Python Scope | Types, LEGB Rule, Use Cases & More (+Code Examples)

0 min read

In Python, scope refers to the accessibility of variables in different parts of the code. Understanding scope is crucial because it determines where variables can be accessed or modified and plays a key role in code organization and debugging. In this article, we’ll explore the various types of scope in Python, including local, global, enclosed, and built-in scopes. By the end, you'll have a clear understanding of how Python handles variable visibility and the best practices for managing scope in your programs.

What Is Scope In Python?

In Python programming, scope refers to the region of your code where a particular variable is accessible. It’s like setting boundaries for where each variable “lives” and can be used. These boundaries help avoid confusion in your program by making sure that variables are only accessed where they’re intended to be.

Types Of Python Scope:

Python has four main types of scope-

• Local Scope – A variable declared inside a function only lives within that function. It can’t be accessed outside.
• Global Scope – A variable declared outside any function is available throughout the entire script.
• Enclosing Scope – This comes into play with nested functions (functions within functions). A nested function can access variables from the function that contains it.
• Built-in Scope – Python has its own set of built-in functions and variables, like print() and len(), accessible from anywhere in your code.

Real-World Example:

Imagine you’re working on a group project in college, and each team member has specific tasks:

• Local Scope: Let’s say you’re responsible for designing the presentation slides. You have your own notes and design choices that no one else needs to access—this is like a local variable inside a function. Only you need this information so it stays within your “scope.”
• Global Scope: Now, your group leader sets a deadline for the project. This deadline applies to everyone, no matter what they’re working on. It’s like a global variable in your program—set once and visible to everyone.
• Enclosing Scope: You decide to help a friend in your team by suggesting some ideas for their part of the project. Your suggestions are relevant only within the context of their section and won’t affect the rest of the project, similar to variables in an enclosing scope that are accessible only within nested functions.
• Built-in Scope: Lastly, you’re using shared campus resources like the library and printers. These resources are available to all students, much like Python’s built-in functions and constants.

Understanding scope is like knowing who has access to which information in your project—keeping everything organized, controlled, and clear for everyone involved.

Explore this amazing course and master all the key concepts of Python programming effortlessly!

Local Scope In Python

The local scope in Python is created whenever you define a variable inside a function. Variables declared within this scope can only be accessed and used within that specific function. Once the function ends, the local variables are no longer accessible—they are essentially “forgotten” by the program. 

• This scope keeps functions isolated, which prevents variables within one function from interfering with those in another. 
• Local scope is important because it allows you to use the same variable names in different parts of a program without causing conflicts. 
• However, if you try to access a local variable outside of its function, Python will raise a NameError because it doesn’t recognize the variable in that context.

Code Example:

ZGVmIGdyZWV0KCk6CiAgICBtZXNzYWdlID0gIkhlbGxvLCBXb3JsZCEiICAjIExvY2FsIHZhcmlhYmxlCiAgICBwcmludChtZXNzYWdlKQoKZ3JlZXQoKQoKcHJpbnQobWVzc2FnZSkgICMgVHJ5aW5nIHRvIGFjY2VzcyBsb2NhbCB2YXJpYWJsZSBvdXRzaWRlIGl0cyBzY29wZQo=

Output: 

Hello, World!
NameError: name 'message' is not defined

Explanation: 

In the above code example: 

• We define a function called greet() that creates a local variable named message.
• Inside greet(), we assign the string "Hello, World!" to message.
• The print() statement within greet() prints this message to the console when we call greet().
• Outside the function, we attempt to print message again.
• Since message is a local variable, it's only accessible within greet().
• Attempting to print message outside greet() causes an error because the variable is not defined in the global scope.
• This code illustrates that local variables are confined to the function where they are declared, ensuring they don’t interfere with variables outside that function.

Global Scope In Python

The global scope applies to variables defined outside of any function or class. These variables are accessible from any part of the code, including within functions, unless a function creates a local variable with the same name. 

• Global variables remain in memory throughout the program’s execution and are widely accessible, which can be helpful when sharing data across multiple functions. 
• However, because global variables are accessible everywhere, they can lead to unintentional changes if modified by multiple functions, which may complicate debugging. 
• Therefore, while global scope offers convenience, it’s best used sparingly to keep the program structure clean and minimize errors from unexpected variable changes.

