`_remove_dups_flatten` in `typing.py` contains dead branch

[sobolevn]

BPO	46470
Nosy	@sobolevn, @Fidget-Spinner
PRs	bpo-46470: remove unused branch from typing._remove_dups_flatten #30780

^{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 2022-01-24.16:10:16.872>
created_at = <Date 2022-01-22.10:16:13.368>
labels = ['type-bug', 'library', '3.11']
title = '`_remove_dups_flatten` in `typing.py` contains dead branch'
updated_at = <Date 2022-01-24.16:10:16.872>
user = 'https://github.com/sobolevn'

bugs.python.org fields:

activity = <Date 2022-01-24.16:10:16.872>
actor = 'kj'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-24.16:10:16.872>
closer = 'kj'
components = ['Library (Lib)']
creation = <Date 2022-01-22.10:16:13.368>
creator = 'sobolevn'
dependencies = []
files = []
hgrepos = []
issue_num = 46470
keywords = ['patch']
message_count = 2.0
messages = ['411244', '411483']
nosy_count = 2.0
nosy_names = ['sobolevn', 'kj']
pr_nums = ['30780']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46470'
versions = ['Python 3.11']

[sobolevn]

Here's how _remove_dups_flatten is defined right now:

def _remove_dups_flatten(parameters):
    """An internal helper for Union creation and substitution: flatten Unions
    among parameters, then remove duplicates.
    """
    # Flatten out Union[Union[...], ...].
    params = []
    for p in parameters:
        if isinstance(p, (_UnionGenericAlias, types.UnionType)):
            params.extend(p.__args__)
        elif isinstance(p, tuple) and len(p) > 0 and p[0] is Union:
            params.extend(p[1:])
        else:
            params.append(p)

    return tuple(_deduplicate(params))

Source:

cpython/Lib/typing.py

Line 274 in 38afeb1

def _remove_dups_flatten(parameters):

It is only used in `def Union():`, source:

cpython/Lib/typing.py

Lines 522 to 523 in 38afeb1

	parameters = tuple(_type_check(p, msg) for p in parameters)
	parameters = _remove_dups_flatten(parameters)

parameters = tuple(_type_check(p, msg) for p in parameters)
parameters = _remove_dups_flatten(parameters)

But, notice that `_remove_dups_flatten` contains this branch: `elif isinstance(p, tuple) and len(p) > 0 and p[0] is Union:`.

It is never executed, removing it does not change test_typing / test_types results. And it is reasonable: _type_check ensures that parameters can only contain types, not tuples.

Proof:

>>> from typing import Union, get_type_hints

>>> Union[int, (Union, str, bool)]
# TypeError: Union[arg, ...]: each arg must be a type. Got (typing.Union, <class 'str'>, <class 'bool'>).

>>> class Some:
...    x: 'Union[int, (Union, str, bool)]'
... 
>>> get_type_hints(Some)
# TypeError: Union[arg, ...]: each arg must be a type. Got (typing.Union, <class 'str'>, <class 'bool'>).

Since it is pretty old, I guess the internal API has changed significantly and it is not needed anymore.

I am going to send a PR to remove it.

[Fidget-Spinner]

New changeset c144d93 by Nikita Sobolev in branch 'main':
bpo-46470: remove unused branch from typing._remove_dups_flatten (GH-30780)
c144d93