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_remove_dups_flatten in typing.py contains dead branch #90628

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sobolevn opened this issue Jan 22, 2022 · 2 comments
Closed

_remove_dups_flatten in typing.py contains dead branch #90628

sobolevn opened this issue Jan 22, 2022 · 2 comments
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3.11 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@sobolevn
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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
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    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.

    @sobolevn sobolevn added 3.11 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Jan 22, 2022
    @Fidget-Spinner
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    New changeset c144d93 by Nikita Sobolev in branch 'main':
    bpo-46470: remove unused branch from typing._remove_dups_flatten (GH-30780)
    c144d93

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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