Level: Basic | Player: Week 8 (All grammar up to now)

Learning Goals

• Define and call the function with def
• Understand the difference between parameters and arguments
• Use default parameters and keyword arguments
• Returns the value as return
• Understand the scope of variables (Region/Area)

1. What is a Function

Instead of using the same action multiple times, name it and reuse it. This makes the code short and clear.

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

greet("Hong Gil-dong")
greet("Kim Young-hee")

2. Parameters and Arguments

• Parameter: A variable in a function definition (name)
• Argument: The value passed when calling a function ("Hong Gil-dong")

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

result = add(3, 5)   #3, 5: arguments

3. return

The function returns a value. Terminates immediately when it encounters return.

def square(x):
    return x * x

n = square(5)   # 25

If return is not available, None is returned automatically.

4. Default Parameters

If arguments are omitted when calling, the default values are used.

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

print(greet("Gildong"))                #Hello, Gildong!
print(greet("Alice", "Hello"))     #Hello, Alice!

5. Keyword Arguments

Deliver by name instead of order.

def make_user(name, age, city):
    return {"name": name, "age": age, "city": city}

#Positional argument
u1 = make_user("Hong Gil-dong", 20, "Seoul")

#Keyword argument (order does not matter, easy to read)
u2 = make_user(city="Busan", name="Kim Young-hee", age=25)

6. Returning Multiple Values (Tuple)

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

lo, hi = min_max([3, 1, 4, 1, 5])
print(lo, hi)   # 1 5

7. Variable Scope

A variable created inside a function is a local variable — it is not visible outside.

def f():
    x = 10        #Region
    print(x)

f()
# print(x)        # NameError

To modify a global variable inside a function, the global keyword:

total = 0
def add(n):
    global total
    total += n

(If possible, avoid global, and it is recommended that the function takes a value and returns it.)

Key Examples (examples/)

⚠️ Common Mistakes

1. Omitting () in function call

   def hi():
       print("hi")
   hi    #Function object. Does not execute
   hi()  #Execute
   

2. Using result without return

   def show(x):
       print(x)
   y = show(5)   # y = None
   

3. Mutable object as default value

   def add_to(item, lst=[]):    #⚠️ Trap!
       lst.append(item)
       return lst
   add_to(1)   # [1]
   add_to(2)   #[1, 2] — The same list is shared
   

→ Set default as None and create a new one inside the function:

def add_to(item, lst=None):
    if lst is None:
        lst = []
    ...

4. Name collision

   def list():        #Internal list hidden
       ...
   

→ Do not use built-in names like print, list, sum for variables or functions.

❓ FAQ

Q1. How many lines are appropriate for a function? A. Recommended 5–30 lines. If it's too long, split into smaller functions.

Q2. What is the difference between print and return? A. print prints to the screen, return returns a value. Use return when the function itself needs the value.

Q3. Can a function call another function inside it? A. Yes. It can also call itself (recursion) (covered in intermediate level).

📝 Exercises (exercises/)

• exercise1.md — collection of calculator functions
• exercise2.md — input validation functions (is_valid_age)
• exercise3.md — prime number detection function (is_prime)

Next Week

Week 10. Comprehensive Practice — mini project