test_tk test_widgets.ScaleTest fails with Tk 8.6.10

[felixonmars]

BPO	41306
Nosy	@terryjreedy, @ned-deily, @ambv, @serhiy-storchaka, @felixonmars, @miss-islington, @E-Paine
PRs	bpo-41306: Allow scale value to not be rounded #21715 [3.8] bpo-41306: Allow scale value to not be rounded (GH-21715) #22596 [3.9] bpo-41306: Allow scale value to not be rounded (GH-21715) #22597

^{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 = None
closed_at = <Date 2020-10-08.17:16:27.973>
created_at = <Date 2020-07-15.19:12:33.815>
labels = ['tests', '3.8', 'expert-tkinter', '3.9', '3.10']
title = 'test_tk test_widgets.ScaleTest fails with Tk 8.6.10'
updated_at = <Date 2020-10-08.17:16:27.972>
user = 'https://github.com/felixonmars'

bugs.python.org fields:

activity = <Date 2020-10-08.17:16:27.972>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-08.17:16:27.973>
closer = 'pablogsal'
components = ['Tests', 'Tkinter']
creation = <Date 2020-07-15.19:12:33.815>
creator = 'felixonmars'
dependencies = []
files = []
hgrepos = []
issue_num = 41306
keywords = ['patch']
message_count = 11.0
messages = ['373710', '373767', '373768', '373780', '374531', '374733', '374741', '377509', '378250', '378254', '378257']
nosy_count = 8.0
nosy_names = ['terry.reedy', 'gpolo', 'ned.deily', 'lukasz.langa', 'serhiy.storchaka', 'felixonmars', 'miss-islington', 'epaine']
pr_nums = ['21715', '22596', '22597']
priority = 'critical'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41306'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[felixonmars]

test_from (tkinter.test.test_tkinter.test_widgets.ScaleTest) is currently failing on Arch Linux, and at least another place: https://python-build-standalone.readthedocs.io/en/latest/status.html

The error looks like:

======================================================================
FAIL: test_from (tkinter.test.test_tkinter.test_widgets.ScaleTest)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/usr/lib/python3.8/tkinter/test/test_tkinter/test_widgets.py", line 939, in test_from
    self.checkFloatParam(widget, 'from', 100, 14.9, 15.1, conv=float_round)
  File "/usr/lib/python3.8/tkinter/test/widget_tests.py", line 106, in checkFloatParam
    self.checkParam(widget, name, value, conv=conv, **kwargs)
  File "/usr/lib/python3.8/tkinter/test/widget_tests.py", line 64, in checkParam
    self.assertEqual2(widget.cget(name), expected, eq=eq)
  File "/usr/lib/python3.8/tkinter/test/widget_tests.py", line 47, in assertEqual2
    self.assertEqual(actual, expected, msg)
AssertionError: 14.9 != 15.0

It's the only failure in the current Python 3.8.4 release's test suite here. Also adding Python 3.9 and 3.10 as I am able to reproduce it on master too.

[ned-deily]

What version of Tk is being used? It's in the output from:
python3 -m test.pythoninfo
or however you invoke python.

[felixonmars]

tkinter.TCL_VERSION: 8.6
tkinter.TK_VERSION: 8.6
tkinter.info_patchlevel: 8.6.10

It's always reproducible in either a real desktop or Xvfb with arbitrary resolution etc as far as I have tested.

[ned-deily]

I can reproduce that test failure with Tk 8.6.10 on macOS, along with a few others. I believe various small things have changed in Tk somewhere between 8.6.8 and 8.6.10 in ways that affect some of the Python Tk gui tests, like test_tk and test_ttk_guionly. Some of our tests are pretty brittle with regard to Tk behavior; they captured how a particular version of Tk worked without necessarily a guarantee on the Tk side that the observed behavior wouldn't change. Someone needs to go through and make the tests work with all recent versions of Tk 8.6.x and someone should also get at least some buildbots running with 8.6.10 and really running the gui tests.

[ned-deily]

This issue should be fixed for upcoming releases. Nosying Łukasz for consideration of "release blocker" status.