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functools_cmp_to_key() doesn't check that the argument is a callable.
>>> table=list(range(3))
>>> table.sort(key=functools.cmp_to_key(3))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable
PyCallable_Check() should be used, something like:
 if (!PyCallable_Check(cmp)) {
 PyErr_SetString(PyExc_TypeError, "parameter must be callable");
 return NULL;
 }