Unit 11.3A · Term 3

Scope & Built-in Functions

Scope determines where a variable can be accessed. Understanding scope prevents subtle bugs. Python also provides powerful built-in functions that handle common tasks without importing anything.

Learning Objectives

  • 11.3.2.2 Understand variable scope and use built-in functions

Lesson Presentation

11.3A-lesson-18-functions-scope.pdf · Slides for classroom use

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Conceptual Anchor

The Room and Building Analogy

Local scope is like items on your desk — only accessible in your room (function). Global scope is like the building's lobby — accessible from any room. You can see the lobby from your room but need explicit permission (global) to change things there.

Rules & Theory

Variable Scope

# Global variable — accessible everywhere message = "Hello" def greet(): # Local variable — only inside this function name = "Ali" print(message) # ✓ Can READ global variable print(name) # ✓ Can use local variable greet() print(message) # ✓ Global is accessible # print(name) # ✗ NameError — local to greet() # Scope Rules: # 1. Local scope: variables defined INSIDE a function # 2. Global scope: variables defined OUTSIDE all functions # 3. A function CAN READ global variables # 4. A function CANNOT MODIFY globals without the 'global' keyword

The global Keyword

count = 0 def increment(): global count # Declare we're using the global variable count += 1 # Now we can modify it increment() increment() print(count) # 2 # WITHOUT global keyword: x = 10 def broken(): x = 20 # Creates a NEW local variable named x! print("Inside:", x) broken() # Inside: 20 print("Outside:", x) # Outside: 10 — global x unchanged!

Essential Built-in Functions

Function Description Example Result
len(x) Length of sequence len("hello") 5
abs(x) Absolute value abs(-5) 5
round(x, n) Round to n decimal places round(3.14159, 2) 3.14
type(x) Type of object type(42) <class 'int'>
isinstance(x, t) Check type isinstance(42, int) True
int(x) Convert to integer int("42") 42
float(x) Convert to float float("3.14") 3.14
str(x) Convert to string str(42) "42"
bool(x) Convert to boolean bool(0) False
input(p) Read user input input("Name: ") User's text
print(*args) Output to screen print(1, 2, 3) 1 2 3
range(a, b, s) Number sequence list(range(0,10,2)) [0,2,4,6,8]
sum(iterable) Sum of elements sum([1,2,3]) 6
min(iterable) Smallest element min(3,1,2) 1
max(iterable) Largest element max(3,1,2) 3
sorted(iterable) New sorted list sorted([3,1,2]) [1,2,3]

Best Practice: Avoid global

Using global is generally discouraged because it makes code harder to debug. Prefer passing values as parameters and using return values instead.

Worked Examples

1 Scope Tracing

x = 10 # Global def func_a(): x = 20 # Local to func_a print("A:", x) def func_b(): print("B:", x) # Reads global x func_a() # A: 20 func_b() # B: 10 print("G:", x) # G: 10 — global unchanged

2 Using Built-in Functions Together

data = [15, -7, 23, -12, 8, 0, 42, -3] print("Length:", len(data)) # 8 print("Sum:", sum(data)) # 66 print("Min:", min(data)) # -12 print("Max:", max(data)) # 42 print("Average:", round(sum(data)/len(data), 2)) # 8.25 # Filter positives using built-ins positives = [x for x in data if x > 0] abs_values = [abs(x) for x in data] print("Positives:", positives) # [15, 23, 8, 42] print("Absolute:", abs_values) # [15, 7, 23, 12, 8, 0, 42, 3]

3 The Right Way — No global Needed

# BAD: using global score = 0 def add_points_bad(): global score score += 10 # GOOD: parameter + return def add_points(current_score, points): return current_score + points my_score = 0 my_score = add_points(my_score, 10) # 10 my_score = add_points(my_score, 5) # 15 print(my_score)

Pitfalls & Common Errors

UnboundLocalError

If you try to read and modify a global variable without global, Python assumes it's local and crashes: x = x + 1 inside a function where x is global causes UnboundLocalError.

Shadowing

Creating a local variable with the same name as a global "shadows" the global — you can no longer access the global version inside that function. Avoid using the same name.

Pro-Tips for Exams

Scope Rules Summary

  • Function parameters are local variables
  • Variables assigned inside a function are local
  • A function can read global variables without global
  • A function needs global x to modify global x
  • Best practice: pass values as parameters, return results

Graded Tasks

Remember

What is the difference between local and global scope?

Understand

Why does modifying a global variable inside a function without global cause an error?

Apply

Trace this code and predict the output: x=5; def f(): x=10; print(x); f(); print(x)

Apply

Write a program using abs(), round(), min(), max(), sum(), and len() to generate statistics for a list of numbers.

Analyze

Why is using global considered bad practice? Give a better alternative using parameters and return values.

Create

Write a quiz program where a function generates questions, another checks answers, and another tracks the score — without using global variables.

Self-Check Quiz

1. What happens if you define x = 5 inside a function?
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2. What does abs(-7) return?
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3. What does round(3.14159, 2) return?
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4. Can a function read a global variable without global?
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5. What does isinstance("hello", str) return?
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