Functions

Learn about functions in Python so that you can package up code into re-usable blocks, avoid repetition and make your code easier to read.

Look at each question and see if you can answer it, then check with the answer we’ve given. Do try these out in the The Python REPL.

Remember: understanding is the most imporant thing, don’t move on until you really understand the question and answer.


What does this expression produce?

2+3
  • It produces the result of 2+3 which is 5

This is a function, what does it do?

def add(a, b):
  return a+b
  • It adds a and b together and returns the result.

How do we call it?

  • We write an expression add(2,5) which would produce 5.

So there’s a difference between defining a function and calling a function?

  • Yes, we use def to define a function then we call it by putting brackets after its name, like add(2,4).

Define a function that returns the number 5.

  • OK:

    def five():
      return 5
    

This function takes no arguments (that’s an odd word!) arguments become variables inside the function.

What arguments does this function take?

def move(angle, direction):
  # to do
  • Two arguments: angle and direction, these become variables inside the function.

How are the arguments set?

  • The caller provides them when they call the function, like in these three examples:

    move(45, 10)
    move(-45, 2)
    move(180, 100)
    

Let’s write a little example program, a number guessing game. The computer generates a random number between 1 and 10 and the user has to guess it.

What does this code do?

import random
number = random.randint(1,10)
print(number)
  • It generates a random number between 1 and 10 using the function random.randint and prints it out. We should keep the number secret until the player guesses correctly!

Here’s an improved version, what does it do?

import random
number = random.randint(1,10)
guess = input("Enter your guess: ")
if guess == number:
  print("Correct!")
else:
  print("Wrong. It was", number)
  • Now it ask the player to input a guess using the input function, which takes a prompt to display to the player
  • Then it checks to see if they have guessed correctly.

What type is the variable number and what type is the variable guess?

  • Ah, number is an int and guess is a string so if we compare them they will never match because 2 does not equal ‘2’.

What does this function do?

def int_input(prompt):
  response = input(prompt)
  return int(response)
  • It asks the user for some input, displaying prompt, converts it to an int and returns it.

Here’s the function in our improved version, does it work now?

import random
number = random.randint(1,5)

def int_input(prompt):
  response = input(prompt)
  return int(response)

guess = int_input("Enter your guess: ")
if guess == number:
    print("Correct!")
else:
    print("Wrong. It was", number)
  • Yes, but you often have to play it several times to get a correct guess!

Can we use a while loop to keep the player guessing?

  • Yes

But when would we stop?

  • When they got the correct answer.