Constructors of some mapping classes don't accept `self` keyword argument

[abacabadabacaba]

BPO	22609
Nosy	@rhettinger, @mdickinson, @vstinner, @larryhastings, @berkerpeksag, @vadmium, @serhiy-storchaka, @1st1
Files	collections_pos_only_params.patch collections_pos_only_params2.patch UserDict_self_and_dict_keywords.patch UserDict_self_and_dict_keywords_2.patch UserDict_self_and_dict_keywords_3.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 = 'https://github.com/rhettinger'
closed_at = <Date 2015-09-29.20:59:53.927>
created_at = <Date 2014-10-11.12:04:09.064>
labels = ['type-bug', 'library']
title = "Constructors of some mapping classes don't accept `self` keyword argument"
updated_at = <Date 2015-09-29.20:59:53.926>
user = 'https://bugs.python.org/abacabadabacaba'

bugs.python.org fields:

activity = <Date 2015-09-29.20:59:53.926>
actor = 'serhiy.storchaka'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2015-09-29.20:59:53.927>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2014-10-11.12:04:09.064>
creator = 'abacabadabacaba'
dependencies = []
files = ['36881', '36886', '38692', '38754', '39821']
hgrepos = []
issue_num = 22609
keywords = ['patch']
message_count = 34.0
messages = ['229076', '229089', '229098', '229118', '229121', '229189', '229248', '229249', '231753', '231763', '231765', '231766', '231769', '231915', '231936', '232353', '234875', '239278', '239655', '239667', '243329', '244470', '244503', '245859', '245877', '245944', '246936', '246954', '246956', '246957', '251778', '251831', '251886', '251888']
nosy_count = 10.0
nosy_names = ['rhettinger', 'mark.dickinson', 'vstinner', 'larry', 'abacabadabacaba', 'python-dev', 'berker.peksag', 'martin.panter', 'serhiy.storchaka', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22609'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[abacabadabacaba]

>>> import collections
>>> collections.Counter(self=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got multiple values for argument 'self'
>>> collections.OrderedDict(self="test")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got multiple values for argument 'self'
>>> collections.UserDict(self="test")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got multiple values for argument 'self'

Python surely should have positional-only parameters.

[serhiy-storchaka]

Here is a patch.

[rhettinger]

I'll spend some time taking this one under consideration.

Keyword arguments for dicts and dict-like classes are already somewhat limited (only non-keyword identifiers) that why Guido resisted adding them in the first place. And, in the context of OrderedDicts, they aren't really useful at all (because the order gets scrambled).

I'm reluctant to make the number of changes required in order to support this corner case. The ship for Python 2.7 sailed a long time ago and it is risky to make changes like this. After the patch, the code is less readable, harder to get right, harder to maintain, and not as performant.

On the other hand, adding "self" would be a nice-to-have, so it is worth thinking about.

[rhettinger]

After some thought, I think we have to fix this.

I'll go through the patch in careful detail this weekend.

Ethan, if you would like to help, it would be great to have a third pair of eyes looking at the patch to make sure it correct it every detail.

[serhiy-storchaka]

I thought that more verbose but straightforward code would be more acceptable.
Well, here is smaller and clever patch. Tests are the same.

[ethanfurman]

I will take a look.

[ethanfurman]

Code looks good.

Only downside is the change in help and inspect.signature output, but that is minor:

Help on dict object:
class dict(object)
[...]
| __init__(self, /, *args, **kwargs)

vs.

Help on class Counter in module collections:
class Counter(builtins.dict)
[...]
| __init__(*args, **kwds)

+1 to accept.

[larryhastings]

FWIW, I agree that it should be fixed:

>>> dict(self=1)
{'self': 1}

[rhettinger]

This looks good. Go ahead and apply the first version of the patch.

[python-dev]

New changeset 816c15fe5812 by Serhiy Storchaka in branch '3.4':
Issue bpo-22609: Constructors and update methods of mapping classes in the
https://hg.python.org/cpython/rev/816c15fe5812

New changeset 88ab046fdd8a by Serhiy Storchaka in branch 'default':
Issue bpo-22609: Constructors and update methods of mapping classes in the
https://hg.python.org/cpython/rev/88ab046fdd8a