[Tcl-bugs] [ tktoolkit-Bugs-2900796 ] ttk widgets do not initialize the textvariable

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Bugs item #2900796, was opened at 2009-11-19 17:52
Message generated for change (Comment added) made by jeremy_c
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Category: 88. Themed Tk
Group: current: 8.5.8
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jeremy Cowgar (jeremy_c)
Assigned to: Joe English (jenglish)
Summary: ttk widgets do not initialize the textvariable

Initial Comment:
% entry .e -textvariable e
% info exists e
1
% ttk::entry .e -textvariable e
.e
% info exists e
0

This requires you to either initialize it ahead of it's use or requires you to check info exists before any use of the variable.

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>Comment By: Jeremy Cowgar (jeremy_c)
Date: 2009-11-19 19:09

Message:
The right thing to do here is beyond me. I'll let others hash it out. If
it's not a bug, you can do as you please with this ticket.

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Comment By: Joe English (jenglish)
Date: 2009-11-19 18:49

Message:
Examples of counterintuitive behaviour:

# (1)
set X "blah"; entry .e -textvariable X
# ...
unset X
info exists X
#      ==> returns 1!

# [2]
set X "B"
spinbox .sb -textvariable X -values {A B C}
set X
# ==> returns "A"!

# [3]
set X 2
spinbox .sb -textvariable X -from 1 -to 3
set X
#  ==> Still 2! 
unset X
set X
# ==> Now it's 1 again!

More examples available on demand.


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Comment By: Joe English (jenglish)
Date: 2009-11-19 18:39

Message:
This behaviour is intentional, sensible, and consistent.

The policy for linked variables (-textvariable, -variable, etc.) is:

(1) The widget state reflects the value of the linked variable; and,
(2) Widgets update linked variables in response to user input.

The ttk::* widgets use a stronger policy:

(2') widgets *only* update linked variables in response to user input.

The core widgets mostly follow this policy as well,
and in the cases where they don't it inevitably leads
to counterintuitive behaviour.


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Comment By: Mark Janssen (mpc_janssen)
Date: 2009-11-19 17:56

Message:
This behaviour is different from the Tk entry widget which does initialize
the textvariable. So even if there is a good reason for this behaviour, the
fact that it's different from the behaviour in Tk adds an extra caveat for
porting Tk code to Ttk. I am curious what reason could be so grave that the
difference is justified.

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You can respond by visiting: 
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