Document dict behavior when setting equal but not identical key

[malthe]

BPO	46925
Nosy	@rhettinger, @methane, @malthe, @JelleZijlstra, @potiuk
PRs	Replace key if not identical to old key in dict #31685

^{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 = None
created_at = <Date 2022-03-04.23:25:53.644>
labels = ['3.11', 'type-bug', '3.9', '3.10', 'docs']
title = 'Document dict behavior when setting equal but not identical key'
updated_at = <Date 2022-03-07.10:13:25.530>
user = 'https://github.com/malthe'

bugs.python.org fields:

activity = <Date 2022-03-07.10:13:25.530>
actor = 'potiuk'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2022-03-04.23:25:53.644>
creator = 'malthe'
dependencies = []
files = []
hgrepos = []
issue_num = 46925
keywords = []
message_count = 9.0
messages = ['414554', '414560', '414562', '414572', '414581', '414586', '414607', '414636', '414652']
nosy_count = 6.0
nosy_names = ['rhettinger', 'methane', 'docs@python', 'malthe', 'JelleZijlstra', 'potiuk']
pr_nums = ['31685']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46925'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry #91499
• [3.11] gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (GH-91499) #100390
• [3.10] gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (GH-91499) #100391

[malthe]

When a key that is equal to an existing key (but not the same object identity) is used to set a new value, the key itself is not replaced.

This manifests perhaps most clearly in weakref.WeakKeyDictionary where keys can mysteriously disappear.

Consider two equal keys, k1 and k2:

d = WeakKeyDictionary()
d[k1] = 1
d[k2] = 2
del k1

We would expect the dictionary to have a single entry k2 => 2. But in fact it is empty now.

[JelleZijlstra]

As @methane also said on the PR, this is a backward compatibility break and we can't just change the behavior. I'd be opposed to a change here: the proposed behavior isn't clearly better than the existing behavior (sometimes you want one behavior, sometimes another), and it will be difficult to find and adjust code that relies on the existing behavior.

[rhettinger]

I concur with Jelle and Methane that we can't do this without breaking code.

Also if you don't care about dict order, the work around is easy. Just remove the old key:

d.pop(k); d[k] = v

[potiuk]

How about if we make PR instead to the documentation about this behaviour?

I think this behaviour is a little surprising (you need to know the internals of how WeakKeyDict keys are constructed and checked) and while I understand you do not want breaking change, maybe this should be explained to the users and example given how to apply the workaround?

Just following the "least surprise" principle.

[JelleZijlstra]

Agree, I couldn't find a place where this behavior is documented. The second paragraph in https://docs.python.org/3.10/library/stdtypes.html#mapping-types-dict comes close. Reopening as a documentation issue.