj) (Py_XDECREF(obj), obj)

since it calls PyFrame_GetCode() twice when proceed by the C preprocessor and so leaks a reference:

    func(Py_XDECREF(PyFrame_GetCode(frame->f_code)), PyFrame_GetCode(frame->f_code));

Attached PR implements Py_Borrow() function as a static inline function.

components: C API
messages: 382237
nosy: vstinner
priority: normal
severity: normal
status: open
title: [C API] Add Py_Borrow() function: call Py_XDECREF() and return the object
versions: Python 3.10

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