How to stop variables becoming local?

Question

I'm trying to create a function to verify if a move in a game is legal. At the top of my code, I declared the variable Valid.

p1 = [1,1]
p2 = [1,1]
turn = 1
move = 0
Valid = False

and later on, I call the function check_swap() and right now, I am just returning that it is True (I will implement the checking later on):

if move == 2:
    print("Swaping selected")
    print("you have " + str(p1[0] + p1[1]) + " in total.")
    want_p1_hand_1 = input("How many do you want hand 1 to have?")
    want_p1_hand_2 = input("How many do you want hand 2 to have?")

    check_swap(p1, p2, turn, want_p1_hand_1, want_p1_hand_2)
    if Valid == True:
        p1[0] = want_p1_hand_1
        p1[1] = want_p1_hand_2
        print_game(p1, p2)
    else:
        print("That didn't work")


def check_swap(p1, p2, turn, want_p1_hand_1, want_p1_hand_2):
    Valid = True
    return Valid

Then, it defaults to the else statement and prints "That didn't work". At first I just had Valid = True, and it said that Valid was called but never used, even though it is. I searched Stackoverflow and adding the return Valid seemed like the right thing to do, but it didn't work.

Unrelated, but you may want to check out [PEP 8](https://peps.python.org/pep-0008/), the Python style guide. — SuperStormer, Jun 24 '22 at 17:57

score 1 · Accepted Answer · answered Jun 24 '22 at 17:56

Valid = True in check_swap actually just creates a new local variable, separate from the global Valid.

Now, you could use the global statement, but a better way to write the code would be to assign the return value of check_swap to Valid.

Valid = check_swap(p1, p2, turn, want_p1_hand_1, want_p1_hand_2)

score 0 · Answer 2 · answered Jun 24 '22 at 18:02

To assign to global variables you'll need the global keyword. Here's an example ran in the Python REPL:

>>> valid = False
>>> 
>>> def not_using_global():
...     valid = True
...     print(valid)
...
>>> def using_global():
...     global valid
...     valid = True
...     print(valid)
...
>>>
>>> not_using_global()
True
>>> print (valid)
False
>>>
>>> using_global()
True
>>> print(valid)
True

How to stop variables becoming local?

2 Answers2