[issue28263] Python 2.7's `-3` flag warns about __eq__ being implemented without __hash__ even if __hash__ is never accessed.

[Christian Heimes]

Christian Heimes added the comment:

Antti is correct. Please add __hash__ = None to your class to silence the 
warning.

--
nosy: +christian.heimes
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue28263] Python 2.7's `-3` flag warns about __eq__ being implemented without __hash__ even if __hash__ is never accessed.

[Ned Batchelder]

Changes by Ned Batchelder :


--
nosy: +nedbat

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[issue28263] Python 2.7's `-3` flag warns about __eq__ being implemented without __hash__ even if __hash__ is never accessed.

[Antti Haapala]

Antti Haapala added the comment:

I am very negative to this idea. Correct code in Python **2** would either set 
`__hash__ = None` or redefine `__hash__` in *any* class that defines `__eq__`. 
That it just wasn't used like that is no excuse.

This warning is even more important if even Ned Batchelder could have a bug 
like that in his code.

It will break as soon as someone "realizes" that "hey I can use a set to remove 
duplicates in my container".

--
nosy: +ztane

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[issue28263] Python 2.7's `-3` flag warns about __eq__ being implemented without __hash__ even if __hash__ is never accessed.

[Roy Williams]

New submission from Roy Williams:

I'm finding the -3 flag to be super useful at identifying problems with code 
when porting to Python 3.  One of the most common failures, however, is 
"DeprecationWarning: Overriding __eq__ blocks inheritance of __hash__ in 3.x".  
While implementing __eq__ without implementing __hash__ is very much an 
anti-pattern, this warning would be much less noisy if it could be thrown at 
time of access (like the __getslice__ warning) instead of time of declaration. 

See Also:
https://github.com/nedbat/coveragepy/pull/17
http://bugs.python.org/issue28260

--
components: Interpreter Core
messages: 277305
nosy: Roy Williams
priority: normal
severity: normal
status: open
title: Python 2.7's `-3` flag warns about __eq__ being implemented without 
__hash__ even if __hash__ is never accessed.
type: behavior
versions: Python 2.7

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