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

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.