Allow dircmp.subdirs to behave well under subclassing

[cjerdonek]

BPO	15450
Nosy	@asvetlov, @mitar, @cjerdonek, @hynek, @NickCrews
PRs	bpo-15450: Allow subclassing of dircmp #5088 bpo-15450: Allow subclassing of filecmp.dircmp #23424
Files	issue-15450-failing-test.patch issue-15450-1.patch issue-15450-2.patch

^{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 2020-11-23.16:31:26.651>
created_at = <Date 2012-07-26.00:00:03.204>
labels = ['easy', 'type-feature', 'library', '3.10']
title = 'Allow dircmp.subdirs to behave well under subclassing'
updated_at = <Date 2020-11-23.16:31:26.650>
user = 'https://github.com/cjerdonek'

bugs.python.org fields:

activity = <Date 2020-11-23.16:31:26.650>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-23.16:31:26.651>
closer = 'asvetlov'
components = ['Library (Lib)']
creation = <Date 2012-07-26.00:00:03.204>
creator = 'chris.jerdonek'
dependencies = []
files = ['26531', '26532', '26551']
hgrepos = []
issue_num = 15450
keywords = ['patch', 'easy']
message_count = 7.0
messages = ['166443', '166503', '166505', '166507', '166618', '381490', '381677']
nosy_count = 7.0
nosy_names = ['eli.bendersky', 'asvetlov', 'mitar', 'chris.jerdonek', 'hynek', 'cbc', 'NickCrews']
pr_nums = ['5088', '23424']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue15450'
versions = ['Python 3.10']

[cjerdonek]

Currently, the subdirs attribute of filecmp.dircmp does not respect subclassing:

>>> from filecmp import dircmp
>>> class MyDirCmp(dircmp):
...   pass
... 
>>> my_dcmp = MyDirCmp('dir1', 'dir2')
>>> for item in my_dcmp.subdirs.values():
...   print(type(item))
...   break
... 
<class 'filecmp.dircmp'>

This is the only place where dircmp does not respect subclassing. It can be corrected here:

def phase4(self): # Find out differences between common subdirectories
    ...
        ...
        self.subdirs[x]  = dircmp(a_x, b_x, self.ignore, self.hide)

This would let one do things like override dircmp.report() and have dircmp.report_full_closure() behave as expected.

[cjerdonek]

Adding a patch with a failing test for the issue.

[cjerdonek]

Attaching full patch (though Misc/NEWS would need to be moved a new section is created for pre-alpha).

[cjerdonek]

Actually, this should also be documented.

[cjerdonek]

Updating patch to include documentation changes.

[NickCrews]

I re-did this at #23424, let me know what you think. This is my first PR here, so I may have pooched something.

[asvetlov]

New changeset 2f2f9d0 by Nick Crews in branch 'master':
bpo-15450: Allow subclassing of dircmp (GH-23424) (bpo-23424)
2f2f9d0