Learn back-end development by writing real code
Boot.dev Blog ยป Stories ยป All the Kinds of Functions in Python

All the Kinds of Functions in Python

By Eteims on Oct 13, 2022

Functions are one of the most versatile tools in any Python programmer’s toolbox. They enable code reuse and provide a form of abstraction. Python offers many different types of functions. In this article, I will be discussing the different types of functions you will encounter as a Python developer.

Impure Functions

num = 0

def sq(x):
    global num
    num = x # side effect
    return x * x

sq_num = sq(2) # returns 4
print(num) # 2
print(sq_num) # 4

Impure functions are one of the most common type of function. they take an input either by value or reference and return a value. They are impure because they perform side effects.

In the example above, the function performs a side effect by altering the global variable num. This is a valid side effect, but you’ll often also encounter “side effects” like HTTP requests, printing to the console or accessing a database.

Impure functions are useful but are often error-prone and hard to test due to their side effects.

Pure functions

def sq(x):
    return x * x

sq_num = sq(2) # returns 4

Pure functions are functions with no side effects. They are similar to functions in mathematics. They take in an input and produce an output without altering any external states.

It makes them easy to test and predictable. Pure functions are widely used in functional programming

Get a back-end job without spending $10k on a bootcamp

  • Learn Python, Javascript and Go
  • Build the professional projects you need to land your first job
  • Spend about 6 months (when done part-time)
  • Pricing as low as $24/month*
  • No risk. Cancel anytime.

Subroutine

def sub(x):
    print(f"The square of {x} is {x * x}")

sub(2)
# The square of 2 is 4

A subroutine is a function that doesn’t return a value. It performs a task, and that task can be an effect. It could take in a value in other to perform it’s task. After execution, it gives control back to the caller.

Coroutine

Coroutines are functions capable of multitasking cooperatively. A function working cooperatively can pause its execution and hand control off to another function when it is idle or performing a blocking task.

Coroutines are a form of concurrency. They are preferred to other concurrency models like multithreading. Thanks to the async and await keywords introduced in PEP 492, they have become common and intuitive. Below is an example of coroutines in Python using the asyncio library.

import asyncio
import time

async def chill(label: str, n: int):
    print(f"{label}: Chilling for {n} seconds")
    await asyncio.sleep(n)
    print(f"Done chilling for {label}")


async def main():
    task1 = asyncio.create_task(chill("A", 2))
    task2 = asyncio.create_task(chill("B", 5))
    task3 = asyncio.create_task(chill("C", 3))

    starttime = time.perf_counter()
    await task1
    await task2
    await task3
    endtime = time.perf_counter()
    print(f"Task finished in {endtime - starttime}")

asyncio.run(main())

""" Sample output:
A: Chilling for 2 seconds
B: Chilling for 5 seconds
C: Chilling for 3 seconds
Done chilling for A
Done chilling for C
Done chilling for B
Task finished in 5.004022927998449
"""

The coroutine chill is called three times, and each instance of it blocks for a certain period. When a particular coroutine starts blocking it passes control to another coroutine. The coroutines in the example above block for a total of 10 seconds but they perform their task in 5 seconds in real-time because they worked cooperatively at the same time.

A simple path to your career in back-end development

The pace of Boot.dev's JavaScript, Python and Go courses has been perfect for me. The diverse community in Discord is a blast, and other members are quick to help out with detailed answers and explanations.

- Daniel Gerep from Cassia, Brasil

Generators

When regular functions are called they perform their task and return to their caller. If they are called again they start execution from the beginning and return to their caller again.

Generators are functions that can pause their execution after being called. If they are called again they resume from where they stopped previously.

def infCount():
    """
    Infinite counter
    """
    i = 0
    while True:
        yield i
        i += 1

# Usage
inf = infCount()
next(inf) # 0
next(inf) # 1
next(inf) # 2

The code above implements an infinite counter using generators in Python. The built-in next() function is used to call a generator. After each call, it pauses its execution and resumes on the next call.

