How can I use a global variable in a function in Python?

To use a global variable inside a function in Python, you need to explicitly declare it with the global keyword if you intend to modify the variable. If you’re only reading its value, no special syntax is required. Here’s how to do it:

1. Modifying a Global Variable

Use the global keyword inside the function to indicate you’re modifying the variable in the global scope.

Example:

x = 10  # Global variable

def update_global():
    global x  # Declare 'x' as global
    x = 20    # Modify the global variable

update_global()
print(x)  # Output: 20

2. Reading a Global Variable

No global keyword is needed if you’re only accessing (not modifying) the variable.

Example:

x = 10

def read_global():
    print(x)  # Access global variable (no modification)

read_global()  # Output: 10

3. Mutable Objects (Lists, Dictionaries)

For mutable objects (like lists), you can modify their contents without global (e.g., appending items).
However, if you reassign the entire object, use global.

Example:

my_list = [1, 2, 3]  # Global list

def modify_list():
    my_list.append(4)  # Modifies the list in place (no 'global' needed)

def reassign_list():
    global my_list
    my_list = [5, 6, 7]  # Reassigns the global variable

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

reassign_list()
print(my_list)  # Output: [5, 6, 7]

Common Pitfalls

Error: Forgetting global when modifying a variable:

x = 10

def bad_example():
    x = 20  # Creates a local variable, leaving the global 'x' unchanged

bad_example()
print(x)  # Output: 10 (global 'x' wasn't modified)

Fix: Declare global x inside the function.

Best Practices

Avoid global variables when possible. Instead:

1. Pass values as arguments and return results.
2. Use class attributes for shared state.

Example (Preferred Approach):

def process_data(value):
    return value * 2  # No globals needed

result = process_data(5)
print(result)  # Output: 10

Summary

Scenario	Syntax	Example
Modify a global variable	global var inside function	global x; x = 20
Read a global variable	No special syntax	print(x)
Modify a mutable global object	No global needed (if not reassigning)	my_list.append(4)
Reassign a mutable object	global var required	global my_list; my_list = [...]

Use global sparingly to avoid unintended side effects!