Right now, in typeobject.c, the call_method and call_maybe utility functions have a fast path for no argument methods, where a NULL or "" format string just calls PyTuple_New(0) directly instead of wasting time parsing Py_VaBuildValue.

Problem is, nothing uses it. Every no arg user (the slot wrappers for __len__, __index__ and __next__ directly, and indirectly through the SLOT0 macro for __neg__, __pos__, __abs__, __invert__, __int__ and __float__) is passing along "()" as the format string, which fails the test for NULL/"", so it calls Py_VaBuildValue that goes to an awful lot of trouble to scan the string a few times and eventually spit out the empty tuple anyway.

Changing the three direct calls to call_method where it passes "()" as the format string, as well as the definition of SLOT0, to replace "()" with NULL as the format string argument should remove a non-trivial number of C varargs function calls and string processing, replacing it with a single, cheap PyTuple_New(0) call (which Py_VaBuildValue was already eventually performing anyway). If I understand the purpose of these slot wrapper functions, that should give a free speed up to all types implemented at the Python level, particularly numeric types (e.g. fractions.Fraction) and container/iterator types (speeding up __len__ and __next__ respectively).

I identified this while on a work machine which I can't use to check out the Python repository; I'll submit a patch later today if no one else gets to it, once I'm home and can use my own computer to make/test the fix.