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
```
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.