(python) Should my variable be local or global? (best practice)

Question

When declaring a constant that is only used one function, should that variable be declared locally since it is only used by that function, or globally since it is never going to change?

IE which is better:

CONSTANT = (1, 3, 5, 8)

##SOME OTHER CODE HERE

def function1(arg):
    if arg in CONSTANT:
        do something

or:

def function1(arg):
    CONSTANT = (1, 3, 5, 8)
    if arg in CONSTANT:
        do something

I know there isn't a lot of difference between these two, but I just wanted to know which of the two practices is preferred since I'm just starting out and want to form good habits.

Answer 1

I would keep it local. You can always just move it global in the future if you need to, or share it between functions by making them methods in a class and turning the constant into a class variable. In these situations, generally speaking, the more local, the better, and best is to hide implementation information within your functions, as in your second example. It doesn't make a huge difference here, but as your projects get bigger, maintainability and modularity will be sustained.

Answer 2

I would put them global because:

1. Your variables are constants
2. In Python, global scope is encapsulated in the module namespace, meaning that your variable is in fact only global inside the module.
3. If you call your function a lot of times, and put your constants local to it, it would reallocate them each time your call the function.
4. Then you can share your constants between different functions.

However, if you move to Object Oriented Programming, then I would put the constants as class variables.