Python - Change variable outside function without return

Question

I just started learning Python and I ran into this problem. I want to set a variable from inside a method, but the variable is outside the method.

The method gets activated by a button. Then I want to get the value from that variable that I set when I press another button. The problem is that the value that I put inside a variable from inside the method doesn't stay. How would I solve this?

The code is underneath. currentMovie is the variable I try to change. When I press the button with the method UpdateText(), it prints out a random number like it is supposed to. But when I press the button that activates UpdateWatched() it prints out 0. So I am assuming the variable never gets set.

import random
from tkinter import *

currentMovie = 0

def UpdateText():
    currentMovie = random.randint(0, 100)
    print(currentMovie)

def UpdateWatched():
    print(currentMovie)

root = Tk()
root.title("MovieSelector9000")
root.geometry("900x600")
app = Frame(root)
app.grid()
canvas = Canvas(app, width = 300, height = 75)
canvas.pack(side = "left")
button1 = Button(canvas, text = "SetRandomMovie", command = UpdateText)
button2 = Button(canvas, text = "GetRandomMovie", command = UpdateWatched)
button1.pack(anchor = NW, side = "left")
button2.pack(anchor = NW, side = "left")
root.mainloop()

Answer 1

Here's a simple (python 2.x) example of how to 1 not use globals and 2 use a (simplistic) domain model class.

The point is: you should first design your domain model independently from your user interface, then write the user interface code calling on your domain model. In this case your UI is a Tkinter GUI, but the same domain model should be able to work with a command line UI, a web UI or whatever.

NB : for python 3.x, replace Tkinter with tkinter (lowercase) and you can get rid of the object base class for Model.

import random
from Tkinter import *


class Model(object):
    def __init__(self):
        self.currentMovie = 0

    def UpdateCurrentMovie(self):
        self.currentMovie = random.randint(0, 100)
        print(self.currentMovie)

    def UpdateWatched(self):
        print(self.currentMovie)

    def ExampleWithArgs(self, arg):
        print("ExampleWithArg({})".format(arg))


def main():
    model = Model()
    root = Tk()
    root.title("MovieSelector9000")
    root.geometry("900x600")
    app = Frame(root)
    app.grid()
    canvas = Canvas(app, width = 300, height = 75)
    canvas.pack(side = "left")
    button1 = Button(canvas, text = "SetRandomMovie", command=model.UpdateCurrentMovie)
    button2 = Button(canvas, text = "GetRandomMovie", command=model.UpdateWatched)
    button3 = Button(canvas, text = "ExampleWithArg", command=lambda: model.ExampleWithArgs("foo"))
    button1.pack(anchor = NW, side = "left")
    button2.pack(anchor = NW, side = "left")
    button3.pack(anchor = NW, side = "left")
    root.mainloop()

if __name__ == "__main__":
    main()

Answer 2

Use global to modify a variable outside of the function:

def UpdateText():
    global currentMovie
    currentMovie = random.randint(0, 100)
    print(currentMovie)

However, don't use global. It's generally a code smell.

Answer 3

Here's a simple but dirty solution: use a mutable variable.

Instead of

currentMovie = 0

def UpdateText():
    currentMovie = random.randint(0, 100)
    print(currentMovie)

you can use a single-cell list for currentMovie and pass it as a (default) argument to UpdateText():

currentMovie = [0]

def UpdateText(cM=currentMovie): # The default value will 'bind' currentMovie to this argument
    cM[0] = random.randint(0, 100) # This will change the *contents* of the variable
    print(cM[0]) # I used a different name for the parameter to distinguish the two

UpdateText() # Calls UpdateText, updating the contents of currentMovie with a random number

Note that setting currentMovie itself (not its contents) with a new value—even with a new list—would cause UpdateText() to stop updating currentMovie unless the def block were run again.

currentMovie = [0]

def UpdateText(cM=currentMovie): # The default value will 'bind' currentMovie to this argument
    cM[0] = random.randint(0, 100) # This will change the *contents* of the list
    print(cM[0]) # I used a different name for the parameter to distinguish the two

currentMovie = 3 # UpdateText() will no longer affect this variable at all

# This will thus not throw an error, since it's modifying the 'old' currentMovie list:
UpdateText() # The contents of this list can also no longer be accessed

This is more of a handy trick if you're building something quick and dirty and don't want to build a class; I find that Python is great for such things, so I think that this is still worthwhile to share despite the other answers.

For more serious purposes, though, creating a class as in bruno's answer would almost certainly be better.

Answer 4

For me, the already mentioned answers did not work for two reasons.

1. In case of an error, I need the variable for a deeper analysis of the data which led to the error.
2. I'm using the function in a pandas.DataFrame.apply() to paste the usual output into a column of the existing DataFrame. Therefore the error information shall not be in the return statement.

Solution for me: Since I did not find a direct solution I decided to write the variable on disk:

    with open('var.pickle', 'wb') as f:
        pickle.dump(var, f)

And then to import it where ever I need it:

    with open('var.pickle', 'rb') as f:
        var = pickle.load(f)