Optional

Optional type

The docs say that this is the preferred way. See page.

from typing import Optional

Allow a variable to be None:

Optional[TYPE]

Declare a type for a variable. This was recommended by the docs.

foo: Optional[str] = None

foo = 'abc'

Use the Union type. This is not preferred but might be needed in some cases.

Union(float, None)

# Python 3.10
float | None

Here we return a string or None.

def foo() -> Optional[str]:
    pass

The second case shows a necessary annotation. Note you must declare the type on the first declaration of a variable and on that one only.

def bar(bazz: bool) -> Optional[str]:
    if bazz:
        buzz: Optional[str] = 'Yes'
    else:
        buzz = None
    
    return buzz

You may can an error from Pylint:

E1136: Value 'Optional' is unsubscriptable (unsubscriptable-object)

You can use use Union instead.

def bar(bazz: bool) -> Union[str, None]:
    if bazz:
        buzz: Union[str, None] = 'Yes'
    else:
        buzz = None
    return buzz

Or you have to use Optional like this.

def bar(bazz: bool) -> Union[str, None]:
    if bazz:
        buzz: Optional[str] = 'Yes'
    else:
        buzz = None
    return buzz

Or, you can use a return None sooner and so don’t define buzz at that point so buzz is never None but the function can return None.

For function parameters which have defaults, you can avoid using Optional or Union by ensuring the default is not set as None.

e.g.

def foo(fizz, buzz:Optional[float]=None):
    pass
    
def foo(fizz, buzz:float=0.0):
    pass