Code Example: 

bWVzc2FnZSA9ICJIZWxsbywgV29ybGQhIiAgIyBHbG9iYWwgdmFyaWFibGUKCmRlZiBncmVldCgpOgogICAgcHJpbnQobWVzc2FnZSkKCmdyZWV0KCkgICMgQWNjZXNzaW5nIHRoZSBnbG9iYWwgdmFyaWFibGUgd2l0aGluIGEgZnVuY3Rpb24KCnByaW50KG1lc3NhZ2UpICAjIEFjY2Vzc2luZyB0aGUgZ2xvYmFsIHZhcmlhYmxlIG91dHNpZGUgYW55IGZ1bmN0aW9uCg==

Output: 

Hello, World!
Hello, World!

Explanation: 

In the above code example:

1. We assign the string "Hello, World!" to a variable named message, making it a global variable.
2. Since message is global, it’s accessible from anywhere in the code, including inside functions.
3. Inside the greet() function, we use print(message) to display the value of message.
4. When we call greet(), the function prints "Hello, World!" by accessing the global message variable.
5. Outside the function, we also print message directly, and it again outputs "Hello, World!".
6. This example shows that global variables are accessible both inside and outside functions, making them available throughout the program.

Nonlocal (Enclosing) Scope In Python 

Nonlocal or the enclosing scope comes into play with nested functions, where one function is defined inside another. The outer function creates an “enclosing” scope for the inner function, allowing the inner function to access variables from the outer function. This is particularly useful when you want to share data between nested functions without making it globally accessible. 

• If you need to modify the outer variable from within the inner function, Python provides the nonlocal keyword to indicate that the variable is in an enclosing scope, not local. 
• Without the nonlocal keyword, reassigning the variable would create a new local variable within the inner function instead of modifying the enclosing one. 
• This scope structure allows for more controlled access to variables, promoting modularity and encapsulation in complex functions.

Code Example: 

ZGVmIG91dGVyKCk6CiAgICBtZXNzYWdlID0gIkhlbGxvIGZyb20gb3V0ZXIgZnVuY3Rpb24hIiAgIyBFbmNsb3NpbmcgdmFyaWFibGUKICAgIAogICAgZGVmIGlubmVyKCk6CiAgICAgICAgbm9ubG9jYWwgbWVzc2FnZQogICAgICAgIG1lc3NhZ2UgPSAiTW9kaWZpZWQgYnkgaW5uZXIgZnVuY3Rpb24iCiAgICAgICAgcHJpbnQoIklubmVyOiIsIG1lc3NhZ2UpCiAgICAKICAgIGlubmVyKCkKICAgIHByaW50KCJPdXRlcjoiLCBtZXNzYWdlKQoKb3V0ZXIoKQo=

Output: 

Inner: Modified by inner function
Outer: Modified by inner function

Explanation: 

In the above code example-

1. We define an outer() function, which contains a variable message set to "Hello from outer function!".
2. Inside outer(), we define another function called inner(), making inner() a nested or inner function.
3. Within inner(), we use nonlocal message to refer to the message variable from the enclosing outer() function.
4. The nonlocal keyword allows inner() to modify the message variable declared in outer(), rather than creating a new local variable.
5. We then assign "Modified by inner function" to message within inner() and print it, so calling inner() will display "Inner: Modified by inner function".
6. After inner() runs, we return to outer() and print message, which now reflects the change made by inner().
7. When we call outer(), it outputs the modified message in both inner() and outer(), illustrating how nonlocal enables a nested function to access and modify variables from its enclosing function.

Built-In Scope In Python

Python’s built-in scope is the highest level of scope and includes Python’s pre-defined functions, variables, and constants, such as print(), len(), and range(). These built-in items are accessible throughout any Python program, without needing to define or import them. They exist in every Python environment as part of the standard library, making them universally available for all programs.

• The built-in scope is particularly useful for commonly used operations, like printing output or calculating the length of a list.
• Because they are universally available, you can rely on these functions and constants regardless of your code’s structure or any specific function or class definitions. 
• However, be cautious about naming your own variables or functions with the same names as built-in ones, as this could lead to unexpected behavior by overwriting the original functionality.

Code Example: 

cHJpbnQoIlVzaW5nIGJ1aWx0LWluIHByaW50IGZ1bmN0aW9uIikKCmxlbmd0aCA9IGxlbihbMSwgMiwgMywgNF0pICAjIFVzaW5nIGJ1aWx0LWluIGxlbiBmdW5jdGlvbgoKcHJpbnQoIkxlbmd0aCBvZiB0aGUgbGlzdDoiLCBsZW5ndGgpCg==

Output: 

Using built-in print function
Length of the list: 4

Explanation: 

In the above code example-

1. We start by using the built-in print() function to display "Using built-in print function" on the console.
2. Next, we use the built-in len() function to find the length of a list containing the elements [1, 2, 3, 4].
3. The len() function returns the count of elements in the list, which is 4 in this case.
4. We store this result in a variable called length.
5. Then, we use print() again to display "Length of the list:" followed by the value of length, which outputs 4.
6. This code demonstrates how we can leverage Python’s built-in functions, like print() for displaying output and len() for calculating the length of a collection.

