Explain the purpose and use of global variables in Python.

In Python, a global variable is a variable declared in the main body of a module, outside of functions and classes, and accessible throughout the module.

To modify the value of a global variable inside a function, the global keyword must be used. Without it, Python will create a local variable with the same name.

Example:

counter = 0  # global variable

def increment():
    global counter  # specify that we want to modify the global variable
    counter += 1

increment()
print(counter)  # Will output 1

Using global variables is convenient for storing state accessible from different parts of the program, but excessive use can lead to debugging and maintenance difficulties.