Functions

Level 5 of Python School: 3 lessons, explained from scratch, with examples.

Play Level 5 in the game

Lesson 1

Your own functions

Wrap up code with def, give it inputs, and get answers back with return.

def makes a function

A function is a block of code with a name, so you can run it whenever you need it.

def (short for define), the name, brackets and a colon, then the indented lines inside. Nothing happens when Python reads the def: the code runs when you call the function, by writing its name with brackets.

Python
def greet():
    print("Hello!")
    print("Welcome to Python School.")

greet()
greet()

Parameters and return

Parameters are the function’s inputs, named inside the brackets. When you call it, you give a value for each one.

return sends a value back to wherever the function was called, and ends the function.

Python
def add(a, b):
    return a + b

result = add(3, 4)
print(result)
print(add(10, 5) * 2)

print is not return

print() shows something to the person using the program. return hands a value back to the program, so it can store it or use it.

A function that never says return gives back None, Python’s word for “nothing”.

Python
def show(n):
    print(n * 2)

def give(n):
    return n * 2

x = show(5)
y = give(5)
print(x)
print(y)

Practise “Your own functions” in the game

Lesson 2

Smarter functions

Default values, giving back two things, and where variables live.

Default values

A parameter can have a default value, used when the call leaves it out: greeting="Hello".

You can also give values by name, in any order: greet(greeting="Hey", name="Mia").

Python
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Priya"))
print(greet("Lucas", "Hi"))
print(greet(greeting="Hey", name="Mia"))

Giving back two things

return low, high gives back two values at once. low, high = min_max(...) puts them in two variables.

And print() can show several things: put commas between them, and it adds a space between each.

Python
def min_max(nums):
    return min(nums), max(nums)

low, high = min_max([4, 9, 1, 7])
print(low, high)

Variables inside stay inside

A variable made inside a function is local: it only exists while the function runs, and the code outside can’t see it.

A variable with the same name outside is a different variable, so the function doesn’t change it.

Python
def make_total():
    total = 100
    return total

total = 5
make_total()
print(total)

Practise “Smarter functions” in the game

Lesson 3

String power tools

Change case, split sentences into words, join them back, and more.

Methods: tools built into strings

Strings come with methods, functions you call with a dot after the string:

  • .upper() and .lower() change the case
  • .strip() removes spaces from both ends
  • .replace("cats", "birds") swaps one piece of text for another

They don’t change the original string: they give you a new one.

Python
word = "  Hello Python  "
print(word.strip())
print(word.upper())
print(word.strip().lower())
print("cats and dogs".replace("cats", "birds"))

Splitting and joining

  • .split() breaks a string into a list of words. Give it something to split on, like .split(","), to split there instead
  • "-".join(words) joins a list back into one string, with - between
  • strings have positions like lists: word[0] is the first letter, and word[:3] the first three
Python
sentence = "the quick brown fox"
words = sentence.split()
print(words)
print(len(words))
print("-".join(words))
print(sentence[:3])

Practise “String power tools” in the game

Recap

What Level 5 covers, in a few lines.

  • def name(parameters): makes a function: a named block of code you can run again and again by calling it, like name(3).
  • return sends a value back to the code that called the function. A function without return gives back None.
  • print() shows something to people; return hands a value to the rest of your program.
  • Parameters can have default values, like def greet(name, greeting="Hello"):. A function can return two values: return low, high.
  • Strings have methods: .upper(), .lower(), .strip(), .replace(), .split() and "-".join(words). They give back new strings.

Now try it in Python School

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