typing.NamedTuple() should prefix parameters with '_'

[GlennGribble]

BPO	38191
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi, @miss-islington, @Glenn Gribble
PRs	bpo-38191: Accept arbitrary keyword names in NamedTuple() and TypedDict(). #16222 [3.8] bpo-38191: Accept arbitrary keyword names in NamedTuple() and TypedDict(). (GH-16222) #16237 bpo-38191: Turn warnings into errors in NamedTuple() and TypedDict(). #16238 [3.7] bpo-38191: Accept arbitrary keyword names in NamedTuple(). (GH-16222) #16239 [3.8] bpo-38191: Use positional-only parameters in TypedDict(). #16240

^{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/serhiy-storchaka'
closed_at = <Date 2019-09-18.06:08:31.725>
created_at = <Date 2019-09-16.19:59:55.784>
labels = ['3.7', '3.8', 'type-feature', 'library', '3.9']
title = "typing.NamedTuple() should prefix parameters with '_'"
updated_at = <Date 2019-09-18.06:08:31.725>
user = 'https://github.com/GlennGribble'

bugs.python.org fields:

activity = <Date 2019-09-18.06:08:31.725>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2019-09-18.06:08:31.725>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2019-09-16.19:59:55.784>
creator = 'gribbg'
dependencies = []
files = []
hgrepos = []
issue_num = 38191
keywords = ['patch']
message_count = 9.0
messages = ['352579', '352620', '352622', '352654', '352655', '352656', '352657', '352659', '352697']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi', 'miss-islington', 'gribbg']
pr_nums = ['16222', '16237', '16238', '16239', '16240']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38191'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[GlennGribble]

At present, it is not possible to use the shorthand notation to define a NamedTuple with typename or fields.  I.e., NamedTuple('MyTuple', typename=str, fields=int) does not work.  Changing the parameter names to _typename and _fields would allow any non-private, legal identifier to be used in the shorthand notation.
  
>>> import typing
>>> typing.NamedTuple('Example', fieldz=int)
<class '__main__.Example'>
>>> typing.NamedTuple('Example2', fields=int)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "C:\Program Files\Python37\lib\typing.py", line 1411, in __new__
    return _make_nmtuple(typename, fields)
  File "C:\Program Files\Python37\lib\typing.py", line 1326, in _make_nmtuple
    types = [(n, _type_check(t, msg)) for n, t in types]
TypeError: 'type' object is not iterable

Of course, it is fairly easy to work around the issue by using fields parameter:

>>> typing.NamedTuple('Example3', [('fields', int)])
<class '__main__.Example3'>

There would be backwards compatibility issues with any code using named arguments for fields or typename.

[serhiy-storchaka]

Looks as a good case for positional-only parameters.

But first passing these arguments by keyword should be deprecated.

[serhiy-storchaka]

PR 16222 adds support for arbitrary keyword argument names in NamedTuple and TypedDict. Passing arguments like "typename", "_typename", "fields" and "_fields" by keyword is still supported, but deprecated. I'm going to backport this to 3.8 and 3.7 (to 3.7 without deprecation warnings), and than convert warnings to error in master (by using positional-only parameters syntax).

[serhiy-storchaka]

New changeset 2bf31cc by Serhiy Storchaka in branch 'master':
bpo-38191: Accept arbitrary keyword names in NamedTuple() and TypedDict(). (GH-16222)
2bf31cc

[miss-islington]

New changeset 54ba5f1 by Miss Islington (bot) in branch '3.8':
bpo-38191: Accept arbitrary keyword names in NamedTuple() and TypedDict(). (GH-16222)
54ba5f1

[serhiy-storchaka]

Oh, when backported to 3.7 I have found that TypedDict is new in 3.8. So I think it is better to get rid of deprecations in TypedDict().

[serhiy-storchaka]

New changeset 69b3718 by Serhiy Storchaka in branch '3.7':
[3.7] bpo-38191: Accept arbitrary keyword names in NamedTuple(). (GH-16222) (GH-16239)
69b3718

[serhiy-storchaka]

New changeset 8fc5839 by Serhiy Storchaka in branch 'master':
bpo-38191: Turn warnings into errors in NamedTuple() and TypedDict(). (GH-16238)
8fc5839

[serhiy-storchaka]

New changeset 2adcd79 by Serhiy Storchaka in branch '3.8':
bpo-38191: Use positional-only parameters in TypedDict(). (GH-16240)
2adcd79