Understand local and global variable scope

What is a Variable? 📌

A variable is like a box that holds a value. The name of the box is the variable name. For example, you can write x = 5 or y = 10 and later use x or y in your calculations.

In math notation, we often write $x = 5$ or $y = 10$ to show the value stored in the variable.

Local Variables 🗂️

Local variables are created inside a function and can only be used inside that function.

Analogy: Think of a local variable as a sticky note you put on a specific page of your notebook. Only you can see it when you look at that page.

def calculate():
    result = 10  # local variable
    return result
  

Here, result exists only inside calculate(). Outside, it is invisible.

The function f(x) = x^2 can be written as:

$$f(x) = x^2$$

Global Variables 🌐

Global variables are defined outside any function and can be accessed anywhere in the program.

Analogy: A global variable is like a whiteboard in the classroom that everyone can see and write on.

total = 0  # global variable

def add(value):
    global total
    total += value
  

Here, total can be read or changed by any function that declares it as global.

Scope Rules in a Nutshell 🥜

• Variables defined inside a function are local to that function.
• Variables defined outside all functions are global.
• Inside a function, if you want to modify a global variable, you must declare it with global.
• Local variables hide global variables of the same name within the function.

Exam Tip 📚

When answering questions about variable scope:

1. Identify where the variable is defined.
2. Determine if it is inside a function (local) or outside (global).
3. Remember that a local variable with the same name as a global one will shadow the global variable within its function.

Use the word shadow to show you understand the concept.