I'm surprised to see the even split on the last case, since my intuition is it should behave the same as the Union case where everyone narrows `x` to `int`. Can anyone from mypy/pyright shed some light on the reasoning there? I assume it has to do with Any being a special case, but I'd like to hear others' thoughts on it.
On Wed, Sep 2, 2020 at 11:26 AM Guido van Rossum firstname.lastname@example.org wrote:
Consider these two similar functions:
from typing import Union def f() -> int: x: Union[int, str] = 42 return x # Error here def g() -> int: x: Union[int, str] x = 42 return x # But not here
Quite a while ago (2016-2017) there was a long discussion about this in the mypy tracker: https://github.com/python/mypy/issues/2008
Not only is it surprising that these two functions are type-checked differently, there are differences across type checkers. I tried mypy, pytype, pyre and pyright. All of them consider the second function correct. But all except pyright produce a type error on the first function, something along the lines of
t.py:5: error: Incompatible return value type (got "Union[int, str]", expected "int")
(It's possible that there are configuration settings to change the behavior; I didn't look into this.)
I think it would be in everybody's interest if all type checkers agreed on this. I also think it's surprising to users that the two functions are treated differently. (But as the old mypy issue shows, not everybody agrees they should be treated the same either. :-)
There's another twist (note the return type is str here):
from typing import Any def h() -> str: x: Any = 42 return x
This is an error in pyre but not in mypy, pytype or pyright. And for good measure the other variant:
def i() -> str: x: Any x = 42 return x
This is accepted by mypy and pyright, but an error to pyre and pytype.
To summarize, it seems the type of x will be as follows:
x: Union[int, str] = 42 # mypy: Union; pytype: Union; pyre: Union; pyright: int # majority doesn't narrow x: Union[int, str] x = 42 # mypy: int; pytype: int; pyre: int; pyright: int # everybody narrows x: Any = 42 # mypy: Any; pytype: Any; pyre: int; pyright: Any # majority doesn't narrow x: Any x = 42 # mypy: Any; pytype: int; pyre: int; pyright: Any # even split
Looking at this, I'd say if we follow the majority, we would have to make the following changes:
- pyright should stop narrowing for `x: Union[..] = value`
- pyre should stop narrowing for `x: Any = value`
- mypy and pyright should start narrowing for `x: Any; x = value`
(The latter recommendation is debatable, since it's an even split, but I like it better if Union and Any are treated similarly in this case.)
Let the debate begin.
-- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/ _______________________________________________ Typing-sig mailing list -- email@example.com To unsubscribe send an email to firstname.lastname@example.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: email@example.com