Python Functions in 5 Minutes: Define, Call, and Reuse Code Efficiently

Core Goal

Master function definition, invocation, parameter passing, and code reuse to avoid duplication, all in a 5‑minute hands‑on session.

What is a Function?

A function packages repeated code under a name so it can be called multiple times, improving efficiency.

# Example: compute the sum of two numbers with a function
def add(a, b):
    return a + b

result1 = add(3, 5)   # 8
result2 = add(10, 20) # 30
print(result1)
print(result2)

Core Function Patterns (Templates to Memorize)

Three common patterns cover most beginner scenarios.

No parameters, no return value (simple action)

Parameters with a return value (most common)

Default parameters (optional arguments)

Comprehensive Practice with Lists and Dictionaries

# Process a list of student records, reuse functions for grading and printing
def get_grade(score):
    if score >= 90:
        return "Excellent"
    elif score >= 80:
        return "Good"
    elif score >= 60:
        return "Pass"
    else:
        return "Fail"

def print_all_students(students):
    for stu in students:
        name = stu["name"]
        score = stu["score"]
        grade = get_grade(score)
        print(f"{name}, Score: {score}, Grade: {grade}")

students = [
    {"name": "Zhang San", "score": 88},
    {"name": "Li Si", "score": 95},
    {"name": "Wang Wu", "score": 76}
]
print_all_students(students)

Four Common Beginner Pitfalls and Fixes

Missing colon after the function header – leads to SyntaxError. Remember the colon and indent four spaces.

Argument count mismatch – call the function with the correct number of arguments or provide default values.

Variables defined inside a function are not accessible outside – return them or declare them global (return is preferred).

Installation errors with certain PyPI mirrors – switch to a reliable mirror such as Aliyun or Douban.

Post‑Lesson Mini‑Tasks (5 Minutes)

# 1. Subtract two numbers
def subtract(a, b):
    return a - b
print(subtract(10, 4))

# 2. Check if a number is even
def is_even(num):
    if num % 2 == 0:
        return True
    else:
        return False
print(is_even(8))
print(is_even(7))

# 3. Use a list to test multiple numbers
nums = [1, 2, 3, 4, 5, 6]
for num in nums:
    print(num, "is even:", is_even(num))

Key Takeaways

Functions are tools for code reuse: define once, call many times.

Essential concepts to master today: def, calling, parameter passing, and return values.

When errors occur, first check for missing colon, argument count, variable scope, or problematic PyPI mirrors.