[New-bugs-announce] [issue18131] Tkinter Variables require a proper master

[Terry J. Reedy]

New submission from Terry J. Reedy:

The signature for tkinter class Variable and its subclasses StringVar, IntVar, DoubleVar, BooleanVar is
  def __init__(self, master=None, value=None, name=None):
However, the None default is invalid because of
        self._tk = master.tk
The preceding lines
        if not master:
            master = _default_root
might suggest that None is acceptible. But _default_root is set to None at the top of the file, so that just replaces None with None.

If Variables must have a gui widget parent, then they cannot be used in gui-free tests, even though they have no graphic representation.

Moveover, not everything is a proper master. configSectionNameDialog.py . GetCfgSectionNameDialog(Toplevel) . Create.Widgets(self) has these lines:
        self.name = StringVar(self)
        self.fontSize = StringVar(self)
However, these are not (always) garbage collectable.

After fixing the dialog test at the bottom of the file, and running it with 3.3 python_d -m idlelib.configSectionNameDialog (#18130), and adding the gc flag, I repeatedly got
[sys:1: ResourceWarning: gc: 2 uncollectable objects at shutdown [<tkinter.StringVar object at 0x02C597E0>, <tkinter.StringVar ...]]
(This only showed when running from the console, not when running within Idle started within the interactive debug interpreter.)

Running the test multiple times within a session, by repeatedly creating new dialogs with the [dialog] button in the test window, multiplied the uncollected objects.

Replacing 'self' with 'self.parent' solved the problem, at least in the test situation.

----------
components: Tkinter
messages: 190574
nosy: gpolo, terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Tkinter Variables require a proper master
type: behavior
versions: Python 2.7, Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18131>
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