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

Asked By: George

||

Source

Answer 1

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.

Answered By: Alex Waygood

Answer 2

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 By: Shital Shah

Answer 3

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()

Answered By: Jason Harrison

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

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