`sum` stub too permissive

[jpy-git]

Currently the stub for the sum built-in allows any iterable as an argument

typeshed/stdlib/builtins.pyi

Lines 1480 to 1489 in ae6ff79

	@overload
	def sum(__iterable: Iterable[_T]) -> _T | Literal[0]: ...
	if sys.version_info >= (3, 8):
	@overload
	def sum(__iterable: Iterable[_T], start: _S) -> _T | _S: ...
	else:
	@overload
	def sum(__iterable: Iterable[_T], __start: _S) -> _T | _S: ...

This means mypy has no issue with:

sum("hello")

Which results in this error at runtime:

TypeError: unsupported operand type(s) for +: 'int' and 'str'

context: I was introducing someone to mypy and used the example above as something that mypy should flag, but turns out it wasn't being flagged so logging here.

[Akuli]

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.

[AlexWaygood]

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'

[JelleZijlstra]

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]).