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That's a child issue of #23883, created to propose a patch fixing configparser module's __all__ list.

Looks good to me

The reason we specifically omitted Error was two-fold:
- the name "Error" is very generic and during a star-import might easily shadow some other class of the same name;
- Error is only a base class for exceptions raised by configparser and as such isn't part of the public API. You can see the same behavior in concurrent.futures for example. However, now I noticed configparser.Error is listed in the documentation so the assertion that "it's not public API" is effectively incorrect.
So I'm torn a little here. On the one hand, it's nice to add Error for completeness. On the other hand, is this change solving a real issue or just satisfying your inner librarian? The reason we have to ask ourselves this question is that this change bears a small risk of breaking user code that was working before. Take a look at this example:
```
from wave import *
from configparser import *
cfg = ConfigParser()
cfg.read('appconfig.ini')
try:
 with Wave_read(cfg['samples']['sad_trombone']) as wav:
 n = wav.getnframes()
 frames = wav.readframes(n)
except Error as e:
 print("Invalid sample:", e)
except KeyError as e:
 print("Can't find {!r} in the config".format(str(e)))
else:
 play_sound(frames)
```
Sure, it's bad code but the point is: it was working before and had a decent error handling strategy. With the change in __all__, it will just crash because wave.Error was never caught.
Is this likely to happen? I don't think so. Knowing my luck, will it happen to somebody? Yeah. So the question remains: why do we want Error in __all__ in the first place? Is it worth it?

My personal opinion is to include all public APIs. Names that are omitted from __all__ may not come up in pydoc, and it is surprising when I use "import * " in the interactive interpreter to play with a module and there is something missing.
To mitigate the risk of breaking code, I have been maintaining a list of the modules affected at <https://docs.python.org/3.6/whatsnew/3.6.html#changes-in-the-python-api>, which warns that extra symbols will be imported in 3.6.
On the other hand, there are other cases where people wanted to exclude APIs from __all__; I pointed out two at <https://bugs.python.org/issue26632#msg266117>.

Łukasz, Martin - I'm not sure how to proceed here, of course both ways out are reasonable. I'd be happy to provide (however small) patch for either one (adding Error to __all__ or just adding test for __all__). :)
My inner librarian would of course either push the Error to fully become part of public API or change its name to non-public _Error or something, but I'd much rather rely on you in this regard.

I'm still not convinced this change is *useful*. The list of incompatibilities in Python 3.6 in whatsnew is going to be used retrospectively by people already affected by a production issue, googling for an explanation as to what went wrong.
I like the idea of adding a unit test that clarifies the omission is deliberate at this point.

That's completely fine for me. I'm attaching the patch that just adds test for __all__, then. :)

New changeset 739528288996 by Martin Panter in branch 'default':
Issue #27106: Add test for configparser.__all__
https://hg.python.org/cpython/rev/739528288996