New issue 2599: Py_XINCREF/Py_XDECREF use argument twice
https://bitbucket.org/pypy/pypy/issues/2599/py_xincref-py_xdecref-use-argument-twice
Marian Beermann:
This isn't documented, but in CPython these are implemented to never use the
macro's argument twice:
```
#define Py_XDECREF(op) \
do { \
PyObject *_py_xdecref_tmp = (PyObject *)(op); \
if (_py_xdecref_tmp != NULL) \
Py_DECREF(_py_xdecref_tmp); \
} while (0)
```
PyPy does:
```
#define Py_XDECREF(op) do { if ((op) == NULL) ; else Py_DECREF(op); } while (0)
```
I didn't notice this in my code, and it's bad style (and I'll fix it in my code
;), but Google suggests that writing `Py_XDECREF(PyObject_CallMethod(...` is
somewhat common.
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