Return False from __contains__ method if object not hashable for set and dict

[heckad]

BPO	45605
Nosy	@ericvsmith, @serhiy-storchaka, @heckad
Superseder	bpo-18510: dict.contains and dict.keys().contains raises exception instead of returning False

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-10-25.16:15:27.466>
created_at = <Date 2021-10-25.15:22:39.835>
labels = ['library', '3.11']
title = 'Return False from __contains__ method if object not hashable for set and dict'
updated_at = <Date 2021-10-25.16:15:27.464>
user = 'https://github.com/heckad'

bugs.python.org fields:

activity = <Date 2021-10-25.16:15:27.464>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-25.16:15:27.466>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2021-10-25.15:22:39.835>
creator = 'heckad'
dependencies = []
files = []
hgrepos = []
issue_num = 45605
keywords = []
message_count = 5.0
messages = ['404971', '404972', '404973', '404975', '404978']
nosy_count = 3.0
nosy_names = ['eric.smith', 'serhiy.storchaka', 'heckad']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '18510'
type = None
url = 'https://bugs.python.org/issue45605'
versions = ['Python 3.11']

[heckad]

Now if do something like [] in set() python raise an exception, but if an object isn't hashable then we know for sure that it isn't in the set. Propose return False for these cases.

P.S. I would like to make a patch

[ericvsmith]

That seems like a pretty big change that could break a lot of existing code. I suggest writing a wrapper around "in" if you want this behavior.

[heckad]

Can you please specify in what cases we need to get an exception instead of False? It just seems very strange to me to rely on this behaviour.

[ericvsmith]

I don't have any code that would care, but if there's one thing I've learned in 15 years of python core development, it's that this will break someone's code.

If you want more feedback, you should probably bring this up for discussion on the python-ideas mailing list.

[serhiy-storchaka]

This is a duplicate of bpo-18510. It was also discussed on the Python-Dev mailing list (maybe more than once). If you have some new arguments or something in past 8 years made the old arguments no longer valid please open a new discussion on the mailing list.