del expects __delitem__ if __setitem__ is defined

[raumzeitkeks]

BPO	31218
Nosy	@rhettinger, @bitdancer, @hongweipeng, @tirkarthi, @iritkatriel
Files	delitem_example.py: Minimal example demonstrating the differing behaviour

^{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 2021-07-08.19:38:45.031>
created_at = <Date 2017-08-16.10:48:18.005>
labels = ['interpreter-core', 'type-bug', '3.11']
title = 'del expects __delitem__ if __setitem__ is defined'
updated_at = <Date 2021-07-08.19:38:45.030>
user = 'https://bugs.python.org/raumzeitkeks'

bugs.python.org fields:

activity = <Date 2021-07-08.19:38:45.030>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-08.19:38:45.031>
closer = 'rhettinger'
components = ['Interpreter Core']
creation = <Date 2017-08-16.10:48:18.005>
creator = 'raumzeitkeks'
dependencies = []
files = ['47085']
hgrepos = []
issue_num = 31218
keywords = []
message_count = 5.0
messages = ['300346', '300354', '327857', '397037', '397167']
nosy_count = 6.0
nosy_names = ['rhettinger', 'r.david.murray', 'raumzeitkeks', 'hongweipeng', 'xtreak', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31218'
versions = ['Python 3.11']

[raumzeitkeks]

I noticed some odd behaviour on classes defining __setitem__.
Using del on a class defining __setitem__ but not __delitem__ results in "AttributeError: __delitem__".
On classes definig neiter __setitem__ nor __delitem__ on the other hand this results in "TypeError: 'WithoutSetItem' object doesn't support item deletion".

See the appended example script.

[bitdancer]

I'm not sure we would consider this a bug (the message is accurate), but I wouldn't object to fixing it, since that would indeed seem more consistent with how __delitem__ and del are defined in the language reference.

[rhettinger]

This is likely an implementation artifact. In the abstract API, both PyObject_DelItem() and PyObject_SetItem() route through the same slot, m->mp_ass_subscript. The set and delete operations are only differentiated in the downstream concrete APIs. When the *value* parameter is NULL, the operation is deemed to be a deletion.

[iritkatriel]

It is still the same in 3.11:

>>> class WithoutSetItem:
...     def __getitem__(self, key):
...         return "foo"
...
>>> class WithSetItem:
...     def __getitem__(self, key):
...         return "foo"
...     def __setitem__(self, key, val):
...         return
...
>>> wo = WithoutSetItem()
>>> del wo[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'WithoutSetItem' object doesn't support item deletion
>>> w = WithSetItem()
>>> del w[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __delitem__
>>>

[rhettinger]

I'm closing this as not being worth changing. It is only a minor irritant and arguably not a bug. "Fixing it" would be disruptive and likely not help anyone.