Chapter 3: Effective Functions

3.1 Python's functions are first class

Functions Can Capture Local State

def get_speak_func(text, volume):
    def whisper():
        return text.lower() + '...'
    def yell():
        return text.upper() + '!'
    if volume > 0.5:
        return yell
    else:
        return whisper

"""
>>> get_speak_func('Hello, World', 0.7)()
'HELLO, WORLD!'
"""

Inner functions whisper and yell can access the text parameter defined in the parent function. Functions that do this are called lexical closures (or just closures, for short). A closure remembers the values from its enclosing lexical scope even when the program flow is no longer in that scope.

Objects can behave like functions

Objects can be made callable using the __call__ method.

class Adder:
    def __init__(self, n):
        self.n = n
    def __call__(self, x):
        return self.n + x

"""
>>> plus_3 = Adder(3)
>>> plus_3(4)
7
"""

Instance, Class, and Static Methods Demystified

Instance methods can modify instance state, as well as class state (via self.__class__ attribute).

Class methods can only modify class state.

Static methods can modify neither.

Class methods as factory methods

Class methods can be used as factory methods as:

class Pizza:
    def __init__(self, ingredients):
        self.ingredients = ingredients
    def __repr__(self):
        return f'Pizza({self.ingredients!r})'
    @classmethod
    def margherita(cls):
        return cls(['mozzarella', 'tomatoes'])
    @classmethod
        def prosciutto(cls):
    return cls(['mozzarella', 'tomatoes', 'ham'])

>>> Pizza.margherita()
Pizza(['mozzarella', 'tomatoes'])
>>> Pizza.prosciutto()
Pizza(['mozzarella', 'tomatoes', 'ham'])