A Python function is a named block of code you can run whenever you need it. Instead of writing the same five lines in three places, you write them once, give them a name, and call that name. Functions are the point where beginner programs become organised programs. This guide explains Python functions for beginners, step by step.

Why functions exist

  • Reuse: write the code once, use it many times.
  • Readability: calculate_tax(price) says what is happening; twelve lines of arithmetic do not.
  • Fixing: a bug in a function is fixed in one place, everywhere at once.
  • Thinking: big problems become a set of small, named steps.

You have been using functions from day one: print(), input(), len(). Now you write your own.

Defining and calling a function

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

greet()   # runs the two lines
greet()   # and again
  • def starts the definition, followed by the name, brackets and a colon.
  • The indented lines are the function's body.
  • Defining a function does not run it. Calling it — greet() — does.

Parameters: giving a function information

Most functions need something to work on. Name it in the brackets:

def greet(name):
    print(f"Hello, {name}!")

greet("Ana")   # Hello, Ana!
greet("Leo")   # Hello, Leo!

Here name is a parameter — a placeholder in the definition. "Ana" is an argument — the actual value passed in when you call it. Functions can take several:

def describe(name, age):
    print(f"{name} is {age} years old.")

describe("Sam", 34)

return versus print: the big one

This confuses almost every beginner, so it is worth slowing down.

  • print() shows a value on the screen. The rest of your program cannot use it.
  • return hands a value back to the code that called the function, so it can be stored and used.
def add_print(a, b):
    print(a + b)

def add_return(a, b):
    return a + b

x = add_print(2, 3)    # shows 5, but x is None
y = add_return(2, 3)   # shows nothing, but y is 5
print(y * 10)          # 50

A rule that serves beginners well: functions that calculate something should return it, and printing should happen in the code that calls them. That makes the function useful in more places — you can print the result, save it, or use it in another calculation.

When Python reaches return, the function stops immediately. A function with no return gives back None.

Default values

Give a parameter a default, and callers can leave it out:

def price_with_tax(price, rate=0.2):
    return price * (1 + rate)

print(price_with_tax(100))        # 120.0
print(price_with_tax(100, 0.05))  # 105.0

Parameters with defaults go after those without.

Scope: where variables live

Variables created inside a function exist only inside it:

def make_total():
    total = 10

make_total()
print(total)   # NameError: name 'total' is not defined

This is a feature, not a limitation: each function has its own private workspace, so functions cannot accidentally overwrite each other's variables. To get a value out, return it:

def make_total():
    total = 10
    return total

result = make_total()
print(result)   # 10

A worked example

Here is a small program that uses functions to keep each job separate:

def average(numbers):
    return sum(numbers) / len(numbers)

def grade(score):
    if score >= 90:
        return "A"
    elif score >= 75:
        return "B"
    elif score >= 60:
        return "C"
    return "Needs work"

scores = [92, 78, 64]
avg = average(scores)
print(f"Average: {avg:.1f} — grade {grade(avg)}")
# Average: 78.0 — grade B

Each function does one thing and has a name that says what. The main code at the bottom reads almost like a sentence. Notice how the functions work with a list; the same style works just as well with dictionaries.

Common mistakes

  • Forgetting the brackets: greet refers to the function; greet() runs it.
  • Printing instead of returning, then wondering why the result is None.
  • Calling a function before defining it — Python reads top to bottom, so definitions come first.
  • Functions that do too much. If you cannot name it in a few words, split it.
  • Missing colon or indentation after def — see Python errors beginners make.

Practise

  1. Write square(n) that returns n * n.
  2. Write is_even(n) that returns True or False.
  3. Write initials(first, last) that returns something like "A.L.".
  4. Rebuild a calculator with one function per operation — project nine in our Python projects for complete beginners.

Functions have a full part of Python for Beginners to themselves: defining and calling, arguments and parameters, return values and scope, lambda functions, and error handling with try/except, finishing with test questions. Graded exercises with reference solutions are in the free Practice Toolkit on the book's page.

Frequently asked questions

What is the difference between a parameter and an argument?

A parameter is the name in the function's definition. An argument is the value you pass in when calling it.

Can a function return more than one value?

Yes: return low, high returns both as a tuple, which you can unpack with a, b = my_function().

When should I write a function?

When you have written the same code twice, or when a block of code does one clear job you can name.