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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.

Adding a patch with a failing test for the issue.

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

Actually, this should also be documented.

Updating patch to include documentation changes.

I re-did this at https://github.com/python/cpython/pull/23424, let me know what you think. This is my first PR here, so I may have pooched something.

New changeset 2f2f9d0b5c96e68ed91cddd6673860ee752eb49c by Nick Crews in branch 'master':
bpo-15450: Allow subclassing of dircmp (GH-23424) (#23424)
https://github.com/python/cpython/commit/2f2f9d0b5c96e68ed91cddd6673860ee752eb49c