Warmup

Write a function join_str that accepts two strings and returns the concatenation of them.

Agenda

• Lesson 4.2
• Photo Day

7:15 - 7:35 Last names A - B
7:35 - 7:55 Last names C - D
7:55 - 8:15 Last names E - F
8:15 - 8:45 Last names G - H
9:00 - 9:20 Last names  K - L 
9:20 - 9:40 Last names M - N
9:45 - 10:05 Last names O - P
10:30 - 10:50 Last Names Q-R
10:50 - 11:10 Last Names  S - T
11:10 - 11:30  Last Names V- Z

Reminders

• Python Essentials Module 4 due this week

  • Module 4 Quiz due Thursday
  • Module 4 Test due Friday

• No Unit 4 exam

  • Midterm (40 questions) will instead be heavily weighted on Unit 4

Lesson 4.2: Function Parameters

Review: Functions

• Functions are reusable blocks of code
• They can take parameters
• They can have return values using return
  • or they can return None

Review: Functions

• When functions are called, they are provided with arguments
  • We can pass positional arguments or keyword arguments

def my_func(a, b):
	return a + b

# keyword arguments
result = my_func(b=2, a=10)
print(result)

# positional arguments
result2 = my_func(10, 2)
print(result2)

Review: Functions

• Writing functions without side effects makes them easier to test
• Side effect = changing something outside the function (e.g., printing, modifying globals, in-place mutations)
• In-place mutation = modifying the variable

Why Parameters?

• Parameters allow us to send information into a function
• Example:

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

• Call: greet("Alex") → "Hello, Alex!"

Positional Parameters

• Parameters filled in order

def power(base, exponent):
    return base ** exponent

print(power(2, 3))   # 8
print(power(3, 2))   # 9

• Order matters!

Keyword Parameters

• Use the parameter name when calling

print(power(exponent=3, base=2))  # 8

• Order doesn’t matter when using keywords

Default Parameters

• Provide fallback values

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

print(greet("Maya"))              # Hello, Maya!
print(greet("Maya", "Welcome"))   # Welcome, Maya!

Variable-Length Parameters

• Collect multiple values
• *args → collects extra positional arguments
• **kwargs → collects extra keyword arguments

def add_all(*nums):
    return sum(nums)

print(add_all(1,2,3,4))   # 10

Variable-Length Keyword Example

def describe_pet(**info):
    return f"{info['name']} is a {info['type']}."

print(describe_pet(name="Buddy", type="dog"))

Side Effects vs. Return Values

• Side effect: changes something outside the function

  • e.g., print, modifying global variables, writing files

• Safer to return values instead

• Example:

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

Quick Check

• What is the difference between positional and keyword arguments?
• Why are default parameters useful?
• Why avoid side effects when possible?

Practice Challenge

Write a function average(*nums) that:

• Accepts any number of numbers
• Returns their average

Next

• Do Assignment 4.2