Warmup

• Write a function that accepts a first and last and returns the full name as one string. Add an optional parameter "last_first" with a default value of False
• If caller passes True for this value, the returned string should have the format <Last>, <First>

Agenda

• Lesson 4.3 (Variable Scope)

Reminders:

• PE1 Module 4 (Quiz due Thurs, Test due Fri)
• Unit quiz on Friday via Canvas Quizzes

Lesson 4.3: Scope

Review

• Functions let us organize code into reusable blocks
• Parameters bring data into a function
• Functions return data back out

What is Scope?

• Scope = where a variable can be accessed in a program
• Two main kinds in Python:
  • Local scope: inside a function
  • Global scope: outside all functions

Local Scope

• Variables created inside a function exist only inside it

def example():
    x = 10  # local variable
    return x

print(example())
# print(x)  # ERROR: x not defined here

Global Scope

• Variables defined outside functions are global
• Can be accessed inside functions (but usually not changed)

y = 5

def show():
    return y

print(show())  # 5

Shadowing

• A local variable with the same name as a global one will shadow it

z = 100

def shadow():
    z = 50  # local z
    return z

print(shadow())  # 50
print(z)         # 100

The global Keyword

• Forces a function to change a global variable
• Not recommended (can make code hard to test/debug)

count = 0

def bad():
    global count
    count += 1

bad()
print(count)  # 1

Why Globals Are Risky

• Harder to test functions
• Changes can affect other parts of the program
• Better: pass values in and return results

Quick Check

• What is the difference between local and global scope?
• What does it mean when a variable is shadowed?
• Why should we avoid changing globals inside functions?

Practice Challenge

Write a function counter(n) that takes a number and returns it increased by 1.

• Don’t use global variables!