Functions

Functions let you put a chunk of code in one place and run it multiple times.

Why use functions?

  • Avoid repeating code
  • Break problems into smaller, testable pieces.
  • Give a name to a task so your code reads like instructions.

Defining and Calling a Function

Use def to define a function, give it a name, parentheses, and a colon. Put the actual code of the function below (make sure to indent it).

def greet():        # define the function
    print("Hello!")

greet()             # call the function

Defining a function does not run it. The code inside runs only when you call it.


Parameters and Return Values

Functions can take parameters (inputs) and return values (outputs).

def greet(name):
    print("Hello", name)

greet("Ada") # Hello Ada

Functions can take multiple parameters:

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

result = add(3, 2)
print(result)  # 5

If a function doesn't use return, it returns None by default.


Default Values

You can give parameters default values and call by name.

def power(x, exp=2):  # without a default this would be "def power(x, exp)""
    return x ** exp   # remember, ** is used to raise to a power

print(power(3))      # 9 (uses default exp=2)
print(power(2, exp=3))  # 8 (override default)

Scope (local vs global)

Variables created inside a function are local to that function (they only exist inside the function and cannot be used anywhere else).

def f():
    x = 10   # local

# x is not available here

Variables defined inside a function are local by default. You can use the global keyword to make a variable accessible anywhere (global name = Ada), but this shouldn't be used often.


Small examples

Example: a function that formats a name:

def format_name(first, last):
    return last + ", " + first

print(format_name("Ada", "Lovelace"))
# Output: Lovelace, Ada

Example: function used inside a loop:

def greet_person(name):
    print("Hello", name)

names = ["Ada", "Grace"]
for n in names:
    greet_person(n)

Challenge

Write a function is_even(n) that returns True when n is even and False otherwise. Then use it to print whether numbers 1..10 are even.

Hint

Use the modulus operator % with 2: n % 2 is 0 when n is even.

Show example solution 
def is_even(n):
    return n % 2 == 0

for i in range(1, 11):
    print(i, "even?", is_even(i))

Key Takeaways

  • If you copy-paste code more than once, it probably should be a function.
  • Use def name(parameters): to define a function and name() to call it.
  • Parameters are the values that a function receives (input); return sends a result back (output).
  • Prefer small functions that do one job; they are easier to test and reuse.
  • Understand local variables: variables defined inside a function are not visible outside it.