Lesson 4 of 40 Foundations Beginner ⏱ 30 min

Functions, Arguments & Closures

Write reusable functions with positional, keyword, *args, **kwargs, default arguments, closures, and first-class function patterns.

Part 1: Defining Functions

# Basic function with type hints
def greet(name: str, greeting: str = "Hello") -> str:
"""Return a formatted greeting string."""
return f"{greeting}, {name}!"

greet("Alice") # "Hello, Alice!"
greet("Bob", "Hi") # "Hi, Bob!"
greet(name="Carol") # keyword arg

Part 2: *args and **kwargs

def total(*args: float) -> float:
    return sum(args)

total(1, 2, 3, 4) # 10

def create_user(name: str, **kwargs):
    print(f"User: {name}")
    for k, v in kwargs.items():
        print(f" {k}: {v}")

create_user("Alice", age=30, role="admin")

Part 3: Lambda & Closures

# Lambda — single-expression function
square = lambda x: x ** 2
sorted_names = sorted(names, key=lambda n: n.lower())

# Closure — inner function captures outer variable
def make_multiplier(factor: int):
    def multiply(x: int):
        return x * factor # captures factor
    return multiply

triple = make_multiplier(3)
triple(10) # 30

Part 4: Positional-Only & Keyword-Only Parameters

# / = positional-only * = keyword-only
def process(x, y, /, mode="fast", *, verbose=False):
    """x, y must be positional; verbose must be keyword."""
    if verbose:
        print(f"Processing ({x}, {y}) in {mode} mode")

process(1, 2, verbose=True) # ✅
process(x=1, y=2) # ❌ TypeError

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