[issue36555] PEP484 @overload vs. str/bytes

[Guido van Rossum]

Guido van Rossum  added the comment:

> Mypy already takes first overload for ambiguous arguments.

This is true, but it requires that the return types match (covariantly, IIUC) 
for the overlapping types. So you can have

@overload
def foo(x: int) -> int: ...
@overload
def foo(x: float) -> float: ...

But you can't have

@overload
def foo(x: int) -> str: ...
@overload
def foo(x: float) -> float: ...

(and the docs explain why -- search for "unsafe_func").

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[issue36555] PEP484 @overload vs. str/bytes

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Mypy already takes first overload for ambiguous arguments. This example is 
however genuinely unsafe from the static typing point of view. Please read the 
docs 
https://mypy.readthedocs.io/en/latest/more_types.html#type-checking-the-variants

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue36555] PEP484 @overload vs. str/bytes

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +gvanrossum, levkivskyi

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[issue36555] PEP484 @overload vs. str/bytes

[Crusader Ky]

New submission from Crusader Ky :

An exceedingly common pattern in many Python libraries is for a function to 
accept either a string or a list of strings, and change the function output 
accordingly.

This however does not play nice with @typing.overload, as a str variable is 
also an Iterable[str] that yields individual characters; a bytes variable is 
also an Iterable[bytes].

The example below confuses tools like mypy:

@overload
def f(x: str) -> int
   ...

@overload
def f(x: Iterable[str]) -> List[int]
   ...

def f(x):
if isinstance(x, str):
return len(x)
return [len(i) for i in x]


mypy output:

error: Overloaded function signatures 1 and 2 overlap with incompatible return 
types


The proposed solution is to modify PEP484 to specify that, in case of 
ambiguity, whatever overloaded typing is defined first wins. This would be 
coherent with the behaviour of @functools.singledispatch.

--
components: Library (Lib)
messages: 339610
nosy: Crusader Ky
priority: normal
severity: normal
status: open
title: PEP484 @overload vs. str/bytes
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7

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