Refactor typing.NamedTuple

[serhiy-storchaka]

BPO	40185
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi, @tirkarthi, @juliusgeo
PRs	bpo-40185: Refactor typing.NamedTuple #19371 bpo-43923: Revert "bpo-40185: Refactor typing.NamedTuple (GH-19371)" #31679 bpo-43923: Allow NamedTuple multiple inheritance #31779

^{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 2020-04-08.08:05:31.438>
created_at = <Date 2020-04-04.21:08:45.136>
labels = ['type-feature', 'library', '3.9']
title = 'Refactor typing.NamedTuple'
updated_at = <Date 2022-03-09.04:16:10.586>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2022-03-09.04:16:10.586>
actor = 'juliusgeo'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-08.08:05:31.438>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2020-04-04.21:08:45.136>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40185
keywords = ['patch']
message_count = 5.0
messages = ['365783', '365788', '365972', '375101', '375105']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'python-dev', 'serhiy.storchaka', 'levkivskyi', 'xtreak', 'juliusgeo']
pr_nums = ['19371', '31679', '31779']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40185'
versions = ['Python 3.9']

[serhiy-storchaka]

typing.NamedTuple is used in two ways.

1. It is a callable which produces a new namedtuple type.
2. It can also be used as a base in the class statement for creating a new namedtuple type.

In both cases it is not a real class. You cannot create an instance of NamedTuple or a subclass of NamedTuple. But it is implemented as a class, and help() shows methods and data descriptors for it, which are useless.

The proposed PR implements NamedTuple like a function. Implementation of the __mro_entries__ method allows to use it as a base in the class statement.

[serhiy-storchaka]

See also bpo-40187.

[serhiy-storchaka]

New changeset a2ec069 by Serhiy Storchaka in branch 'master':
bpo-40185: Refactor typing.NamedTuple (GH-19371)
a2ec069

[tirkarthi]

This seems to have caused a test failure in astroid project. I am not sure if the fix needs to be done from their end probably relying on some implementation detail or from the patch itself. Feel free to ignore if it's a third-party only error.

Reproducer script : /tmp/namedtuple_astroid.py

from astroid import parse

mod = parse('''from typing import NamedTuple; NamedTuple("A")''')
print(str(next(mod.body[-1].value.infer())))
assert str(next(mod.body[-1].value.infer())) != 'Uninferable'

Before this commit :

python /tmp/namedtuple_astroid.py
Instance of typing.NamedTuple

After this commit :

python /tmp/namedtuple_astroid.py
Uninferable
Traceback (most recent call last):
  File "/tmp/namedtuple_astroid.py", line 4, in <module>
    assert str(next(mod.body[-1].value.infer())) != 'Uninferable'
AssertionError

[serhiy-storchaka]

$ echo 'from typing import NamedTuple; NamedTuple("A")' | ./python -m ast
Module(
   body=[
      ImportFrom(
         module='typing',
         names=[
            alias(name='NamedTuple')],
         level=0),
      Expr(
         value=Call(
            func=Name(id='NamedTuple', ctx=Load()),
            args=[
               Constant(value='A')],
            keywords=[]))],
   type_ignores=[])

So mod.body[-1].value is a call:

Call(func=Name(id='NamedTuple', ctx=Load()), args=[Constant(value='A')], keywords=[])

I do not know what Astroid does with this node, but seem the problem is in its infer() method.