Variable outside of value not permanently changing

Question

I have the following code for a maze solving bot:

def main():
    axioms = {
        "Y":"F",
        "X":"R",
        "-Y":"B",
        "-X":"L"
        }

    def rotate():
        axioms = list(axioms.items())
        new = []
        new.append((axioms[0][0], axioms[1][1]))
        new.append((axioms[1][0], axioms[2][1]))
        new.append((axioms[2][0], axioms[3][1]))
        new.append((axioms[3][0], axioms[0][1]))
        return dict(new)

    def move(dir):
        if dir == "F":
            log("Moving forward.")
            API.moveForward()
        elif dir == "L":
            log("Turning left.")
            axioms = rotate()
            API.turnLeft()
            API.moveForward()
        elif dir == "R":
            log("Turning right.")
            axioms = rotate()
            axioms = rotate()
            axioms = rotate()
            API.turnRight()
            API.moveForward()
        elif dir == "B":
            log("Turning back.")
            axioms = rotate()
            axioms = rotate()
            API.turnLeft()
            API.turnLeft()
            API.moveForward()

    def calculateNext(current):
        #calculate possible turns
        if current[3] == False:
            if sensor.front() == True:
                del current[2][(next(key for key, value in axioms.items() if value == "F"))]
            if sensor.left() == True:
                del current[2][(next(key for key, value in axioms.items() if value == "L"))]
            if sensor.right() == True:
                del current[2][(next(key for key, value in axioms.items() if value == "R"))]
            current[3] = True

        #find preferred turn
        values = []
        for temp in current[2]:
            values.append(current[2][temp][1])

        #check for inconsistency
        def retry():
            global preferred
            if all(x==values[0] for x in values) == True and values[0] > current[1]:
                current[1] += 2
                retry()
            else:
                preferred = min(values)
        retry()

        preferedTurns = []
        #for temp in current[2]:
        preferedTurns.append(next(key for key, value in current[2].items() if value[1] == preferred))

        return preferedTurns[0]

The problem is the axioms variable. I have it outside of the calculateNext function and when its values change, within calculateNext such as,

    def rotate():
        axioms = list(axioms.items())
        new = []
        new.append((axioms[0][0], axioms[1][1]))
        new.append((axioms[1][0], axioms[2][1]))
        new.append((axioms[2][0], axioms[3][1]))
        new.append((axioms[3][0], axioms[0][1]))
        return dict(new)
...
...
...

    def move(dir):
        if dir == "F":
            log("Moving forward.")
            API.moveForward()
        elif dir == "L":
            log("Turning left.")
            axioms = rotate(axioms)
            API.turnLeft()
            API.moveForward()
        elif dir == "R":
            log("Turning right.")
            axioms = rotate(axioms)
            axioms = rotate(axioms)
            axioms = rotate(axioms)
            API.turnRight()
            API.moveForward()
        elif dir == "B":
            log("Turning back.")
            axioms = rotate(axioms)
            axioms = rotate(axioms)
            API.turnLeft()
            API.turnLeft()
            API.moveForward()

they do not save to the variable and the same values are used instead of the new ones.

yes, because `axioms` are two *different* local variables, one exists in the `rotate` function, the other in `move` — juanpa.arrivillaga, Feb 14 '20 at 22:41
oh sorry thats an error in the code ill update it but the problem still occurs with the proper code — JojoTheCodeDude, Feb 14 '20 at 22:43
If you would've produced a [minimal example](https://stackoverflow.com/help/minimal-reproducible-example), you would have been able to figure out the problem — user8408080, Feb 14 '20 at 22:43
the thing is axioms cant be a local variable for `rotate` and `move` becuase it is defined in the same outside scope of the 2 functions — JojoTheCodeDude, Feb 14 '20 at 22:49
just like `a = 5 def b(): print(a) ` works becuase `a` is defined on the outside of `b` — JojoTheCodeDude, Feb 14 '20 at 22:53
Why are you defining those three functions inside `main` in the first place? — chepner, Feb 14 '20 at 23:05
It may be helpful to read and understand global scope. https://stackoverflow.com/questions/10588317/python-function-global-variables — Stuart, Feb 14 '20 at 23:24
That code is a sample of a bigger problem so there is a reason to the nested functions — JojoTheCodeDude, Feb 15 '20 at 10:49

score 0 · Accepted Answer · answered Feb 14 '20 at 23:36

To maintain the value of a variable across functions you have several options, such as: use a global variable, use a class to share properties along methods, use a mutable variable and mutate rather than reassign it, or pass the variable as a parameter and return it at the end of the function.

As axioms is a dict and therefore mutable, rewriting rotate to change its values in place (rather than reassigning) might work best for you

def rotate():
    values = list(axioms.values())
    for k, v in zip(axioms, values[1:] + [values[0]]):
        axioms[k] = v

You would then just call it as rotate()

Variable outside of value not permanently changing

1 Answers1