How does Python decide whether a value is truthy or falsy?

Short answer

An object is falsy if it defines __bool__ returning False or __len__ returning zero; the built-in falsy values are None, False, 0, 0.0, the empty string and empty collections. Everything else is truthy.

Explanation

This lets you write if items: instead of if len(items) > 0. Be careful, though, when 0 or an empty string is a valid value: if not quantity treats a quantity of zero as if it were missing. In that case test for None explicitly. When defining a class, define __bool__ only when truthiness has a clear meaning.

Code example

def apply_discount(price, discount=None):
    if discount is None:  # And not: if not discount
        return price
    return price - discount  # discount=0 Valid value

class Cart:
    def __init__(self):
        self.items = []
    def __len__(self):
        return len(self.items)  # Empty basket falsy

Common mistakes

• Using if not value to check for None, which also rejects valid values such as 0 and "".
• Comparing lists with == [] instead of relying on truthiness.

Follow-up questions

• What is the difference between is and == in Python?

Sources

• Python documentation — Truth value testing