tkinter: Canvas().keys returns empty strings.

[terryjreedy]

BPO	26177
Nosy	@terryjreedy, @serhiy-storchaka
Files	tkinter_keys.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2016-03-09.08:55:11.946>
created_at = <Date 2016-01-22.06:51:29.409>
labels = ['expert-tkinter', 'type-bug', 'library']
title = 'tkinter: Canvas().keys returns empty strings.'
updated_at = <Date 2016-03-09.08:55:11.946>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2016-03-09.08:55:11.946>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2016-03-09.08:55:11.946>
closer = 'serhiy.storchaka'
components = ['Library (Lib)', 'Tkinter']
creation = <Date 2016-01-22.06:51:29.409>
creator = 'terry.reedy'
dependencies = []
files = ['41686']
hgrepos = []
issue_num = 26177
keywords = ['patch']
message_count = 6.0
messages = ['258789', '258790', '258791', '258843', '258856', '261407']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26177'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[terryjreedy]

tkinter wraps tk widgets as option-value mappings with subscripting and .keys(). The latter does not work for Canvas (subscripting does). First tested with 3.5.1, Win 10. First Text versus Canvas contrast below was confirmed with 2.7.11.

>>> import tkinter as tk
>>> tk.Text().keys()
['autoseparators', 'background', 'bd', 'bg', 'blockcursor', 'tabstyle', 
...
'takefocus', 'undo', 'width', 'wrap', 'xscrollcommand', 'yscrollcommand']
>>> tk.Canvas().keys()
['', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '']

Explanation: Widget classes inherit keys from Misc via BaseWidget and Widget. Misc.keys calls into tk.

    def keys(self):
        """Return a list of all resource names of this widget."""
        return [x[0][1:] for x in
                self.tk.splitlist(self.tk.call(self._w, 'configure'))]

The call returns different types for Text and Canvas.

>>> self = tk.Text()
>>> self.tk.call(self._w, 'configure')  # tuple of tuples
(('-autoseparators', 'autoSeparators', 'AutoSeparators', 1, 1),
 ('-background', 'background', 'Background', <border object: 'SystemWindow'>, 'SystemWindow'), <etc>)
>>> self = tk.Canvas()
>>> self.tk.call(self._w, 'configure')  # tcl list as single string
'{-background background Background SystemButtonFace SystemButtonFace} {-bd borderWidth} {-bg background} {-borderwidth borderWidth BorderWidth 0 0} ...'

This difference between widgets seems odd. A bug in tcl/tk/_tkinter?

tk.splitlist appears to return the Text tuple of tuples as is. It splits the Canvas string into a tuple of strings

('-background background Background SystemButtonFace SystemButtonFace', '-bd borderWidth', ... )

but does not split the inner strings. As a consequence, x[0] is the first character of each string ('{' as it turns out, though not relevant) and the slice [1:] of a single char string is always ''.

Possible remedy: tk.split also leaves tuple of tuples alone *and* splits the inner strings to a tuple.

>>> self.tk.split(self.tk.call(self._w, 'configure'))
(('-background', 'background', 'Background', 'SystemButtonFace', 'red'), ('-bd', 'borderWidth'), ...)

So a fix (if the different returns are not changed) is to change 'splitlines' to 'split'. With this change, keys (extracted as a function) gives the expected list.

>>> keys(tk.Canvas())
['background', 'bd', ..., 'yscrollincrement']

A test for Canvas().keys should be added somewhere.

[terryjreedy]

Bug came from https://stackoverflow.com/questions/34911069/tk-canvas-options-keys-list-empy. Bryan Oakley answered that tk.Canvas().configure().keys() does work, making sorted(x.configure.keys()) a replacement for x.keys that should work for all widgets.

[serhiy-storchaka]

split() has many warts, it shouldn't be used. It is enough just call splitlist() for elements of a list.

Proposed patch fixes Misc.keys(). It also adds tests for missed options (some of them are not explicitly documented).

[terryjreedy]

With patch applied to 3.5, tk.Canvas().keys() works and test_tk and other three files pass with -ugui.

Is avoiding splitlist calls when not needed worthwhile? Tests also pass with

    def keys(self):
        """Return a list of all resource names of this widget."""
        config = self.tk.call(self._w, 'configure')
        if type(config) == tuple:
            return [x[0][1:] for x in config]
        else:  # str
            splitlist = self.tk.splitlist
            return [splitlist(x)[0][1:] for x in
                splitlist(config)]

I am a bit curious why Canvas gives a different return. Is the special casing in _tkinter or tk itself? Otherwise, looks good to me.

[serhiy-storchaka]

Is avoiding splitlist calls when not needed worthwhile?

We can't guarantee that future release of Tk wouldn't return a list of strings. Calling splitlist() for a tuple is cheap (but looks cumbersome).

I am a bit curious why Canvas gives a different return. Is the special casing in _tkinter or tk itself?

Tk is full of such inconsistencies. A result for one widget can be a list of numbers, but for other it is a list of strings or a list of special Tcl objects. One method can return empty list, other returns empty string. One widgets truncate floating point parameters, others round them up or down.

I'll commit the patch after testing with Tk 8.5 and 8.4.

[python-dev]

New changeset d86891dee68a by Serhiy Storchaka in branch '3.5':
Issue bpo-26177: Fixed the keys() method for Canvas and Scrollbar widgets.
https://hg.python.org/cpython/rev/d86891dee68a

New changeset f0c895fb0374 by Serhiy Storchaka in branch '2.7':
Issue bpo-26177: Fixed the keys() method for Canvas and Scrollbar widgets.
https://hg.python.org/cpython/rev/f0c895fb0374

New changeset 3a0bc2efed26 by Serhiy Storchaka in branch 'default':
Issue bpo-26177: Fixed the keys() method for Canvas and Scrollbar widgets.
https://hg.python.org/cpython/rev/3a0bc2efed26