3.3 - Global and Local Variables
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Understanding Global Variables
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Today, we will discuss global variables. Can anyone tell me what a global variable is?
Isn't it a variable that can be accessed from anywhere in the program?
Exactly! Global variables are defined outside of functions and can be accessed from any point in the code. For example, if I say `x = 10` outside any function, I can use `print(x)` inside any function to get 10. Great memory aid to remember this: the letter G for Global reminds us that it's 'General' and can be accessed anywhere!
Got it! So, can we modify a global variable inside a function?
Good question! We will cover that shortly, but you have to use the `global` keyword to modify it inside a function.
What happens if you try to use a global variable that wasn't defined yet?
If you attempt to access a global variable before defining it, you will face a `NameError`. Remember that variables must be defined before use!
In summary, global variables are those defined in the general scope, and their accessibility is extensive throughout the program.
Understanding Local Variables
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Now, letβs talk about local variables. Who can tell me what they are?
Are they the ones you define inside functions only?
Exactly! Local variables are created when a function is called and are destroyed once the function exits. For example, `def func(): x = 5` will make `x` local to `func()`. You canβt access `x` outside that function.
So, what happens if I try to print `x` outside the function?
You will receive an error because `x` is not defined in the global scope. So always remember: Local is Localized!
To summarize, local variables only exist within the function scope and cannot be accessed elsewhere.
Using Global and Nonlocal Keywords
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Let's discuss the `global` and `nonlocal` keywords. Why do you think they're important?
Are they used to control access to variables across functions and scopes?
Perfect! The `global` keyword tells Python to use the global variable instead of creating a new one. For example, if I have `global x` in my function, it modifies the global `x`.
What about `nonlocal`?
Great that you asked! If you have nested functions, `nonlocal` allows you to modify a variable from the enclosing function without declaring it global. So, just remember: Global for globals, Nonlocal for neighbors!
Can you give an example of `nonlocal` in action?
Certainly! If you have a nested function, you can modify the variable of the outer function using `nonlocal`! That way, we respect the scope rules while allowing flexibility. To recap, `global` changes globals, and `nonlocal` deals with outer function scopes.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we explore the distinction between global and local variables in Python, how they are declared, and the implications of their scopes in programming. The use of global and nonlocal keywords to modify variables across different scopes is also discussed.
Detailed
Global and Local Variables
In programming, the concept of variable scope is crucial as it determines where and how variables can be accessed within the code. Specifically, local variables are defined within a function and are only accessible inside it, while global variables are defined outside of any function and can be accessed throughout the entire program.
Key Points:
- Global Variable: A variable defined outside any function, like
x = 10, which is accessible in any function as shown in the example whereprint(x)will output 10 when called from thedisplay()function. - Local Variable: A variable defined within a function, such as
x = 5infunc(), which cannot be accessed outside that function, leading to a potential error if trying to printxelsewhere. - Keywords
globalandnonlocal: Theglobalkeyword allows modification of global variables from within a function, whilenonlocalis used in nested functions to reference variables from an enclosing function.
Understanding these concepts is essential for managing variable visibility and preventing errors in Python programming.
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Global Variable Example
Chapter 1 of 3
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Chapter Content
x = 10
def display():
print(x)
display()
Detailed Explanation
In this example, a global variable 'x' is defined with a value of 10. It is defined outside of any function, which means it has global scope and can be accessed anywhere in the program. The function 'display' is defined, and inside this function, the value of 'x' is printed. When we call 'display()', it outputs the value of 'x' which is 10, demonstrating how global variables can be accessed within a function.
Examples & Analogies
Think of a global variable like a public notice board in a classroom. Everyone in the classroom (all the functions in the program) can see and interact with the notices (the global variable) posted on that board.
Local Variable Example
Chapter 2 of 3
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Chapter Content
def func():
x = 5
print(x)
func()
print(x) # This will give an error if x is not defined globally
Detailed Explanation
Here, 'func' is defined with a local variable 'x' set to 5. This variable is only accessible within the function 'func'. When we call 'func()', it prints the value of 'x' which is 5. However, if we try to print 'x' outside of the function after the function call, we will receive an error if 'x' is not defined globally, because 'x' is local to 'func' and cannot be accessed outside of it.
Examples & Analogies
Imagine a local variable as a secret held by a friend in a group. Only that friend knows it and can talk about it while the others (the rest of the program) cannot access it unless it's shared explicitly.
Global vs Local Variable Concept
Chapter 3 of 3
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Chapter Content
Local variables are defined inside functions and have a limited scope, whereas global variables are defined outside any function and are accessible throughout the program.
Detailed Explanation
The key difference between global and local variables lies in their scope. A global variable can be accessed from any part of the program, while a local variable is confined to the function in which it is defined. This distinction is crucial because it helps manage the namespace in larger programs where different parts might have variables of the same name.
Examples & Analogies
Think of the local variable like a clubhouse where only certain friends can enter (the function), whereas the global variable is like a neighborhood where everyone can roam freely. If everyone started using the same name for their clubhouses (variables) without the local scope, it would create confusion about whose is whose.
Key Concepts
-
Global Variables: Defined outside any function; accessible everywhere in the code.
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Local Variables: Defined inside functions; restricted to that function's scope.
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Global Keyword: Modifies a global variable within a function.
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Nonlocal Keyword: Accesses and modifies a variable in the outer (non-global) scope.
Examples & Applications
Example of Global Variable: x = 10 and def display(): print(x) outputs 10.
Example of Local Variable: def func(): x = 5; print(x) will give an error if print(x) is called outside func().
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Global variables are everywhere, Local variables are confined, keep the scopes clear, and you'll do just fine!
Stories
Imagine a town (global scope) where everyone can see the town square (global variable) but there are private rooms (local variables) in each house that no one else can enter.
Memory Tools
Remember G for Global and L for Local; G is for General access, L is for Limited access.
Acronyms
G-L stands for Global-Local, distinguishing where you can access variables.
Flash Cards
Glossary
- Global Variable
A variable that is defined outside of any function and accessible throughout the program.
- Local Variable
A variable defined within a function, only accessible within that function's scope.
- global keyword
A keyword used in a function to refer to a global variable.
- nonlocal keyword
A keyword used in nested functions to refer to variables in the enclosing scopes.
Reference links
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