functions_as_objects.py

# Functions as objects — in Python, functions are first-class objects.
#
# This means a function can be:
# - Assigned to a variable
# - Stored in a list or dictionary
# - Used as an attribute of an object
# - Passed as an argument to another function
# - Returned as the result of a function call


def greet(name):
    return f'Hello, {name}!'


# --- Assign a function to a variable ---

say_hello = greet
print(say_hello('Alice'))


# --- Function as an attribute of an object ---

class Greeter:
    def __init__(self, func):
        self.greet_func = func

my_greeter = Greeter(greet)
print(my_greeter.greet_func('Bob'))


# --- Functions in a list ---

def add1(x):
    return x + 1

def subtract1(x):
    return x - 1

func_list = [greet, add1, subtract1]
for func in func_list:
    if func == greet:
        print(func('Charlie'))
    else:
        print(func(10))


# --- Functions as dictionary keys ---

func_dict = {
    greet:     'Greeter function',
    add1:      'Add one function',
    subtract1: 'Subtract one function',
}
for func, description in func_dict.items():
    if func == greet:
        print(f'{description}: {func("Dave")}')
    else:
        print(f'{description}: {func(10)}')


# --- Passing a function as an argument ---

def execute(func, value):
    return func(value)

print(execute(greet, 'Eve'))


# --- Returning a function ---

def get_greeting_function():
    return greet

greet_func = get_greeting_function()
print(greet_func('Frank'))


# --- Higher-order functions ---
# Functions that take or return other functions.

def add(x, y):
    return x + y

def subtract(x, y):
    return x - y

def apply(f, x, y):
    return f(x, y)

print(apply(add, 10, 1))         # 11
print(apply(subtract, 10, 1))    # 9


# --- Function composition ---
# compose(f, g) returns a new function that applies g then f.

def compose(f, g):
    return lambda x: f(g(x))

add2 = compose(add1, add1)
print(add2(10))            # 12

do_nothing = compose(add1, subtract1)
print(do_nothing(10))      # 10