IDLE: improve handling of int entry in settings dialog

[terryjreedy]

BPO	39472
Nosy	@terryjreedy, @csabella

^{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/terryjreedy'
closed_at = None
created_at = <Date 2020-01-27.23:34:59.719>
labels = ['expert-IDLE', 'type-bug', '3.9']
title = 'IDLE: improve handling of int entry in settings dialog'
updated_at = <Date 2020-01-28.12:32:21.133>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-01-28.12:32:21.133>
actor = 'cheryl.sabella'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-01-27.23:34:59.719>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 39472
keywords = []
message_count = 2.0
messages = ['360821', '360867']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39472'
versions = ['Python 3.9']

Linked PRs

• gh-83653: Do not save a blank int entry in IDLE Settings #152743

[terryjreedy]

Spinoff from bpo-31414, about int entry fields. It claims: Note: a deeper problem is attaching a tracer that get called with each keystroke. Using a StringVar avoids the error when the entry is blanked, but currently allows non-ints to be saved. A better solution would be to not do the auto tracing, but use a IntVar and only call var_changed when the user 'leaves' the box, after checking for a count in a sane range.

Verify claim and proposed solution.

[csabella]

With keeping it a StringVar, there is an issue where it writes a blank (ie, bad data) to the config file. Steps to reproduce:

• Go into config dialog and click 'General' tab.
• Delete value from 'Auto squeeze min line'
• Click apply or OK. A warning message appears in the terminal
  Warning: config.py - IdleConf.GetOption -
  invalid 'int' value for configuration option 'auto-squeeze-min-lines'
  from section 'PyShell': ''
• Close out of IDLE.
• Start IDLE again - the warning reappears, but on the General tab, it shows the original value.

The issue here is that the custom config-main.cfg is being written to (and then read back in) with the blank.

I picked the auto-squeezer because it has validation on it - validatecommand=self.digits_only.