1.3 · Sub-programs & Parameter Passing

Goal: define functions cleanly; understand value vs reference, local vs global.

Defining a function

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

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

Parameters and arguments

• Parameter = name in the function definition.
• Argument = value passed at call time.

Positional vs keyword

def order(food, drink, size):
    print(food, drink, size)

order("burger", "cola", "large")          # positional
order(drink="tea", food="rice", size="M") # keyword

Default values

def power(base, exp=2):
    return base ** exp

power(5)        # 25
power(5, 3)     # 125

Variable scope

Scope	Where
Local	Inside a function
Enclosing	Inside an outer function (closures)
Global	Top of module
Built-in	Python's pre-defined names

total = 0          # global

def add(x):
    total = x      # local — does NOT change global
    return total

add(10)
print(total)       # still 0

Use global keyword to modify (rarely needed):

def reset():
    global total
    total = 0

Pass by value vs reference

In Python, everything is "pass by object reference":

• Immutable types (int, str, tuple): function cannot modify original.
• Mutable types (list, dict, set, custom objects): function can modify in place.

def double_each(lst):
    for i in range(len(lst)):
        lst[i] *= 2

nums = [1, 2, 3]
double_each(nums)
print(nums)             # [2, 4, 6] — original changed

def increment(n):
    n += 1
    return n

x = 5
y = increment(x)
print(x, y)             # 5 6  — original unchanged

Return values

def divmod_(a, b):
    return a // b, a % b      # returns a tuple

q, r = divmod_(17, 5)
print(q, r)                   # 3 2

Good function design

• One responsibility.
• Short (≤ 30 lines).
• Meaningful name (verb-noun).
• Document with a docstring.

Common student mistakes

• Modifying a global from inside a function by accident.
• Returning print() (which returns None).
• Mutating a list parameter unintentionally.

Exam-style question

Q (5 marks): Write a function stats(numbers) that returns the average, minimum and maximum of a list. Demonstrate calling it.

Sample answer:

def stats(numbers):
    if not numbers:
        return None
    return sum(numbers) / len(numbers), min(numbers), max(numbers)

avg, mn, mx = stats([4, 7, 1, 9, 5])
print(f"avg={avg}, min={mn}, max={mx}")

Key takeaways

• Functions encapsulate logic.
• Default parameters, keyword args, multiple returns.
• Mutable arguments can be modified inside the function.

➡️ Next: 1.4 File Handling