sum stub too permissive #7574

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1. AlexWaygood

   added

   stubs: false negativeType checkers do not report an error, but shouldType checkers do not report an error, but should

   on Apr 1, 2022
2. Akuli commented on Apr 1, 2022

   

   Akuli

   on Apr 1, 2022

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   It's easy to put a TypeVar bound on the first overload, but sum() doesn't work for strings even if you define start="". It intentionally works with everything except strings.

   >>> sum("asd", start="")
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: sum() can't sum strings [use ''.join(seq) instead]
   

   Fixing this properly would require python/typing#1043.

   👍React with 👍2Reacted by Alex Waygood and Joe Young
3. AlexWaygood commented on Apr 1, 2022

   

   AlexWaygood

   on Apr 1, 2022

   Last edited by AlexWaygood

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   It's easy to put a TypeVar bound on the first overload

   Even that's actually a little tricky, because you get into the question of what is a number.

   For example, this works fine, even though there's not really anything that these classes all have in common either from a nominal or structural point of view:

   >>> import numpy as np
   >>> from fractions import Fraction
   >>> sum((3, np.float64(), Fraction(7, 22), complex(8, 0), 9.83))
   (21.14818181818182+0j)

   This does too:

   >>> from decimal import Decimal
   >>> sum((3, Decimal('0.98')))
   Decimal('3.98')

   But these all break:

   >>> from decimal import Decimal
   >>> sum((2.34, Decimal('3.87')))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: unsupported operand type(s) for +: 'float' and 'decimal.Decimal'
   >>> from fractions import Fraction
   >>> sum((Fraction(2, 3), Decimal('2.3')))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: unsupported operand type(s) for +: 'Fraction' and 'decimal.Decimal'
   >>> sum((complex(2, 0), Decimal('2.34')))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: unsupported operand type(s) for +: 'complex' and 'decimal.Decimal'
   >>> import numpy as np
   >>> sum((np.float64(), Decimal('2.34')))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: unsupported operand type(s) for +: 'float' and 'decimal.Decimal'
4. JelleZijlstra commented on Apr 1, 2022

   

   JelleZijlstra

   on Apr 1, 2022

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   We could at least bound the TypeVar to a Protocol defining __add__. That wouldn't help with Alex's mixed numeric types, but it would help type checkers catch plausible mistakes like sum([1, None]).

   👍React with 👍3Reacted by Akuli, Alex Waygood and Joe Young
5. AlexWaygood

   closed this as completedin #8000on Jun 13, 2022