08: Funcții

Organize your code into reusable blocks with Python functions.

python functions def parameters return scope

Lesson 08 · Functions

What you'll learn

  • How to define and call a function with def
  • Parameters and arguments
  • The return value with return
  • Scope: local vs global
  • Parameters with default values

Why functions?

Without functions, if you want to compute the area of a rectangle three times, you write the code three times. With functions, you write it once and call it as many times as you need:

def rectangle_area(length, width):
    return length * width

print(rectangle_area(4, 5))    # 20
print(rectangle_area(10, 3))   # 30
print(rectangle_area(7, 7))    # 49

Basic syntax

def greet():
    print("Good day!")

greet()   # function call → Good day!

Anatomy of a function

def calculate_average(grades):     # 'grades' is the parameter
    total = sum(grades)
    average = total / len(grades)
    return average                 # the returned value

result = calculate_average([8, 9, 7, 10])
print(result)   # 8.5
  • def — the keyword for defining a function
  • calculate_average — the function name
  • grades — the parameter (the input data)
  • return — sends back a value

Parameters and arguments

def introduce(name, age):
    print(f"My name is {name} and I am {age} years old.")

introduce("Anna", 14)        # Anna, 14
introduce("Michael", 16)     # Michael, 16

Keyword arguments

introduce(age=15, name="Helen")   # order doesn't matter

Parameters with default values

def greet(name, message="Good day"):
    print(f"{message}, {name}!")

greet("Anna")              # Good day, Anna!
greet("Michael", "Hi")     # Hi, Michael!

Functions without return

A function without return implicitly returns None:

def show_title(text):
    print("=" * len(text))
    print(text)
    print("=" * len(text))

show_title("Leaderboard")
# ===========
# Leaderboard
# ===========

Scope

Variables created inside a function are local — they don't exist outside it:

def calc():
    x = 10   # local variable
    print(x)

calc()
print(x)   # NameError — x doesn't exist here!

Variables defined outside any function are global:

name = "Global"   # global variable

def show():
    print(name)   # can be read

show()   # Global

Avoid modifying global variables from inside functions

Modifying global variables from inside functions makes the code hard to follow. Prefer passing values as parameters and returning them.

Multiple return values

Python can return multiple values at once (as a tuple):

def min_max(values):
    return min(values), max(values)

minimum, maximum = min_max([3, 7, 1, 9, 4])
print(minimum)   # 1
print(maximum)   # 9

Exercises

Exercise 1 — Simple function

Write a function square(n) that returns the square of n.

Solution 
def square(n):
    return n ** 2

print(square(5))    # 25
print(square(12))   # 144

Exercise 2 — Celsius to Fahrenheit

Write a function celsius_to_fahrenheit(c) that converts the temperature.

Solution 
def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

print(celsius_to_fahrenheit(0))    # 32.0
print(celsius_to_fahrenheit(100))  # 212.0

Exercise 3 — Validation

Write a function is_even(n) that returns True if n is even, False otherwise.

Solution 
def is_even(n):
    return n % 2 == 0

print(is_even(4))   # True
print(is_even(7))   # False

Mini-project: Password generator

Write a function that generates a random password using uppercase letters, lowercase letters, and digits.

import random
import string

def generate_password(length=8):
    characters = string.ascii_letters + string.digits
    password = ""
    for _ in range(length):
        password += random.choice(characters)
    return password

print(generate_password())      # e.g., "aB3xKp9Q"
print(generate_password(12))    # e.g., "kR7mNq2xPa5L"
What are string.ascii_letters and random.choice?
  • string.ascii_letters = all lowercase and uppercase letters: abcde...XYZ
  • string.digits = the digits: 0123456789
  • random.choice(s) = picks a random character from the string s

Summary

  • def name(param): defines a function
  • return value sends back the result
  • Parameters with default values: def f(x, y=0):
  • Local variables only exist inside the function
  • Functions can return multiple values: return a, b

Next step: → Lesson 09: Files

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