Question

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

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

 46  25445  46
1 Jan 1970

Solution

 70

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



  1. Use assert:

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

  2. Raise some other exception:

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

  3. Use an if statement:

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

  4. 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()
    

  5. 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.

2021-07-21

Solution

 10

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.

2023-01-21