[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Javier Dehesa]

Javier Dehesa added the comment:

With the additional hint of garbage collection I have found now a number of 
examples of this behaviour (e.g. 
https://stackoverflow.com/questions/7439432/python-themed-tkinter-entry-variable-will-not-set).
However, I haven't found actual documentation warning of this, so I'm not sure 
if this is really expected or just something people has gotten used to live 
with. One could argue that you normally wouldn't need a variable in the first 
place if you are not keeping a reference to it, but I'm not sure what is the 
benefit of having a (I assume) weak reference in the widget. Maybe there are 
solid technical reasons but, from what I have seen, it seems to have caused 
more than one confusion.

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Javier Dehesa]

Javier Dehesa added the comment:

I see what you mean. Looking at TkDocs (not sure if this is an "official" or 
"officially endorsed" source or not), one of the Python examples in the "Tk 
Concepts" section (http://www.tkdocs.com/tutorial/concepts.html) says:

> Whether or not you save the widget object in a variable is entirely up to 
> you, and depends of course whether you'll need to refer to it later. Because 
> the object is inserted into the widget hierarchy, it won't be garbage 
> collected even if you don't keep your own reference to it.

However, it very much looks like it may be related to what you said.

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Paul Moore]

Paul Moore added the comment:

I suspect that the problem is somehow related to garbage collection - in your 
failing example you don't keep any references to v or cb. I don't know what 
references tkinter keeps internally, it may be that this is OK because the root 
window is guaranteed to keep references to everything, but it's certainly worth 
a look.

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Javier Dehesa]

Javier Dehesa added the comment:

Yeah is quite subtle, I should have pointed it out...
The difference is that `root.mainloop()` is called inside `make_var_cb(root)` 
in the first example and under `if __name__ == '__main__'` in the second one. I 
have experience the problem on Windows 10, and I'd say there is a good chance 
it's a platform-specific thing...

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Louie Lu]

Louie Lu added the comment:

Sorry, but I can't see the different between your first and second code. On my 
Linux, it all set to check the box when it run up. Do I miss anything?

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nosy: +louielu

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Javier Dehesa]

Javier Dehesa added the comment:

Note, this is not something specific to check buttons, the same happens with 
other widgets such as entries.

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[issue30678] Widget variable binding does not work if mainloop is called from a different function

[Javier Dehesa]

New submission from Javier Dehesa:

When you build a Tkinter interface with variables, if tkinter.Tk.mainloop is 
called from a different function that the one creating the variable objects, 
then in some cases the widgets associated with the variables will not be set to 
the right value. I have not been able to find a consistent pattern, but I have 
some examples.

The following script works correctly:
```
import tkinter as tk
from tkinter import ttk

def make_var_cb(root):
v = tk.BooleanVar(root, True)
cb = ttk.Checkbutton(root, text='Checkbutton', variable=v)
cb.pack()
root.mainloop()

if __name__ == '__main__':
root = tk.Tk()
make_var_cb(root)
```

But the following does not (the result is shown in the attached image):
```
import tkinter as tk
from tkinter import ttk

def make_var_cb(root):
v = tk.BooleanVar(root, True)
cb = ttk.Checkbutton(root, text='Checkbutton', variable=v)
cb.pack()

if __name__ == '__main__':
root = tk.Tk()
make_var_cb(root)
root.mainloop()
```

However, the following _does_ work again:
```

def make_var(root):
return tk.BooleanVar(root, True)

def make_cb(root, v):
return ttk.Checkbutton(root, text='Checkbutton', variable=v)

if __name__ == '__main__':
root = tk.Tk()
v = make_var(root)
cb = make_cb(root, v)
cb.pack()
root.mainloop()
```

--
components: Tkinter, Windows
files: checkbutton_bad.png
messages: 296100
nosy: Javier Dehesa, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Widget variable binding does not work if mainloop is called from a 
different function
type: behavior
versions: Python 3.5
Added file: http://bugs.python.org/file46954/checkbutton_bad.png

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