Tkinter - StringVar() trace not executing command

Question

I wrote this code to be a version manager, but it doesn't execute the command changeDir(). Why?

Please don't link to off-site code. See [How to create a Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve) — Bryan Oakley, May 13 '17 at 20:47

score 1 · Accepted Answer · edited May 23 '17 at 11:47

You forgot to pass a 'name' argument to changeDir function. And there's no exception because your statement has no effect!

Snippet to represent the problem:

import sys


def exec_smth():
    # execution without effect
    exec('write_smth')

    try:
        # execution with exception because of missing argument
        exec('write_smth()')
    except TypeError as error:
        # now we pass an argument
        exec('write_smth("I failed because of %s" % error )')


def write_smth(smth):
    sys.stdout.write(smth)

exec_smth()

Anyway, outside your __init__ function there're no StringVars at all thanks to garbage collector, so your code would fail anyway!

There're even more problems, because you never bind any of your sv{} to a widget and expect something in return! But ok, let's try to do things with exec:

try:
    import tkinter as tk
except ImportError:
    import Tkinter as tk


class App(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        self.entries = []

        for _ in range(5):
            exec('self.sv{} = tk.StringVar()'.format(_))
            exec('self.sv{}.trace("w", self.change_sv)'.format(_))
            exec('self.entries.append(tk.Entry(self, text="", textvariable=self.sv{}))'.format(_))

        for entry in self.entries:
            entry.pack()

    def change_sv(*args):
        # get id of a variable (you can't rely on that (0-9)!)
        idx = args[1][-1:]
        # get new value
        value = getattr(args[0], 'sv{}'.format(idx)).get()
        # result
        print('Value changed in self.sv%s to %s!' % (idx, value))

app = App()
app.mainloop()

Output:

As you see - we always need a reference to StringVars and I think that option with a list of them is far better!

Note: If you need to pass something to callback function - use a lambda function! All code tested with Python 3.

Links:

Tkinter - StringVar() trace not executing command

1 Answers1