global var in python: Python for Beginners

In Python, a variable is a named location in memory where data can be stored and retrieved later. Variables are essential components of any programming language as they allow the program to store and manipulate data dynamically during runtime. In Python, you can declare and initialize a variable using a single line of code.

For example, consider the following line of code:

name = "John"

Here, we have declared a variable named “name” and assigned it the string value “John”. The equal sign (=) is used to assign a value to a variable.

However, in Python, there are two types of variables: local variables and global variables. Local variables are declared and used within a function or a block of code, while global variables are declared outside of any function and can be accessed and modified from anywhere within the program.

In this article, we will focus on global variables in Python and how they differ from local variables. We will explore their scope, declaration, and usage in detail, along with some best practices for using them in your programs. So let’s dive in and learn about global variables in Python.

What are global variables in Python?

Global variables in Python are variables that are declared outside of any function or block of code, which means they can be accessed and modified from anywhere within the program. In other words, global variables have a scope that extends to the entire program.

Here are some examples of global variables in Python:

Example 1:

x = 10

def add_numbers(y):
    return x + y

print(add_numbers(5))  # Output: 15

In this example, x is a global variable that is declared outside of the add_numbers function. The add_numbers function takes a parameter y and returns the sum of x and y. Since x is a global variable, it can be accessed within the function without passing it as a parameter.

Example 2:

count = 0

def increment_count():
    global count
    count += 1

increment_count()
print(count)  # Output: 1

In this example, count is a global variable that is initialized to 0. The increment_count function uses the global keyword to indicate that it wants to access the global variable count. It then increments the value of count by 1. After calling the increment_count function once, the value of count is 1.

Example 3:

PI = 3.14

class Circle:
    def __init__(self, radius):
        self.radius = radius

    def get_area(self):
        return PI * self.radius * self.radius

circle = Circle(5)
print(circle.get_area())  # Output: 78.5

In this example, PI is a global variable that is used in the get_area method of the Circle class. The Circle class takes a radius parameter and has a get_area method that returns the area of the circle using the formula PI * r^2. Since PI is a global variable, it can be accessed within the method without passing it as a parameter.

How global variables are different than local variables

Variables can be either global or local, depending on where they are defined and how they are used. Here are some examples that illustrate the differences between global and local variables:

Example 1:

x = 10

def print_x():
    print("x is", x)

print_x()  # Output: x is 10

In this example, x is a global variable that is defined outside of the print_x function. When we call the print_x function, it can access and print the value of the global variable x.

Example 2:

def print_x():
    x = 10
    print("x is", x)

print_x()  # Output: x is 10

In this example, x is a local variable that is defined inside the print_x function. When we call the print_x function, it can access and print the value of the local variable x.

Example 3:

x = 10

def print_x():
    x = 5
    print("x is", x)

print_x()  # Output: x is 5
print("x is still", x)  # Output: x is still 10

In this example, we define a global variable x with a value of 10, and a function print_x that defines a local variable x with a value of 5. When we call the print_x function, it prints the value of the local variable x. However, when we print the value of x outside of the function, it still refers to the global variable with a value of 10.

Example 4:

def print_x():
    print("x is", x)

x = 10
print_x()  # Output: x is 10

In this example, we define a global variable x with a value of 10, and a function print_x that uses the global variable x. When we call the print_x function, it can access and print the value of the global variable x.

Defining global var in Python

Global variables can be defined outside of any function or block of code, making them accessible from anywhere in the program. To define a global variable, you simply need to declare it outside of any function or block of code.

Here’s an example:

name = "John"
age = 25

def print_person():
    print("Name:", name)
    print("Age:", age)

print_person()

In this example, we define two global variables name and age outside of any function. We then define a function print_person that prints the values of these variables. When we call the print_person function, it can access and print the values of the global variables because they are in the global scope.

It’s important to note that if you want to modify the value of a global variable inside a function, you need to use the global keyword to indicate that you want to use the global variable instead of creating a new local variable with the same name. Here’s an example:

count = 0

def increment_count():
    global count
    count += 1

increment_count()
print(count)

In this example, we define a global variable count and a function increment_count that increments the value of count by 1. We use the global keyword inside the function to indicate that we want to use the global variable count instead of creating a new local variable with the same name. When we call the increment_count function and print the value of count, it has been incremented by 1.

Modifying global variables in Python

Global variables can be modified from anywhere in the program, including inside functions or blocks of code. However, when modifying a global variable inside a function or block of code, it’s important to use the global keyword to indicate that you want to use the global variable instead of creating a new local variable with the same name.

Here’s an example to illustrate how to modify a global variable in Python:

count = 0

def increment_count():
    global count
    count += 1

increment_count()
increment_count()
print(count)  # Output: 2

After calling the increment_count function twice, the value of count has been incremented twice and is now 2. This demonstrates how global variables can be modified from inside a function.

It’s important to note that modifying global variables can lead to unexpected results and make your code harder to debug and maintain. When modifying global variables, it’s best to use them sparingly and keep them well-documented to avoid confusion.

In addition, if you have multiple threads or processes running in your program, modifying global variables can lead to race conditions and other synchronization issues.

In these cases, it’s often better to use synchronization primitives such as locks or semaphores to ensure that only one thread or process can modify the global variable at a time.

