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default argument for generic function #2078

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naphatkrit opened this issue Sep 1, 2016 · 1 comment
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default argument for generic function #2078

naphatkrit opened this issue Sep 1, 2016 · 1 comment

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@naphatkrit
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naphatkrit commented Sep 1, 2016 •
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I'm trying to do something like this:

T = TypeVar('T')

def identity(string):
  # type: (str) -> str
  return string

def f(normalize=identity):
  # type: (Callable[[str], T] -> str)

But mypy says this does not type-check. After adding a cast (normalize=cast(Any, identity), or even normalize=cast(Callable[[str], T], identity), this works fine.
@rwbarton
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rwbarton commented Oct 7, 2016

This is intentional, as the caller of f gets to choose T and identity does not have type Callable[[str], T] for most types T. Also see the discussion at #1721.

Have you simplified the type of your real function f? It's hard to see how you could use normalize with this signature for f anyways.

@ddfisher ddfisher added this to the Questions milestone Oct 13, 2016
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@rwbarton @ddfisher @gvanrossum @naphatkrit