Sharing list between functions - windows

Question

I recently asked another question asking how to pass a list from one function to another, which was kindly answered by #Modred. Essentially what I've got now is this:
def run_command():
machines_off = []
# Some stuff .....
machines_off.append(machineName)
# Some stuff ....
wol_machines(machines_off)
def wol_machines(machines_off):
# Some stuff ....
(I've cleared off all the non-essential code for this example as it's 300+ lines).
Now, each function is called by clicking on a tkinter button; the run_command is always run and sometimes will add items to the list 'machines_off'. I only want it to action machines_off if the second function button is clicked. At the moment after clicking on the run_command button, it runs through the whole script, including the second function when I don't want it to. I assume as I'm forwarding the list onto the second function (with the 5th line), it's bypassing the need to click the second functions button.
What do I need to change/add to allow the list from the first function to be available to the second, but not action it until needed??
Many thanks,
Chris.

Answer 1

I'm guessing your code looks something like:
from Tkinter import Tk, Button
def run_command():
machines_off = []
# Some stuff .....
machineName = "foo"
machines_off.append(machineName)
# Some stuff ....
wol_machines(machines_off)
def wol_machines(machines_off):
print "wol_machines was called"
print "contents of machines_off: ", machines_off
# Some stuff ....
root = Tk()
a = Button(text="do the first thing", command=run_command)
b = Button(text="do the second thing", command=wol_machines)
a.pack()
b.pack()
root.mainloop()
If you want the functions to execute independently of one another, you shouldn't call wol_machines from within run_command. You'll have to find some other way for both functions to see the list. One way of doing this is to use a global value.
from Tkinter import Tk, Button
machines_off = []
def run_command():
#reset machines_off to the empty list.
#delete these next two lines if you want to retain old values.
global machines_off
machines_off = []
# Some stuff .....
machineName = "foo"
machines_off.append(machineName)
# Some stuff ....
def wol_machines():
print "wol_machines was called"
print "contents of machines_off: ", machines_off
# Some stuff ....
root = Tk()
a = Button(text="do the first thing", command=run_command)
b = Button(text="do the second thing", command=wol_machines)
a.pack()
b.pack()
root.mainloop()
This is the simplest change to your original code that can give you your desired behavior, but global values are generally considered a sign of bad design. A more object oriented approach could localize the global into a class attribute.
from Tkinter import Tk, Button
class App(Tk):
def __init__(self):
Tk.__init__(self)
self.machines_off = []
a = Button(self, text="do the first thing", command=self.run_command)
b = Button(self, text="do the second thing", command=self.wol_machines)
a.pack()
b.pack()
def run_command(self):
#reset machines_off to the empty list.
#delete this next line if you want to retain old values.
self.machines_off = []
# Some stuff .....
machineName = "foo"
self.machines_off.append(machineName)
# Some stuff ....
def wol_machines(self):
print "wol_machines was called"
print "contents of machines_off: ", self.machines_off
# Some stuff ....
root = App()
root.mainloop()