Idle: debugger source line highlighting fails again

[terryjreedy]

BPO	25254
Nosy	@terryjreedy, @roseman

^{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 2015-10-12.02:01:21.574>
created_at = <Date 2015-09-28.03:16:01.025>
labels = ['expert-IDLE', 'type-bug', 'invalid']
title = 'Idle: debugger source line highlighting fails again'
updated_at = <Date 2015-10-12.16:45:10.174>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2015-10-12.16:45:10.174>
actor = 'markroseman'
assignee = 'none'
closed = True
closed_date = <Date 2015-10-12.02:01:21.574>
closer = 'terry.reedy'
components = ['IDLE']
creation = <Date 2015-09-28.03:16:01.025>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 25254
keywords = []
message_count = 4.0
messages = ['251746', '251982', '252834', '252872']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'markroseman']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25254'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[terryjreedy]

Re-occurrence of bpo-14146. On that issue, msg225380, 2014-08-15, I said "Debugger source line highlighting works fine." I am really sure it was true when I posted previously, 2013-05-29. In 3.4.3, Feb 24 2015, it is not working. Ditto for 2.7.10 and 3.5.0, released subsequently, and repository versions of 2.7 and 3.4, The problem is not the fix in bpo-14146, which I tested separately before replacing it in bpo-24972.

[roseman]

Was the 'source' checkbox in the debugger checked?

[terryjreedy]

No. Turning it on, I discovered why off is default. When it on, it sometimes does surprising things on IDLE. For instance, print('x') is (usually) a builtin and can not be stepped into. However, it calls sys.stdout.write('x'+'\n'). Under Idle, user process sys.stdout is PyShell.Pseudofile, so when source is checked, 'step' on a line with print opens the file and highlights lines in the write method.

Step should ask before opening a file, and/or a 'smart step' is needed.

[roseman]

FYI, the new debugger UI has an option to only show highlights in already open files (i.e. don't open new ones)