Jelle Zijlstra <jelle.zijls...@gmail.com> added the comment:
Thanks for your report, but I would appreciate a more concise explanation. Let
me try to rephrase the problem.
Given this function:
def mean(x: list[float]) -> float:
return sum(x) / len(x)
We want to provide a guarantee that if x is a nonempty list containing only
floats, the function returns successfully and returns a float.
But the type system currently doesn't give this guarantee, because PEP 484
specifies that ints are compatible with floats, and `mean([0.0, 1.25, 10**1000,
0.0])` will throw OverflowError.
---
We generally discuss issues with the general type system over at
https://github.com/python/typing/issues, but here are a few thoughts:
- The type system doesn't generally try to cover exceptions. Your function
could also raise MemoryError, or KeyboardInterrupt, and the type system can't
tell you.
- The concrete proposal here would be to make int no longer implicitly
compatible with float. That might be nice, but at this point it would be a big
break in backwards compatibility, so I'm not sure we can do it.
----------
nosy: +AlexWaygood, JelleZijlstra, gvanrossum, kj
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<https://bugs.python.org/issue47234>
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