Adventures in Machine Learning

Demystifying Global Variables in Python Functions

Global Variables in Python Functions: Understanding Scope and Mutability

Python is a high-level programming language that’s known for being simple, elegant, and versatile. One of its most useful features is the ability to work with global variables in functions, which can help make code more efficient and easier to read.

In this article, we’ll explore the concept of global variables in Python functions, covering everything from scope to mutability.

Global Scope and Accessing Variables

A global variable is a variable that’s defined outside of a function, and can be accessed from anywhere in the code. Python has four scopes: local, enclosing, global, and built-in.

  • Local variables are defined within a function and can only be accessed from within that function.
  • Enclosing variables are defined in the enclosing function of a nested function and can be accessed from both the enclosing function and the nested function.
  • Global variables are defined outside of any function, and can be accessed from anywhere in the code, including within a function.
  • Built-in variables are predefined by Python and can be accessed from anywhere in the code as well.

To access a global variable within a function, you simply refer to it by name. For example:

x = 10
def my_function():
    print(x)

my_function()   # Output: 10

In this example, we define a global variable x with a value of 10, and then define a function my_function that prints the value of x. We then call the function, which prints the value of x (10).

Modifying Global Variables and the global keyword

Accessing global variables within a function is straightforward, but what about modifying them? By default, Python assumes that any variable being assigned a value within a function is a local variable, which means that if you try to assign a value to a global variable within a function, Python will create a new local variable with the same name instead.

To modify a global variable within a function, you need to use the global keyword. The global keyword tells Python that you want to use the global variable, rather than creating a new local variable with the same name.

For example:

x = 10
def my_function():
    global x
    x = 15

my_function()
print(x)    # Output: 15

In this example, we define a global variable x with a value of 10. We then define a function my_function that uses the global keyword to modify the value of x to 15.

We call the function, which modifies the value of x, and then print the value of x, which is now 15.

Using the globals() Function

Another way to access and modify global variables within a function is to use the globals() function. The globals() function returns a dictionary containing all global variables defined in the module.

For example, consider the following code snippet:

x = 10
def my_function():
    globals()['x'] = 15

my_function()
print(x)    # Output: 15

In this example, we define a global variable x with a value of 10. We then define a function my_function that modifies the value of x using the globals() function.

We call the function, which modifies the value of x, and then print the value of x, which is now 15.

Understanding How Mutability Affects Global Variables

So far, we’ve been dealing with immutable global variables – that is, variables whose values cannot be changed. But what about mutable global variables, such as lists or dictionaries?

Mutability Overview

Mutable data types are those whose values can be changed after they’ve been created. These include lists, dictionaries, and sets.

Immutable data types, on the other hand, are those whose values cannot be changed after they’ve been created. These include strings, integers, and tuples.

Modifying Mutable Global Variables

If you have a mutable global variable, such as a list or dictionary, you can modify its contents within a function without using the global keyword. For example:

my_list = [1, 2, 3]
def my_function():
    my_list.append(4)

my_function()
print(my_list)    # Output: [1, 2, 3, 4]

In this example, we define a global variable my_list with a value of [1, 2, 3]. We then define a function my_function that appends the value 4 to the list.

We call the function, which modifies the contents of my_list, and then print the value of my_list, which is now [1, 2, 3, 4].

Creating Global Variables Inside a Function

If you need to create a global variable inside a function, you can do so using the global keyword. For example:

def my_function():
    global x
    x = 10

my_function()
print(x)    # Output: 10

In this example, we define a function my_function that creates a global variable x with a value of 10 using the global keyword. We then call the function, which creates the global variable x, and then print the value of x, which is 10.

Creating global variables inside functions can be useful if you have a dynamic variable that needs to be set based on the input to the function, or if you want to use a configuration file to set global variables.

Conclusion

In this article, we’ve covered everything you need to know about working with global variables in Python functions. We’ve discussed the different scopes in Python, how to access and modify global variables, and how mutability affects global variables.

By understanding these concepts, you can write more efficient and flexible Python code. In this article, we’ve examined the concept of global variables in Python functions.

We’ve discussed the different scopes in Python, how to access and modify global variables, and how mutability affects global variables. By understanding these concepts, we can write more efficient and flexible Python code.

The main takeaways are learning how to create global variables, accessing them, and modifying mutable global variables. Knowing how to use these techniques can make your code more efficient, easier to read and more flexible in terms of dynamic variables.

By keeping these concepts in mind, you can create functions and modules in Python with greater ease and precision.

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