Sharpen your coding skills with Unstop's 100-Day Coding Sprint and compete now for a top spot on the leaderboard!

The LEGB Rule For Python Scope 

In Python, the LEGB Rule—an acronym for Local, Enclosing, Global, Built-in—defines the order in which Python searches for variables when they are referenced in your code. Understanding this rule is crucial as it helps you predict where Python will search for a variable and how it handles variable access and modification.

Here’s how the LEGB Rule works:

Local Scope (L):

Python first checks the local scope, which refers to the current function or method in which the variable is called. If a variable is defined within the function, Python will use that version of the variable, ignoring any versions in higher-level scopes. This makes the local scope the most immediate and specific scope.

Enclosing Scope (E):

If Python doesn’t find the variable in the local scope, it checks the enclosing scope, which applies to nested functions. If a variable is defined in an outer function (the enclosing function), but not in the inner one, Python will use the variable from the outer function. Modifying a variable in the enclosing scope requires the nonlocal keyword.

Global Scope (G):

If the variable isn’t found in the local or enclosing scope, Python moves to the global scope. This is where Python looks for variables defined outside of any function, typically at the top level of the script. Global variables are accessible throughout the entire program. However, modifying a global variable inside a function requires the global keyword.

Built-in Scope (B):

Finally, if Python hasn’t found the variable in any of the previous scopes, it checks the built-in scope. This scope contains Python’s predefined functions and constants, such as print(), len(), int(), and others. These functions and constants are available globally across all Python programs. Python will only search here if the variable hasn’t been found in any other scope.

Let’s look at an example to illustrate the LEGB rule in Python-

Code Example: 

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

Output: 

Hello from the local scope!
<built-in function len>

Explanation: 

In the above code example-

1. We start by defining a variable message in the global scope, assigning it the value "Hello from the global scope!". This message is accessible throughout the entire script, unless shadowed by other message variables within local or nested scopes.
2. Next, we define a function outer() where we declare another message variable, this time with the value "Hello from the enclosing scope!". This message is in the enclosing scope of any functions defined within outer().
3. Inside outer(), we define a nested function called inner(). In inner(), we declare yet another message variable, setting it to "Hello from the local scope!". This variable is local to inner() and will override message from the enclosing scope when accessed within inner().
4. We print message inside inner(), which outputs "Hello from the local scope!" since the innermost message variable takes precedence due to Python's scope resolution order (LEGB: Local, Enclosing, Global, Built-in).
5. When we call outer(), it executes inner(), printing "Hello from the local scope!" as expected.
6. Finally, we print len() outside the function. The len() is a built-in function in Python, demonstrating the built-in scope level in Python’s LEGB resolution model.

The LEGB Rule thus enables Python to prioritize variable access in an organized way, allowing us to manage and structure our code with predictable variable behavior.

Python Scope And Variable Lifetime

The lifetime of a variable refers to how long the variable exists in memory. In Python, the lifetime of a variable depends on its scope.

• Local Scope: Variables are created and exist only during the function's execution. They are accessible only within the function or block where they are defined.
• Global Scope: Variables are created outside any function, typically at the top of the script. They exist throughout the program's execution and are accessible from any part of the program.
• Enclosing Scope: Variables defined in an outer function can be accessed by an inner (nested) function. The variable’s lifetime is tied to the outer function’s execution, i.e. variables live as long as the outer function is running.
• Built-in Scope: It contains Python’s pre-defined functions, exceptions, and objects. Available globally throughout the program and doesn’t need to be defined or imported.

Garbage Collection And Memory Management In Python

Python manages memory through an automatic process called garbage collection. The primary goal is to reclaim memory by removing objects that are no longer referenced by any variables.

• Reference Counting: Every object in Python has an associated reference count. When a variable points to an object, the reference count increases. If no variables are referencing the object, the reference count becomes zero, and Python will automatically remove the object.
• Garbage Collector: Python's garbage collector is responsible for cleaning up memory used by objects involved in circular references (where objects reference each other but aren't used anymore). It runs periodically to clear such objects.

