What is Truthy and Falsy? How is it different from True and False?

Question

I just learned there are truthy and falsy values in python which are different from the normal True and False.

Answer 1

Python determines the truthiness by applying bool() to the type, which returns True or False which is used in an expression like if or while.

Here is an example for a custom class Vector2dand it's instance returning False when the magnitude (lenght of a vector) is 0, otherwise True.

import math
class Vector2d(object):
    def __init__(self, x, y):
        self.x = float(x)
        self.y = float(y)

    def __abs__(self):
        return math.hypot(self.x, self.y)

    def __bool__(self):
        return bool(abs(self))

a = Vector2d(0,0)
print(bool(a))        #False
b = Vector2d(10,0)    
print(bool(b))        #True

Note: If we wouldn't have defined __bool__ it would always return True, as instances of a user-defined class are considered truthy by default.

Example from the book: "Fluent in Python, clear, concise and effective programming"