Using and creating global variables:¶

In Python, you’ll typically define global variables at the module level. Once you’ve defined a global variable, you can use it from within the module itself or from within other modules in your code.

Accessing a global variable works, but directly modifying a variable doesn’t work. You can’t directly modify a variable from a high-level scope like global in a lower-level scope like local.

UnboundLocalError:¶

In the below example, python first tries to find variables a, b, c inside the print_globas function. It doesn't find any of the variables inside the function, but it finds it outside the function in the global scope.

a = 10
b = 20
c = 30

def print_globals():
    print(a, b, c) 

>> 10 20 30

In the below example, python first tries to find variables a, b, c inside the print_globas function and it finds c but the value 100 doesn't get assigned to it in the first line. So UnboundLocalError: cannot access local variable 'c' where it is not associated with a value error is seen on the terminal.

c is both global and local variable in this function. Python gives priority to the local variable and shows error.

a = 10
b = 20
c = 30

def print_globals():
    print(a, b, c) 
    c = 100

Below code resolves the UnboundLocalError exception by using global keyword.

a = 10
b = 20
c = 30

def print_globals():
    global c
    print(a, b, c)
    c = 100
    print(c)

print_globals()
print(a,b,c)

>>> 10 20 30
>>> 10 20 100
>>> 10 20 100

If you want to modify a global variable inside a function, then you need to explicitly tell Python to use the global variable rather than creating a new local one. To do this, you can use one of the following:

The global keyword
The built-in globals() function
The nonlocal keyword

Using `nonlocal` keyword:¶

nonlocal keyword is similar to global keyword. Only, it is used to in case of nested functions where a outer scoped variable is made use inside a inner function.

    def trial():
        a = 10
        print(a)
        def inner_func():
            print(a)
            a=100
        inner_func()
        print(a)

trial()
>>> UnboundLocalError

Fixing UnboundLocalError above:¶

    def trial():
        a = 10
        print(a)
        def inner_func(): 
            nonlocal a 
            print(a)
            a=100
        inner_func()
        print(a)


trial() 
>>> 10
>>> 10
>>> 100

globals() functions:¶

This function comes in handy when you have local variables with the same name as your target global variables, and you still need to use the global variable inside the function:

>>> a = 10
>>> b = 20
>>> c = 30

>>> def print_globals():
...     print(a, b, globals()["c"])
...     c = 100
...     print(c)
...

>>> print_globals()
10 20 30
100

>>> c
30

Best practices:¶

Use only constant as globals, which are not going to change within the script.
python USER = 'oia' ID = '1234235356'
Use a function that takes argument and modified it and returns it if you need a global kind of variable which needs to be modified.
Create a class and use the class attribute as a global variable throughout the class.