Just a minor performance issue.

The C functions _PyFunction_FastCallDict() and _PyFunction_FastCallKeywords() (branch 'master', Objects/call.c) and their predecessors fast_function() and _PyFunction_FastCallDict() in Python/ceval.c all contain the following sub-expression in the "if"-statement for the fast-path. For instance Objects/call.c:318

 co->co_flags == (CO_OPTIMIZED | CO_NEWLOCALS | CO_NOFREE)

Now, if co_flags has any of the CO_FUTURE_... bits set, the expression is always False and the fast path is not used.

Currently this affects only Python 3.6 and Python 2.7, because other Python versions do not use the __future__ mechanism.

The fix is simple. Replace the faulty sub-expression by

 (co->co_flags & (~PyCF_MASK)) == (CO_OPTIMIZED | CO_NEWLOCALS | CO_NOFREE))

I discovered this issue while debugging reference leaks in Stackless Python a few month ago. It is hard to write a test case, but one can compare C call stacks using a debugger.

$ ulimit -c unlimited  # enable core dumps
$ python3.6 -c 'from __future__ import generator_stop; import os; (lambda: os.abort())()'
$ gdb -batch -ex bt  python3.6 core > trace_with_future
$ python3.6 -c 'import os; (lambda: os.abort())()'
$ gdb -batch -ex bt  python3.6 core > trace_without_future

If you compare the traces, the difference is in stack frame #9. Same for python2.7.

> The fix is simple. Replace the faulty sub-expression by
> (co->co_flags & (~PyCF_MASK)) == (CO_OPTIMIZED | CO_NEWLOCALS | CO_NOFREE))

I proposed PR 4087 to implement this optimization.

I wouldn't call it a "fix", since the "co->co_flags == (CO_OPTIMIZED | CO_NEWLOCALS | CO_NOFREE)" check exists since Python 2.7 at least (whereas Python 2.7 also has CO_FUTURE_xxx flags).

> Just a minor performance issue.

I prefer to call it a performance opportunity :-)

I reset Versions to Python 3.7. I don't consider this issue as a bug, but only as a new optimization. So it can only go into the future Python 3.7.

New changeset 086c3ae5f0995a62092b9080f32dd118c2923453 by Victor Stinner in branch 'master':
bpo-31835: Optimize also FASTCALL using __future__ (#4087)
https://github.com/python/cpython/commit/086c3ae5f0995a62092b9080f32dd118c2923453

Thank you Anselm Kruis for spotting this nice optimization opportunity! Sadly, as I wrote, I don't want to backport the optimization to the stable Python 3.6 branch.