bdb.Bdb.format_stack_entry: checks for obsolete __args__

[blueyed]

BPO	39649
Nosy	@gvanrossum, @terryjreedy, @blueyed, @miss-islington
PRs	bpo-39649: Remove obsolete check for __args__ in bdb.Bdb.format_stack_entry #18531 [3.8] bpo-39649: Remove obsolete check for __args__ in bdb.Bdb.format_stack_entry (GH-18531) #18635 [3.7] bpo-39649: Remove obsolete check for __args__ in bdb.Bdb.format_stack_entry (GH-18531) #18636

^{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 2020-02-24.04:53:40.210>
created_at = <Date 2020-02-16.13:41:44.635>
labels = ['type-bug', 'library', '3.9']
title = 'bdb.Bdb.format_stack_entry: checks for obsolete __args__'
updated_at = <Date 2020-02-24.04:53:40.210>
user = 'https://github.com/blueyed'

bugs.python.org fields:

activity = <Date 2020-02-24.04:53:40.210>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-02-24.04:53:40.210>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2020-02-16.13:41:44.635>
creator = 'blueyed'
dependencies = []
files = []
hgrepos = []
issue_num = 39649
keywords = ['patch']
message_count = 6.0
messages = ['362070', '362109', '362111', '362561', '362563', '362564']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'terry.reedy', 'blueyed', 'miss-islington']
pr_nums = ['18531', '18635', '18636']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39649'
versions = ['Python 3.9']

[blueyed]

It does:

        if '__args__' in frame.f_locals:
            args = frame.f_locals['__args__']
        else:
            args = None
        if args:
            s += reprlib.repr(args)
        else:
            s += '()'

However that appears to be wrong/unnecessary since the following likely, but
maybe also others:

commit 75bb54c3d8
Author: Guido van Rossum <guido@python.org>
Date:   Mon Sep 28 15:33:38 1998 +0000

    Don't set a local variable named __args__; this feature no longer
    works and Greg Ward just reported a problem it caused...

diff --git a/Lib/bdb.py b/Lib/bdb.py
index 3ca25adbbf..f2cf4caa36 100644
--- a/Lib/bdb.py
+++ b/Lib/bdb.py
@@ -46,7 +46,7 @@ def dispatch_line(self, frame):
                return self.trace_dispatch

            def dispatch_call(self, frame, arg):
    -               frame.f_locals['__args__'] = arg
    +               # XXX 'arg' is no longer used
                    if self.botframe is None:
                            # First call of dispatch since reset()
                            self.botframe = frame

Code ref:

cpython/Lib/bdb.py

Lines 551 to 558 in 1ed6161

	if '__args__' in frame.f_locals:
	args = frame.f_locals['__args__']
	else:
	args = None
	if args:
	s += reprlib.repr(args)
	else:
	s += '()'

.

So it should either get removed, or likely be replaced with actually displaying
the args.

For this the part could be factored out of do_args maybe, adjusting it for
handling non-current frames.

Of course somebody might inject/set __args__ still (I've thought about doing that initially for pdb++, but will rather re-implement/override format_stack_entry instead), so support for this could be kept additionally.