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configparser.ConfigParser.clean() always fails:
 >>> cfg = configparser.ConfigParser()
 >>> if not hasattr(configparser.ConfigParser, 'clear'):
 ... configparser.ConfigParser.clear = configparser_clear_compat
 >>> cfg.clear() #doctest: +ELLIPSIS
 Traceback (most recent call last):
 ...
 ValueError: Cannot remove the default section.
configparser.ConfigParser.update() overwrites all sections except DEFAULT instead of updating them.
See attached test file-

New changeset dc5adc08f1a8 by Łukasz Langa in branch '3.2':
Fixes `parser.clean()` reported in issue #16820.
http://hg.python.org/cpython/rev/dc5adc08f1a8
New changeset 4fc2fea807e6 by Łukasz Langa in branch '3.3':
Merged `parser.clean()` fix (issue #16820) from 3.2.
http://hg.python.org/cpython/rev/4fc2fea807e6
New changeset c6f9bc5a0cf1 by Łukasz Langa in branch 'default':
Merged `parser.clean()` fix (issue #16820) from 3.2 through 3.3.
http://hg.python.org/cpython/rev/c6f9bc5a0cf1 

Thanks for your report, Wolfgang. The `clean()` method is now fixed. The
`update()` situation is more complicated, however.
The mapping protocol defines that
 mapping.update({'a': 'b', 'c': 'd'})
and
 mapping.update(a='b', c='d')
are equivalent to
 mapping['a'] = 'b'
 mapping['c'] = 'd'
For `configparser` we decided that setting a section on a parser is
a destructive operation (e.g. it overwrites previous options). If the user
finds this undesirable, she can either use `parser['section'].update()` or
`parser.read_dict()` like you suggested.
The bug here is that __setitem__ for the DEFAULTSECT should also clear options
previously set on this section.

New changeset 459a23083b66 by Łukasz Langa in branch '3.3':
Fixes `__setitem__` on parser['DEFAULT'] reported in issue #16820.
http://hg.python.org/cpython/rev/459a23083b66
New changeset f6fb5a5748f0 by Łukasz Langa in branch 'default':
Merged `parser['DEFAULT'].__setitem__` fix (issue #16820) from 3.3.
http://hg.python.org/cpython/rev/f6fb5a5748f0 

For the record, the bug that caused the following to be equivalent:
 parser['DEFAULT'] = {'option': 'value'} 
 parser['DEFAULT'].update({'option': 'value'})
has been fixed for 3.3.1+ only. This way it's going to be easier for users to reason about the fix ("it was broken in 3.2.0 - 3.3.0").
Note that the bug only affected __setitem__ on the default section.

Thanks, works for me.
I only noted the discrepancy and did not give it much thought.
I will just implement a merge method on top of read_dict.
That gives me all options that could be desired :).
However, after implementing the entire compatibility layer, I found one more issue:
Using self.remove_section() changes the section order during an update.
I would prefer that the section be cleared instead of removed in order to preserve the section order. Since OrderedDict does indeed preserve the key order during update, I think that this does not violate the Mapping Protocol.
If this is not desired, just go ahead and close the issue.
See also attached example bug_configparser_update_order.py:
OrderedDict does not change the order of keys upon .update():
 >>> od = OrderedDict((('section1', {}), ('section2', {})))
 >>> list(od.keys())
 ['section1', 'section2']
 >>> od.update((('section1', {}), ('section3', {})))
 >>> list(od.keys())
 ['section1', 'section2', 'section3']
But ConfigParser changes the order of sections upon .update():
 >>> cfg = configparser.ConfigParser()
 >>> cfg.update((('section1', {}), ('section2', {})))
 >>> cfg.sections()
 ['section1', 'section2']
 >>> cfg.update((('section1', {}), ('section3', {})))
 >>> cfg.sections()
 ['section2', 'section1', 'section3']

This is a fair point. Stay tuned.

New changeset f580342b63d8 by Łukasz Langa in branch '3.3':
configparser: preserve section order when using `__setitem__` (issue #16820)
http://hg.python.org/cpython/rev/f580342b63d8
New changeset a758f561a280 by Łukasz Langa in branch 'default':
Merged section order preservation fix when using `__setitem__` (issue #16820)
http://hg.python.org/cpython/rev/a758f561a280 

New changeset 6f0cee62f0c6 by Łukasz Langa in branch '3.2':
configparser: preserve section order when using `__setitem__` (issue #16820)
http://hg.python.org/cpython/rev/6f0cee62f0c6
New changeset 2f5320497017 by Łukasz Langa in branch '3.3':
Null-merged 3.2 section order preservation fix when using `__setitem__` (issue
http://hg.python.org/cpython/rev/2f5320497017
New changeset 27b698395d35 by Łukasz Langa in branch 'default':
Null-merged 3.2 section order preservation fix when using `__setitem__` (issue
http://hg.python.org/cpython/rev/27b698395d35 

Breaking section order when using `__setitem__` was a bug and as such was fixed in 3.2.4 and 3.3.1.
In the DEFAULTSECT case above someone could potentially use the broken behaviour as a feature so that change was only made for 3.3.1+. In the section order case there is no such controversy.

New changeset d5c45089df2d by Łukasz Langa in branch '3.3':
Misc/NEWS updated to tell about #14590 and #16820
http://hg.python.org/cpython/rev/d5c45089df2d
New changeset 7938847b2641 by Łukasz Langa in branch '3.2':
Misc/NEWS updated to tell about #16820
http://hg.python.org/cpython/rev/7938847b2641