Best practices for using global variables in Python

Avoid using global variables excessively: Global variables can make your code harder to understand and maintain, especially if they are used excessively. Try to limit the use of global variables and instead use function parameters or return values to pass data between functions.
Use descriptive names: When defining global variables, use descriptive names that clearly indicate their purpose. This will make your code more readable and easier to understand.

Define global variables at the top of your file: By convention, global variables should be defined at the top of your Python file, before any functions or other code. This makes it easier to see all the global variables used in your program and avoids issues with variable scoping.
Use constants instead of mutable data structures: If you need to use a global variable to store a constant value that will not change during the execution of your program, consider using a constant instead. This can help prevent accidental modifications to the value and make your code more reliable.
Avoid modifying global variables from multiple threads or processes: If you have multiple threads or processes running in your program, modifying global variables can lead to race conditions and other synchronization issues. Instead, use synchronization primitives such as locks or semaphores to ensure that only one thread or process can modify the global variable at a time.

Use the global keyword to indicate when you are modifying a global variable: When modifying a global variable from within a function or block of code, always use the global keyword to indicate that you want to use the global variable instead of creating a new local variable with the same name.

Examples of using global variables in Python

Here are a few examples of using global variables in Python and how they can help solve programming problems:

Configuration settings: If your program has configuration settings that need to be accessed from multiple parts of the code, you can store them in global variables. This can make it easier to modify the settings in one place, and have those changes propagate throughout the rest of the code. For example:

# Define global configuration variables
MAX_CONNECTIONS = 100
TIMEOUT = 10

def connect_to_server():
    # Use global configuration variables
    global MAX_CONNECTIONS, TIMEOUT
    # Connect to server with maximum number of connections and timeout
    print("Connecting to server with", MAX_CONNECTIONS, "connections and a timeout of", TIMEOUT, "seconds...")

In this example, MAX_CONNECTIONS and TIMEOUT are global configuration variables that are used in the connect_to_server function. By storing these variables as global variables, we can easily change their values if needed and have those changes reflected throughout the program.

Caching data: If your program needs to store data in memory for later use, you can use global variables to cache the data. This can help improve performance by reducing the need to retrieve the data from disk or another source each time it is needed. For example:

# Define global cache variable
CACHE = {}

def get_data(key):
    # Use global cache variable
    global CACHE
    if key in CACHE:
        # Return cached data
        return CACHE[key]
    else:
        # Retrieve data from disk or another source
        data = retrieve_data_from_disk(key)
        # Cache data for future use
        CACHE[key] = data
        return data

In this example, CACHE is a global variable that is used to store data that has been retrieved from disk or another source. By caching the data in memory, we can avoid the need to retrieve it again in the future, which can improve performance.

Managing state: If your program needs to maintain state information, such as a user’s login status or a game’s score, you can use global variables to store that information. This can help simplify the code by avoiding the need to pass state information between functions or classes. For example:

# Define global state variables
IS_LOGGED_IN = False
SCORE = 0

def login(username, password):
    # Use global state variable
    global IS_LOGGED_IN
    # Verify username and password
    if verify_credentials(username, password):
        IS_LOGGED_IN = True
    else:
        print("Invalid username or password")

def add_points(points):
    # Use global state variable
    global SCORE
    SCORE += points
    print("Score is now", SCORE)

In this example, IS_LOGGED_IN and SCORE are global state variables that are used to maintain the user’s login status and the game’s score, respectively. By storing this information in global variables, we can avoid the need to pass it between functions or classes, which can simplify the code and make it easier to understand.

Potential issues with using global variables

While global variables can be useful in certain situations, there are some potential issues with using them. Here are a few examples:

Name collisions: If you use the same name for a global variable and a local variable, it can cause name collisions and make it difficult to know which variable is being used in a particular context. This can lead to bugs and make your code harder to debug. For example:

x = 10

def foo():
    x = 5
    print("x is", x)

foo()  # Output: x is 5
print("x is still", x)  # Output: x is still 10

In this example, the foo function defines a local variable x with a value of 5, which overrides the global variable x with a value of 10. This can cause confusion when trying to understand which variable is being used in a particular context.

Side effects: Modifying global variables can have side effects that affect other parts of your program. This can make it harder to reason about your code and make it more difficult to debug. For example:

count = 0

def increment_count():
    global count
    count += 1

def print_count():
    global count
    print("Count is", count)

increment_count()
print_count()  # Output: Count is 1
increment_count()
print_count()  # Output: Count is 2

In this example, we define a global variable count and two functions increment_count and print_count. The increment_count function modifies the value of count, and the print_count function prints the value of count. However, because count is a global variable, modifying it in one function can affect the behavior of other functions that use it.

Concurrency issues: If your program has multiple threads or processes running at the same time, modifying global variables can lead to race conditions and other synchronization issues. This can cause your program to behave unpredictably and make it harder to debug. For example:

import threading

count = 0

def increment_count():
    global count
    count += 1

def worker():
    for i in range(1000):
        increment_count()

threads = [threading.Thread(target=worker) for i in range(10)]
for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

print("Final count is", count)  # Output: Final count is not deterministic

In this example, we define a global variable count and a function increment_count that increments the value of count. We then spawn 10 worker threads that each call the increment_count function 1000 times. Because the threads are modifying the global variable count at the same time, it can lead to race conditions and make the final value of count unpredictable.