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[Python-Dev] Python semantic: Is it ok to replace not x == y with x != y? (no)

Victor Stinner victor.stinner at gmail.com
Tue Dec 15 08:04:43 EST 2015 
Hi,

I implemented more constant folding optimizations in my FAT Python
project, but it looks like I made a subtle change in the Python
semantic.

Replacing "not x == y" with "x != y" changes the behaviour of Python.
For example, this optimization breaks test_unittest because
unittest.mock._Call implements __eq__() but not __ne__().

Is it expected that "not x.__eq__(y)" can be different than
"x.__ne__(y)"? Is it part of the Python semantic?

IMHO it's a bug in the unittest.mock module, but it's "acceptable"
because "it just works" :-) So FAT Python must not replace "not x ==
y" with "x != y" to not break the code.

Should Python emit a warning when __eq__() is implemented but not __ne__()?

Should Python be modified to call "not __eq__()" when __ne__() is not
implemented?

For me, it can be an annoying and sublte bug, hard to track.

Victor
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