11. Variables Scope

11.1. Scope

Scope is a section of code in which a variable is defined, can be accessed and written to.

There are two types of variables: global and local.

By default, all variables defined within a function are local - they cannot be accessed outside of the function.

Since the scope within the function is different from the global one,

a variable with the same name outside of the function is different to the variable within a function.

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11.2. Local Variables

The code below attempts to use a name_age_greeting() function and increment age every time the function is called:

name = "Joe"
age = 12

def name_age_greeting():
    age += 1
    return "Hi " + name + ", you are " + str(age) + " years old"


print(name_age_greeting())
print(name_age_greeting())
print(name_age_greeting())

This results in an error message: local variable 'age' referenced before assignment.

This is because the variable age within the function is not the same as the variable age outside of the name_age_greeting() function and the variable within the function has not yet had a value assigned to it.

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11.3. Global Variables

To access the global variable inside the function, declare the variable age as global within the function:

This will let Python know, that the age variable in the function is the same one in the global namespace.

name = "Joe"
age = 12

def name_age_greeting():
    global age
    age += 1
    return "Hi " + name + ", you are " + str(age) + " years old"


print(name_age_greeting())  # Hi Joe, you are 13 years old
print(name_age_greeting())  # Hi Joe, you are 14 years old
print(name_age_greeting())  # Hi Joe, you are 15 years old

Warning

Using global variables for functions like this is generally a bad practice and should be avoided, since it makes the purpose of your functions less obvious and you can end up with messy code. A better way to do this is to pass variable age as one of the arguments of the function (example below).

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11.4. Use arguments instead of global variables

Here is an example for a function that passes variables as arguments.

The age variable is incremented outside of the function.

name = "Joe"
age = 12

def name_age_greeting(name, age):
    return "Hi " + name + ", you are " + str(age)

print(name_age_greeting(name, age))   # Hi Joe, you are 12
age += 1
print(name_age_greeting(name, age))   # Hi Joe, you are 13
age += 1
print(name_age_greeting(name, age))   # Hi Joe, you are 14

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11.5. Nonlocal variables in nested functions

The nested functions below has a variable x which if different to the x in the outer function:

def outer_func():
    x = 3
    print(x)  # 3

    def inner_func():
        x = 7
        print(x) # 7

    inner_func()
    print(x) # 3

outer_func()

Nested functions can use the keyword: nonlocal, to make the variable in the nested function the same variable as in the outer function:

def outer_func():
    x = 3
    print(x)  # 3

    def inner_func():
        nonlocal x
        x = 7
        print(x) # 7

    inner_func()
    print(x) # 7

outer_func()