How do I get Pylance to ignore the possibility of None?

Question

I love Pylance type checking.

However, If I have a variable var: Union[None, T], where T implements foo, pylance will throw an error at:

var.foo() since type None doesn't implement foo.

Is there any way to resolve this? A way to tell Pylance "This variable is None sometimes but in this case I'm 100% sure it will be assigned

1

does `var: Optional[T]` work? – dzshn Jul 20 '21 at 01:36

Alex Waygood · Accepted Answer · 2021-10-10T10:14:00.053

51

There are many ways of forcing a type-checker to accept this.

Use assert:

from typing import Union

def do_something(var: Union[T, None]):
    assert var is not None
    var.foo()

Raise some other exception:

from typing import Union

def do_something(var: Union[T, None]):
    if var is None:
        raise RuntimeError("NO")
    var.foo()

Use an if statement:

from typing import Union

def do_something(var: Union[T, None]):
    if var is not None:
        var.foo()

Use typing.cast, a function that does nothing at runtime but forces a type-checker to accept that a variable is of a certain type:
```
from typing import Union, cast

def do_something(var: Union[T, None]):
    var = cast(T, var)
    var.foo()
```

Switch off the type-checker for that line:

from typing import Union

def do_something(var: Union[T, None]):
    var.foo()  # type: ignore

Note also that, while it makes no difference to how your type annotation is interpreted by a type-checker (the two are semantically identical), you can also write typing.Union[T, None] as typing.Optional[T], which is arguably slightly nicer syntax. In Python >=3.10 (or earlier if you have from __future__ import annotations at the top of your code), you can even write Union types with the | operator, i.e. T | None.

edited Oct 10 '21 at 10:14

answered Jul 21 '21 at 11:37

Alex Waygood

6,304
3
24
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@George I rolled back your edit as `Optional[T]` is semantically identical to `Union[T, None]`. While I agree that it is nicer syntax, it *should* make no difference to how the annotation is interpreted by a type-checker. – Alex Waygood Oct 06 '21 at 14:31
1

Do you mind letting it in? I get that it's the same but I think the syntax is nicer, it's what I ended up using in my codebase. I guess the question is not so popular anyway, so up to you in the end. – George Oct 07 '21 at 13:16
@George does pylance not complain if you use `var.foo()` after annotating `var` as being of type `Optional[T]`? Mypy sure does, so it's not a solution that's universal for all type-checkers :/ E.g. https://mypy-play.net/?mypy=latest&python=3.10&flags=show-error-codes%2Cstrict&gist=66569c9e53af328165f9259591bedf1f – Alex Waygood Oct 07 '21 at 13:22
I've added a paragraph at the bottom regarding syntax options :) – Alex Waygood Oct 07 '21 at 13:22
1

@George I've just tested the linked snippet here on Pylance, and it fails on Pylance even with type-checking set to "basic" rather than "strict" (as it should). So I don't see how writing it as `Optional[T]` is a solution to the problem you posed in the question :) https://gist.github.com/AlexWaygood/f9ae35eb43418a2e674826db516dc654 – Alex Waygood Oct 08 '21 at 15:48
2

I was looking for an answer like this. It's neat to see different options on forcing the type checker. Kind of wish this was in the docs for [mypy errors about None/Optional types](https://mypy.readthedocs.io/en/stable/common_issues.html#unexpected-errors-about-none-and-or-optional-types) – Ehtesh Choudhury Mar 18 '22 at 09:05
1

Hmm. I can maybe submit a docs PR to mypy. No promises about when though. (@EhteshChoudhury feel free to submit your own PR to mypy! Some form of credit in the PR description would be nice, if you do want to do so :) I'm @AlexWaygood on github.) – Alex Waygood Mar 18 '22 at 11:18

score 6 · Answer 2 · answered Jan 21 '23 at 06:08

6

Please don't use blanket # type: ignore. Instead be specific on what linting error you want to ignore:

myOptionalVar.foo()         # pyright: ignore[reportOptionalMemberAccess]

Above works in VSCode default linter Pylance which uses Pyright.

answered Jan 21 '23 at 06:08

Shital Shah

63,284
17
238
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score -3 · Answer 3 · answered Mar 07 '23 at 23:39

from https://stackoverflow.com/a/71523301/54745

In a class, use a private member and a property:

class something:
    def __init__(self):
        self._var = None

    @property
    def var(self) -> T:
        assert self._var is not None
        return self._var

    def do_something(self)
        self.var.foo()

How do I get Pylance to ignore the possibility of None?

3 Answers3

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