_remove_dups_flatten in typing.py contains dead branch #90628

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1. sobolevn commented on Jan 22, 2022

   

   sobolevn

   on Jan 22, 2022

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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):

   See More

   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.
2. sobolevn

   added

   3.11only security fixesonly security fixes

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jan 22, 2022
3. Fidget-Spinner commented on Jan 24, 2022

   

   Fidget-Spinner

   on Jan 24, 2022

   Member

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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
4. Fidget-Spinner

   closed this as completedon Jan 24, 2022
5. Fidget-Spinner

   closed this as completedon Jan 24, 2022
6. ezio-melotti

   transferred this issue fromon Apr 10, 2022