Why does an empty list evaluates to False on a while loop in Python

Question

What is process that occurs for a while-loop to evaluate to False on an empty list ?

For instance:

a=[1, 2, 3]
while a:
    a.pop()

Essentially, I want to know which method or attribute of the list object the while-loop is inspecting in order to decide wether to terminate or not.

Answer 1

Loops and conditionals implicitly use bool on all their conditions. The procedure is documented explicitly in the "Truth Value Testing" section of the docs. For a sequence like a list, this usually ends up being a check of the __len__ method.

bool works like this: first it tries the __bool__ method. If __bool__ is not implemented, it checks if __len__ is nonzero, and if that isn't possible, just returns True.

As with all magic method lookup, Python will only look at the class, never the instance (see Special method lookup). If your question is about how to change the behavior, you will need to subclass. Assigning a single replacement method to an instance dictionary won't work at all.

Answer 2

Great question! It's inspecting bool(a), which (usually) calls type(a).__bool__(a).

Python implements certain things using "magic methods". Basically, if you've got a data type defined like so:

class MyExampleDataType:
    def __init__(self, val):
        self.val = val

    def __bool__(self):
        return self.val > 20

Then this code will do what it looks like it'll do:

a = MyExampleDataType(5)
b = MyExampleDataType(30)

if a:
    print("Won't print; 5 < 20")
if b:
    print("Will print; 30 > 20")

For more information, see the Python Documentation: 3.3 Special Method Names.