20 February 2022

Functions Refactor

Exercise 37 (and Solution)

One area of confusion for new coders is the concept of functions (which have been addressed on this blog in exercise 11 for example). So in this exercise, we will be stretching our functions muscle by refactoring an existing code snippet into using functions.

Here is the code snippet to refactor (taken from a correct but very repeated solution to exercise 24 on this website):

print(" --- --- ---")
print("|   |   |   |")
print(" --- --- ---")
print("|   |   |   |")
print(" --- --- ---")
print("|   |   |   |")
print(" --- --- ---")

Hint: Think about a way to refactor this using functions where generating an 8x8 or a 19x19 grid is a single change to a function call!

Discussion

Oftentimes as we write a program, we find ourselves wanting to reuse the same piece of code over and over again. This is a great use case for a function, and in particular, re-writing a piece of code (known as refactoring) to use a function. We often refactor code to use a function or functions to not repeat similar code segments, and make the code less error-prone.

As an example of how to do this, let’s take the following code snippet for playing the Rock, Paper, Scissors game from exercise 8 and refactor.

The following snippet is a complete piece of code for playing the game Rock Paper Scissors.

while True:
	player1 = ""
	while player1 not in ["rock", "paper", "scissors"]:
		player1 = input("player1 make your move (rock, paper, scissors): ")
	player2 = ""
	while player2 not in ["rock", "paper", "scissors"]:
		player2 = input("player2 make your move (rock, paper, scissors): ")
	winner_message = ""
	if player1 == player2:
		winner_message = "The outcome is a tie"
	elif player1 == "rock" and player2 == "scissors":
		winner_message = "player1 is the winner"
	elif player1 == "paper" and player2 == "rock":
		winner_message = "player1 is the winner"
	elif player1 == "scissors" and player2 == "paper":
		winner_message = "player1 is the winner"
	elif player2 == "rock" and player1 == "scissors":
		winner_message = "player2 is the winner"
	elif player2 == "paper" and player1 == "rock":
		winner_message = "player2 is the winner"
	elif player2 == "scissors" and player1 == "paper":
		winner_message = "player2 is the winner"
	print(winner_message)
	again = ""
	while again not in ["y", "n"]:
		again = input("Do you want to play again? (y/n): ")
	if again == "n":
		break

When looking at this piece of code, we see two things that stand out as “repeated blocks”:

Asking for player 1’s input and asking for player 2’s input is an identical piece of code
The if/else checking for who is the winner has many doubled checks

So let’s use these two as inspiration for refactoring. First, getting input from either player - let’s put that code into a separate function:

def get_move():
    move = ""
    while move not in ["rock", "paper", "scissors"]:
    	move = input("player2 make your move (rock, paper, scissors): ")
    return move

Now we can use this function in our new gameplay for Rock Paper Scissors:

def get_move():
    move = ""
    while move not in ["rock", "paper", "scissors"]:
    	move = input("player2 make your move (rock, paper, scissors): ")
    return move

while True:
	player1 = get_move()
	player2 = get_move()
	winner_message = ""
	if player1 == player2:
		winner_message = "The outcome is a tie"
	elif player1 == "rock" and player2 == "scissors":
		winner_message = "player1 is the winner"
	elif player1 == "paper" and player2 == "rock":
		winner_message = "player1 is the winner"
	elif player1 == "scissors" and player2 == "paper":
		winner_message = "player1 is the winner"
	elif player2 == "rock" and player1 == "scissors":
		winner_message = "player2 is the winner"
	elif player2 == "paper" and player1 == "rock":
		winner_message = "player2 is the winner"
	elif player2 == "scissors" and player1 == "paper":
		winner_message = "player2 is the winner"
	print(winner_message)
	again = ""
	while again not in ["y", "n"]:
		again = input("Do you want to play again? (y/n): ")
	if again == "n":
		break

When looking at the big while True loop we can see two calls of the function get_move(), which makes it clear what we are doing for each player’s move.

Now let’s tackle the winner-checking as a function. This is a bit more subtle for how to refactor. One thing we notice from the existing code is that we have two sets of conditions: one to check whether player 1 is the winner, and one to check whether player 2 is the winner. However, this code is duplicated. Instead, what we will do is write a function that will return “is the first player the winner” and call that function twice.

def is_first_the_winner(first, second):
	if first == "rock" and second == "scissors" or \
			first == "paper" and second == "rock" or \
			first == "scissors" and second == "paper": 
		return True
	return False

Now we refactor our main code with this helper function:

def get_move():
    move = ""
    while move not in ["rock", "paper", "scissors"]:
    	move = input("player2 make your move (rock, paper, scissors): ")
    return move

def is_first_the_winner(first, second):
	if first == "rock" and second == "scissors" or \
			first == "paper" and second == "rock" or \
			first == "scissors" and second == "paper": 
		return True
	return False

while True:
	player1 = get_move()
	player2 = get_move()
	winner_message = ""
	if player1 == player2:
		winner_message = "The outcome is a tie"
	elif is_first_the_winner(player1, player2):
		winner_message = "player1 is the winner"
	elif is_first_the_winner(player2, player1):
		winner_message = "player2 is the winner"
	print(winner_message)
	again = ""
	while again not in ["y", "n"]:
		again = input("Do you want to play again? (y/n): ")
	if again == "n":
		break