“First class” means a function can be treated like any other object or variable.

An example:

def square(x):
    return x * x 

f = square(5)

# print(square)
print(f)

# OR

f = square # assigns variable equal to function

^ Now f is effectively the same as the function.

print(f(5)) # 25

Pass function as argument #

map() is an example - takes function and array as arguments, returns a new array.

Writing a map functiion from scratch:

def my_map(func, arg_list):
    result = []
    for i in arg_list:
        result.append(func(i))
    return result

Colloquially, this takes a function and a list of arguments, runs the function on each argument, and appends each individual result to a results list.

squares = my_map(square, [1, 2, 3, 4, 5])

Note no parethesis - that would execute the function, which is not what we want when running a map.

With map, easy to swap out functions.

def cube(x):
    return x * x * x

cubes = my_map(cube, [1, 2, 3, 4, 5])

Return function as results #

Return a function from another function.

This one can get tricky.

An example:

def logger(msg):

    def log_message():
        print('Log:', msg)

    return log_message

log_hi = logger('Hi!')
log_hi()