Macro Py_CLEAR references argument two times. · Issue #98724 · python/cpython · GitHub

Bug report

The macro Py_CLEAR(op) references the argument op two times. If the macro is called with an expression it will be evaluated two times, for example Py_CLEAR(p++).

Your environment

x86_64

CPython versions tested on:
Python 3.7m
Operating system and architecture:
Debian Stable

I suggest a fix similar to this (old version commented out with #if 0):

#if 0
#define Py_CLEAR(op)                            \
    do {                                        \
        PyObject *_py_tmp = (PyObject *)(op);   \
        if (_py_tmp != NULL) {                  \
            (op) = NULL;                        \
            Py_DECREF(_py_tmp);                 \
        }                                       \
    } while (0)
#else
#define Py_CLEAR(op)                                         \
    do {                                                     \
        PyObject **_py_tmp = (PyObject **)&(op);             \
        if (*_py_tmp != NULL) {                              \
            PyObject *_py_tmp2 = *_py_tmp;                   \
            (*_py_tmp) = NULL;                               \
            Py_DECREF(_py_tmp2);                             \
        }                                                    \
    } while (0)
#endif

I am not sure if this has happened anywhere, but I see a possible problem here. I think the compiler will optimize out the additional temporary variable in most cases.