Add generic version of OrderedDict to typing module

[itoijala]

BPO	35341
Nosy	@gvanrossum, @rhettinger, @ilevkivskyi, @miss-islington, @tirkarthi, @itoijala
PRs	bpo-35341: Add generic version of OrderedDict to typing #10850 [3.7] bpo-35341: Add generic version of OrderedDict to typing (GH-10850) #10851

^{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 2018-12-02.16:18:57.270>
created_at = <Date 2018-11-28.17:17:38.210>
labels = ['3.7', '3.8', 'type-feature', 'library']
title = 'Add generic version of OrderedDict to typing module'
updated_at = <Date 2018-12-02.16:18:57.269>
user = 'https://github.com/itoijala'

bugs.python.org fields:

activity = <Date 2018-12-02.16:18:57.269>
actor = 'levkivskyi'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-02.16:18:57.270>
closer = 'levkivskyi'
components = ['Library (Lib)']
creation = <Date 2018-11-28.17:17:38.210>
creator = 'itoijala'
dependencies = []
files = []
hgrepos = []
issue_num = 35341
keywords = ['patch']
message_count = 9.0
messages = ['330616', '330661', '330672', '330673', '330861', '330873', '330875', '330887', '330889']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'rhettinger', 'levkivskyi', 'miss-islington', 'xtreak', 'itoijala']
pr_nums = ['10850', '10851']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35341'
versions = ['Python 3.7', 'Python 3.8']

[itoijala]

The other collections from the collections module (namedtuple, deque, ChainMap, Counter, defaultdict) have generic versions in the typing module for use in type annotations.

The problem is currently the following:

from __future__ import annotations
import typing
from collections import OrderedDict
# Understood by mypy
def f(d: OrderedDict[str, str]) -> None:
    pass
typing.get_type_hints(f)

gives:

Traceback (most recent call last):
  File "foo.py", line 9, in <module>
    typing.get_type_hints(f)
  File "/usr/lib/python3.7/typing.py", line 1001, in get_type_hints
    value = _eval_type(value, globalns, localns)
  File "/usr/lib/python3.7/typing.py", line 260, in _eval_type
    return t._evaluate(globalns, localns)
  File "/usr/lib/python3.7/typing.py", line 464, in _evaluate
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
TypeError: 'type' object is not subscriptable

To fix this, a line like the following could be added to Lib/typing.py near line 1250:

OrderedDict = _alias(collections.OrderedDict, (KT, VT))

There might be a reasoning for why this has not been done yet, but I have not been able to find any.

If this is acceptable, I could prepare a PR.
I suppose there is no hope of a backport to 3.7, since this is a new feature (though very minor).

[gvanrossum]

Given that this is in collections, I don't object. Ivan, what do you say?

[ilevkivskyi]

Yes, since we already have DefaultDict, Counter, and ChainMap from collections, we can also add OrderedDict.

[ilevkivskyi]

Also typing is technically still provisional, we can backport this to previous versions (at least to 3.7).

[rhettinger]

Now that regular dicts are ordered, my expectation is that OrderedDict() is going to mostly fall into disuse (much like UserDict, UserList, UserString).

That said, it would be nice if all of the collections classes had generic counterparts in the typing module.

[itoijala]

My reason for using OrderedDict was that the normal dict is not reversible. I needed to get the last element added, so I ended up doing something like next(reversed(ordered_dict)).

Perhaps this would be something to add to the normal dict (for 3.8?). Then I could migrate away from OrderedDict.

Shall I open a new issue for this?

[tirkarthi]

Shall I open a new issue for this?

@itoijala Please see https://bugs.python.org/issue33462. It's on master.

[ilevkivskyi]

New changeset 68b56d0 by Ivan Levkivskyi (Ismo Toijala) in branch 'master':
bpo-35341: Add generic version of OrderedDict to typing (GH-10850)
68b56d0

[miss-islington]

New changeset 6cb0486 by Miss Islington (bot) in branch '3.7':
bpo-35341: Add generic version of OrderedDict to typing (GH-10850)
6cb0486