Question

I have a question about the following statement in Python

if not x % y: 
    # do something

After seeing this in a piece of code and experimenting I found that if modulo evaluates to anything but zero it’ll skip the “do something” code.

My question is, is there a general rule about If and If not statements with implied conditions and are there any good references for Python “tricks” like this?

I apologize about the broad question but this threw me for a loop when I first saw it. I would like to learn as many of these short hand tricks as I can!

Answer 1

• None is false.
• Numbers that are not zero are considered true; 0 is false
• Strings with any content are true; "" is false
• Containers with anything in them are true; [], (), and {} (and other empty containers) are false

This can be overridden on your own types by defining __len__() or __nonzero__() (the latter is named __bool__() in Python 3). You could even define, for example, a zero that evaluates as true:

class trueint(int):
    def __nonzero__(self):
        return True
    __bool__ = __nonzero__    # Python 3

truezero = trueint(0)

if truezero:
    print("yep, this zero is true!")

You probably shouldn’t do this, as it will confuse Python programmers, but you could.