Python Language Testing if an object is None and assigning it


Example

You'll often want to assign something to an object if it is None, indicating it has not been assigned. We'll use aDate.

The simplest way to do this is to use the is None test.

if aDate is None:
    aDate=datetime.date.today()

(Note that it is more Pythonic to say is None instead of == None.)

But this can be optimized slightly by exploiting the notion that not None will evaluate to True in a boolean expression. The following code is equivalent:

if not aDate:
    aDate=datetime.date.today()

But there is a more Pythonic way. The following code is also equivalent:

aDate=aDate or datetime.date.today()

This does a Short Circuit evaluation. If aDate is initialized and is not None, then it gets assigned to itself with no net effect. If it is None, then the datetime.date.today() gets assigned to aDate.