>>> 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.

Here is a patch.

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.

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.

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

I will take a look.

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.

FWIW, I agree that it should be fixed:

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

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

New changeset 816c15fe5812 by Serhiy Storchaka in branch '3.4':
Issue #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 #22609: Constructors and update methods of mapping classes in the
https://hg.python.org/cpython/rev/88ab046fdd8a

New changeset cd1ead4feddf by Serhiy Storchaka in branch '3.4':
Issue #22609: Revert changes in UserDict. They conflicted with existing tests.
https://hg.python.org/cpython/rev/cd1ead4feddf

New changeset 167d51a54de2 by Serhiy Storchaka in branch 'default':
Issue #22609: Revert changes in UserDict. They conflicted with existing tests.
https://hg.python.org/cpython/rev/167d51a54de2

Unfortunately there is existing test for current behavior of UserDict:

    self.assertEqual(collections.UserDict(dict=[('one',1), ('two',2)]), d2)

This looks wrong to me and I think we should break compatibility here.

New changeset 3dfe4f0c626b by Serhiy Storchaka in branch '2.7':
Issue #22609: Constructors and update methods of mapping classes in the
https://hg.python.org/cpython/rev/3dfe4f0c626b

So what to do wish UserDict? Should we break backward compatibility and add support for "self" and "dict" keywords as in other dict-like types?

Fix `self` now, add a warning and single minor cycle deprecation period for 'dict'.

[Serhiy]
> So what to do wish UserDict?

I'm leaning in favor of leaving UserDict as-is.   AFAICT, in the very long history of UserDict, this has never been a problem.  So, I don't think there is an issue worth breaking the published API and possibly breaking code that currently works.  (This is a case of practicality beating purity).

So may be close this issue?

See also issue22958.

Here is a patch for UserDict, that keep current behavior, but allows to pass keys "self" and "dict" if positional parameter dict is specified.

>>> UserDict(self=42)
{'self': 42}
>>> UserDict({}, dict=42)
{'dict': 42}
>>> UserDict(dict={'a': 42})
{'a': 42}

UserDict_self_and_dict_keywords.patch looks good to me. Is there a reason not to use assertWarnsRegex?

Also, there are already collections.UserDict() usages in the test file, so I'd remove the "from collections import UserDict" import.

Thank you for your review Berker.

> Is there a reason not to use assertWarnsRegex?

Initially the patch was written for 2.7. Fixing WeakValueDictionary in 2.7 
needs first fix UserDict (issue22958). That is why I returned to this issue.

> Also, there are already collections.UserDict() usages in the test file, so
> I'd remove the "from collections import UserDict" import.

These tests originally was written for test_collections.

Updated patch addresses Berker's comment.

Could you please look at the new patch Raymond? This is the dependency for issue22958.

Ping.

Left a couple pedantic comments.

Updated patch addresses Martin's comments.

Left some feedback in the code review.

Patch looks fine to me. I understand Yury withdrew his comment.

Ping again.

Who are you pinging? I did just notice a minor English grammar problem (“one arguments”). But as far as I am concered you could have already committed the patch.

I will look at this more when I get a chance (likely this week).

I was pinging Raymond. He is maintainer of the collections module, this issue is assigned to his, and he had valid objections to previous version of the patch. Even one of this reason is enough to wait his review before committing.

Thank you Raymond.

Ping.

UserDict_self_and_dict_keywords_3.patch looks good to me.

New changeset 4c5407e1b0ec by Serhiy Storchaka in branch '2.7':
Issue #22609: Constructor and the update method of collections.UserDict now
https://hg.python.org/cpython/rev/4c5407e1b0ec

New changeset 1869f5625392 by Serhiy Storchaka in branch '3.4':
Issue #22609: Constructor of collections.UserDict now accepts the self keyword
https://hg.python.org/cpython/rev/1869f5625392

New changeset ab7e3f1f9f88 by Serhiy Storchaka in branch '3.5':
Issue #22609: Constructor of collections.UserDict now accepts the self keyword
https://hg.python.org/cpython/rev/ab7e3f1f9f88

New changeset 901964295066 by Serhiy Storchaka in branch 'default':
Issue #22609: Constructor of collections.UserDict now accepts the self keyword
https://hg.python.org/cpython/rev/901964295066

Thank you all for your reviews.