Digging deeper into python if statements

Question

It might be a really silly question, but I was not able to find the answer to it anywhere else, I've looked at SO, but there is nothing I could find related to my question.

Question:

In python, don't know about other languages, whenever we call if statements for a builtin class it returns something which the if statement interprets, For example,

a = 0
if a: print("Hello World")

The above statement does not print anything as the if a is False. Now which method returned that it is False, or is there a method the if statement calls in order to know it ??

Or more precisely, how does the if statement work in python in a deeper level ?

Answer 1

Objects have __bool__ methods that are called when an object needs to be treated as a boolean value. You can see that with a simple test:

class Test:
    def __bool__(self):
        print("Bool called")
        return False

t = Test()
if t:   # Prints "Bool Called"
    pass

bool(0) gives False, so 0 is considered to be a "falsey" value.

A class can also be considered to be truthy or falsey based on it's reported length as well:

class Test:
    def __len__(self):
        print("Len called")
        return 0

t = Test()
if t:
    pass