make inspect Signature hashable

[1st1]

BPO	20334
Nosy	@brettcannon, @ncoghlan, @larryhastings, @1st1, @anntzer
Files	hashable_signature_01.patch issue20334-2.01.patch signature-hash-and-equality.patch

^{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 2014-04-08.15:47:22.795>
created_at = <Date 2014-01-21.16:50:24.937>
labels = ['type-feature', 'library']
title = 'make inspect Signature hashable'
updated_at = <Date 2014-09-12.19:50:37.497>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2014-09-12.19:50:37.497>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2014-04-08.15:47:22.795>
closer = 'yselivanov'
components = ['Library (Lib)']
creation = <Date 2014-01-21.16:50:24.937>
creator = 'yselivanov'
dependencies = []
files = ['33595', '36607', '36610']
hgrepos = []
issue_num = 20334
keywords = ['patch']
message_count = 12.0
messages = ['208671', '208673', '208674', '214962', '215768', '225343', '225361', '225364', '226820', '226831', '226840', '226841']
nosy_count = 6.0
nosy_names = ['brett.cannon', 'ncoghlan', 'larry', 'python-dev', 'yselivanov', 'Antony.Lee']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20334'
versions = ['Python 3.5']

[1st1]

inspect.Signature and inspect.Parameter are immutable structures, and it makes sense to make them hashable too.

Patch is attached.

[brettcannon]

This is a new feature so it can't go into Python 3.4.

[1st1]

Fair enough.

[1st1]

If nobody has any objections on this, I'm going to commit this in 3.5 soon.

[python-dev]

New changeset 932d69ef0c63 by Yury Selivanov in branch 'default':
inspect: Make Signature and Parameter hashable. Issue bpo-20334.
http://hg.python.org/cpython/rev/932d69ef0c63

[anntzer]

The hash function of the Signature class is actually incompatible with the definition of Signature equality, which doesn't consider the order of keyword-only arguments:

>>> from inspect import signature
>>> s1 = signature(lambda *, x, y: None); s2 = signature(lambda *, y, x: None)
>>> s1 == s2
True
>>> hash(s1) == hash(s2)
False

Actually the implementation of Signature.__eq__ seems way too complicated; I would suggest making a helper method returning (return_annotation, tuple(non-kw-only-params), frozenset(kw-only-params)) so that __eq__ can compare these values while __hash__ can hash that tuple.

[1st1]

Thanks, Antony, this is a good catch. Your suggestion seems like a good idea. I'll look into this more closely soon.

[anntzer]

Actually, that specific solution (using a helper method) will fail because there may be unhashable params (due to unhashable default values or annotations) among the keyword-only arguments, so it may not be possible to build a frozenset (rather, one should compare the {param.name: param if param.kind == KEYWORD_ONLY} dict). The frozenset approach still works for computing the hash as this requires all params to be hashable anyways.

[1st1]

Antonie, I'm attaching a patch (bpo-20334-2.01.patch) to this issue which should fix the problem. Please review.

[anntzer]

While your patch works, I think it is a good opportunity to simplify the implementation of Signature.__eq__, which is *much* more complicated than what it should be.
Please comment on the attached patch, which uses the helper method approach I suggested.

[python-dev]

New changeset 3b974b61e74d by Yury Selivanov in branch 'default':
inspect.Signature: Fix discrepancy between __eq__ and __hash__.
http://hg.python.org/cpython/rev/3b974b61e74d

[1st1]

Antony, I've tweaked the patch a bit and it's now in default branch. Thank you!