Methods

An Object is a collection of related data and functions. Functions in an object are called methods. Rather than manipulating the data directly, methods are used. This is known as Encapsulation. The method definition is contained in the class of the object along with its related data. When an instance of the class has been created the method can be accessed via the syntax instance-name.method-name.

class Position:

    def __init__(self, x, y):
        self.x = x
        self.y = y

    def pos(self):
        return (self.x, self.y)

    def set_pos(self, x, y):
        self.x = x
        self.y = y
        print(f"New positiom {self.x} and {self.y}")

p = Position(2,3)
p.pos() # (2, 3)
p.set_pos(4, 5) # New positiom 4 and 5
p.pos() # (4, 5)

The class above defines 3 methods. The __init__() method is the constructor, pos() returns a tuple of the object’s position and set_pos() changes the object’s position. self.x and self.y are member variables mutated by the various methods.

Anonymous function

Anonymous functions are functions without an assigned name. They are used to perform one-off tasks. The code below is an example of and anonymous function in Python.

lambda x: x * x

Anonymous functions are also called lambda expressions. The functions can then be saved in variables.

sq = lambda x: x * x
sq(2) # returns 4

Learn back-end development the right way

Computer science skills will get you interviews. A portfolio of projects will get you hired.

Higher-Order Functions

Higher-order functions take in other functions as input or return other functions. Here’s a list of popular higher-order functions:

  • map()
  • filter()
  • reduce()

map()

The map() higher-order function takes in an array and another function. It then applies the function across the array.

data = map(lambda x: x*x, [1,2,3,4,5])
list(data) # [1, 4, 9, 16, 25]

The code above uses the map higher-order function in Python to square all the values in the array via an anonymous function.

The map function returns an iterator which needs to be converted to a list to get the new values.

filter()

The filter() higher-order function takes in an array and another function, which is called the predicate. It selects the entries in the array that are true based on the predicate function.

data = filter(lambda x: x % 2 == 0, [1,2,3,4,5,6,7,8,9,10])
list(data) # [2, 4, 6, 8, 10] 

The filter function also returns an iterator.

reduce()

The reduce() higher-order function reduces an array of values to a single value using another function called the reducer.

from functools import reduce

reduce(lambda a, b: a + b, ["H", "E", "L", "L", "O"])
# "HELLO"

The reduce higher-order function needs to be imported from functools.

Closures

def factory():
    num = 10
    def mult_by_10(inp):
        return num * inp
    return mult_by_10

clos = factory()
clos(2) # returns 20

A closure is a function capable of capturing variables from where it was created. They are functions with internal state. They are created by a higher-order function.

The example above has a higher-order function called factory. The function has a variable called num. Another function is defined within the factory function called mult_by_10.

The factory function returns the mult_by_10 function which is the closure. When the returned function is called, it still has access to the num value.

Recursive function

A recursive function is a function that can call itself. It has a base case, which serves as it’s termination point. Recursion is often an alternative to iteration.

def nthSum(n):
    if n == 0: # Base case
        return 0
    else: # else case
        return n + nthSum(n - 1)

nthSum(5) # returns 15

nthSum() is a recursive function that calculates the sum of n natural numbers.

Curried function

A curried function is a function whose inputs can be partially applied. Curried functions are a form of closures.

def add(x):
    def add2(y):
        return x + y
    return add2

add10 = add(10)
add10(2) # returns 12

Through this partial application, new functions can be created.

from functools import partial

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

add10 = partial(add, y=10)
add10(2) # returns 12

A non-curried function can be partially applied via the partial function from functools.

Decorators

Decorators are functions which add extra functionality to previously existing functions. They are similar to closures but have a special syntax.

def decor(func):
    def wrapp():
        print("############")
        func()
        print("############")
    return decor

@decor
def hello():
    print("Hello World!")

"""Sample Output

############
Hello world!
############

"""

The example above defines a decorator called decor which takes in a function func as a parameter. Another function called wrapp is defined within decor. The wrapp function serves as a wrapper to the func function.

The decor is then used to decorate the hello function. Decorators are just syntactic sugar.

decor(hello)

"""Sample Output

############
Hello world!
############

"""

Python offers a wide range of functions and techniques for creating functions. Knowing the different types of functions helps you write better code, and also understand other programmers' code.

Learn back-end without spending $10,000+ on a bootcamp

  • Write modern code in JavaScript, Python and Go
  • Build and deploy real backend projects to your personal portfolio
  • Compete in the job market by mastering computer science fundamentals

Related Reading