IDLE configdialog: reduce multiple references to Var names

[terryjreedy]

BPO	27388
Nosy	@terryjreedy, @csabella
Dependencies	bpo-22115: Add new methods to trace Tkinter variables

^{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 2016-06-26.02:23:35.928>
labels = ['expert-IDLE', 'type-feature', '3.10']
title = 'IDLE configdialog: reduce multiple references to Var names'
updated_at = <Date 2020-06-07.22:02:26.163>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-06-07.22:02:26.163>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2016-06-26.02:23:35.928>
creator = 'terry.reedy'
dependencies = ['22115']
files = []
hgrepos = []
issue_num = 27388
keywords = []
message_count = 3.0
messages = ['269271', '296497', '296608']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27388'
versions = ['Python 3.10']

[terryjreedy]

ConfigDialog uses nearl 20 tkinter Vars, and I expect to add more. The name of each appears in at least 6 places.

In one of 'create tab frame' methods:

1. create the Var
2. use the Var with at least 1 tk widget
   In AttachVarCallbacks method:
   3, 4. attach trace with callback with derived name
   self.varname.trace_add('write', self.VarChanged_varname)
   In remove_var_callbacks method (with delete call already factored out):
3. include Var in for loop tuple, to remove callback with
   "var.trace_remove('write', var.trace_info()[0][1])"
   In callback definition:
4. def VarChanged_varname

The above uses the new trace method names that will be introduced in bpo-22115, which I expect will be applied first. (I might also replace
'VarChanged_' with 'var_changed_' or something shorter, but this is not relevant here.)

I propose to consolidate 1, 3, & 4 by replacing AttachVarCallbacks with

def add_traced_var(varname):
    """Create var; bind to varname, append to list, and trace with callback.

varname must match use in caller and callback def.
"""
var = tk.StringVar(self.root)
setattr(self, varname, var)
self.vars.append(var)
cbsuffix = 'font' if varname.startswith('font') else varname
var.trace_add('write', self.getattr('VarChanged_' + cbsuffix))

The cbsuffix local takes care of the complication that the 3 'fontWxyz' vars need the same callback. In any Var are not StringVars, add vartype parameter.

Each tab frame creation method would call add_traced_var within a varname loop. The current varname tuple for 5. would be replaced with self.vars, which should then be cleared. This will leave 3 name occurrences that must match instead of 6.

---
I believe 1-6 is complete. Varnames are translated to config item names within the callbacks, so do not have to match. For instance,

def VarChanged_spaceNum(self, *params):
    value = self.spaceNum.get()
    self.AddChangedItem('main', 'Indent', 'num-spaces', value)

translated 'spaceNum' to 'num-spaces'. On the other hand, I an not sure why the difference. For back compatibility, config names are fixed. The varnames, like method names, are internal to configdialog and can lowercased (PEP-8) and otherwised changed.
---

For testing, I will embed the add and delete methods in a dummy class with a couple of dummy callbacks. Then add, introspect, delete, and introspect again.

[csabella]

Created a pull request for this - "The varnames, like method names, are internal to configdialog and can lowercased (PEP-8) and otherwised changed."

[terryjreedy]

I moved PR 2307 to a new issue, bpo-30728 Modernize configdialog. My comment that internal Var names "can lowercased (PEP-8) and otherwised changed" was anticipating such an issue. The point for this issue was to contrast Var names with the cross-version config names that we must not change.

I don't expect anyone else to fully understand this without a couple of hours of code study.

Once bpo-22115 was merged, I could have gone ahead. Since I did not, I should now wait for at least part of PR 2307 and bpo-30728 since the patch for this would break parts of the existing new patch. I could work on a new test though.