can't reference global variable inside a function

Question

Good day to all. I am trying to build a game called 2048.I made a variable named box and added 4 lists each list containing a 4 empty values to be replaced later.Then I declared it global(tried reverse too:declared global and then made the values inside them).when I call global box in one function(to do changes to the box),it works.then I called another function which changes position of box and it too passed without a error.but the third where I assign the box to another variable with list comphrehension like this:

box = [[[]]*4 for i in range(4)] # this is box
box1 = [box[i][::-1] for i in range(4)] # tried to do this

the second function didn't require box1 but I called it to see if any error occurs.Nothing occurs. but when I do same with the next function error "NameError:free variable box referenced before assignment in enclosing scope" occurs.

why is that.what did I do wrong? here is the code

import random
global box
box = [[[]]*4 for i in range(4)]


def adding():
    digit = random.choice([2,4])
    empty = [index for index,i in enumerate(box[0]) if i==[]]
    add = random.choice(empty)
    try:
        for i in range(3):
            if box[i][add] == [] and box[i+1][add] != []:
                box[i][add] = digit
                break
            elif box[-1][add] == []:
                box[-1][add] = digit
                break
    except:
        print('Error Occured')

def right():
    box1=[box[i][::-1] for i in range(4)]
    for i in range(4):
        take = box[i]
        box[i]=[[] for x in take if x ==[]]
        for j in [x for x in take if x != []]:
            box[i].append(j)

def left():
    box1 = [box[i][::-1] for i in range(4)]
    right()
    box1 = [box[i][::-1] for i in range(4)]
    box= box1

error occurs at function left() tried in different os too.

thanks.works when I declared global inside function but ca anyone explain why did it work for right() but not for left()?

Answer 1

You must declare it global inside the function, not outside. Module level variables are always global:

box = ...    

def left():
    global box  # unnecessary, if not assigned a value
    box1 = [box[i][::-1] for i in range(4)]

def foo():
    global box  # necessary, otherwise box is local
    box = bar 

The global declaration is unnecessary, as you are only using the variable and not assigning it a new value. It i therefore considered global by default. See the docs:

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.