Searching for someone to answer your programming-related queries? Find the perfect mentor here.

Best Practices For Managing Python Scope

To avoid common pitfalls related to Python scope, here are some best practices that one must follow:

• Limit Use of Global Variables: Use global variables sparingly. If a variable only needs to be accessed within a function, keep it local to avoid unintended side effects.
• Use global and nonlocal Keywords Correctly: If you need to modify a global variable inside a function, use the global keyword to avoid confusion. Similarly, use nonlocal to modify enclosing scope variables inside nested functions.
• Maintain Clear Function Interfaces: Pass values into functions as arguments instead of relying on global variables. This makes your code more modular and avoids unexpected interactions between functions.
• Avoid Modifying Built-in Functions: Don’t use names for your variables that are the same as built-in functions (e.g., len, print). This can lead to confusion and bugs, as you may accidentally override the functionality of the built-in functions.
• Understand Function Scopes: Be mindful of local and enclosing scopes, especially when dealing with nested functions. Be sure to differentiate between local variables and those in the enclosing scope.

Conclusion

Understanding Python scope is crucial for writing efficient and bug-free code. By grasping the concepts of local, global, enclosing, and built-in scopes, along with the LEGB rule, we can predict how Python will access and resolve variables. Managing scope properly ensures that variables are only accessible where needed, helping avoid unwanted side effects and making code easier to maintain.

In addition to avoiding common pitfalls like unintended global variable modifications or scope-related errors, following best practices such as minimizing the use of global variables and understanding variable lifetime will contribute to writing more efficient and readable code. With a solid understanding of Python's scoping rules, developers can design more organized, maintainable, and error-free programs.

Frequently Asked Questions

Q. What is scope in Python?

Scope in Python refers to the region of the program where a particular variable is accessible. It defines the visibility and lifetime of variables, and it helps determine where you can access or modify a variable in your code. Python uses a hierarchy of scopes, including local, enclosing, global, and built-in scopes, which are checked in a specific order to resolve variable references.

Q. What are the different types of scope in Python?

Python has four main types of scope:

• Local Scope: Variables defined within a function or block of code, accessible only within that function.
• Enclosing Scope: Variables in an outer function that are accessible to inner (nested) functions.
• Global Scope: Variables defined at the top level of a script or module, accessible throughout the program.
• Built-in Scope: Predefined functions and objects in Python, such as print(), len(), and int(), available globally across all Python programs.

Q. Can I modify a global variable inside a function?

Yes, you can access and modify global variables inside a function, but to modify them directly, you must use the global keyword. Without it, Python will create a new local variable instead of modifying the global one. For Example-

x = 10  # Global variable
def modify_global():
    global x
    x = 20  # Modify global variable
modify_global()
print(x)  # Output: 20

Q. What is the difference between local and global scope?

Here’s a table that highlights the differences between local and global scope in Python:

Q. What is the LEGB rule?

The LEGB rule (Local, Enclosing, Global, Built-in) defines the order in which Python looks for variables when they are referenced. Python checks:

• Local scope (current function),
• Enclosing scope (outer functions for nested functions),
• Global scope (top-level script),
• Built-in scope (Python's pre-defined objects and functions). This rule ensures Python resolves variable references in a structured manner.

Q. What is an enclosing scope?

An enclosing scope refers to the scope of an outer function that is accessible to its inner (nested) functions. If a variable is defined in an outer function but not in the inner function, the inner function can access the outer function's variable. The enclosing scope is not as immediate as the local scope, but it is checked before the global scope. For Example-

def outer():
    message = "Hello from the outer function!"
    def inner():
        print(message)  # Accessing variable from enclosing scope
    inner()
outer()  # Output: Hello from the outer function!

Q. How does Python handle variable lifetime in different scopes?

Local variables are created when a function is called and destroyed when the function exits. Global variables exist throughout the program’s execution, from the start of the program until it ends. Enclosing variables exist as long as the outer function is running, and they are accessible to any nested functions. Built-in variables are available as long as the Python interpreter is running.

With this, we conclude our discussion on the scope of variables in Python. Here are a few other topics that you might be interested in reading: 

1. 12 Ways To Compare Strings In Python Explained (With Examples)
2. How To Reverse A String In Python? 10 Easy Ways With Examples!
3. 10 Important Features Of Python That You Must Know!
4. Flask vs Django: Understand Which Python Framework You Should Choose
5. 20+ Best Python Projects Ideas