Global vs Local Variables

Global Variables

• Global variables are defined outside any function.
• They are accessible throughout the program, including inside functions (for reading).
• By default, a function can read a global variable but cannot modify it directly.

Example: Accessing a Global Variable

a = 10   # global variable

def something():
    print("inside :", a)

something()
print("outside :", a)

Output:

inside : 10
outside : 10

Here, the function can read the global variable a without any issue.

Local Variables

• Local variables are defined inside a function.
• They are accessible only within that function.
• Attempting to access a local variable outside its function results in a NameError.

Example: Local Variable Scope

def something():
    a = 15   # local variable
    print("inside :", a)

something()
print("outside :", a)

Output:

inside : 15
NameError: name 'a' is not defined

The variable a exists only inside the function, so it cannot be accessed outside.

Same Variable Name in Global and Local Scope

• Python allows global and local variables to have the same name.
• They are treated as separate and independent variables.
• Modifying the local variable does not affect the global variable.

Example: Separate Global and Local Variables

a = 10   # global variable

def something():
    a = 15   # local variable
    print("inside :", a)

something()
print("outside :", a)

Output:

inside : 15
outside : 10

This demonstrates that the local a shadows the global a inside the function.

Modifying Global Variables Inside a Function

By default, Python treats variables assigned inside a function as local. To modify a global variable from within a function, Python provides two approaches.

1. Using the global Keyword

• The global keyword tells Python that the variable refers to the global scope.
• This allows direct modification of the global variable.

a = 10

def something():
    global a
    a = 20
    print("inside :", a)

something()
print("outside :", a)

Output:

inside : 20
outside : 20

2. Using the globals() Function

• globals() returns a dictionary of all global variables.
• You can modify a global variable by accessing it using its string name as a key.
• This method modifies the global variable, not the local one.

a = 10

def something():
    a = 15      # local variable
    globals()['a'] = 20
    print("inside :", a)

something()
print("outside :", a)

Output:

inside : 15
outside : 20

Here:

• Local a remains 15
• Global a is updated to 20