[Python-ideas] SealedMock proposal for unittest.mock

[Mario Corchero]

Hello Everyone!

First time writing to python-ideas.

*Overview*
Add a new mock class within the mock module
<https://github.com/python/cpython/blob/master/Lib/unittest/mock.py>,
SealedMock (or RestrictedMock) that allows to restrict in a dynamic and
recursive way the addition of attributes to it. The new class just defines
a special attribute "sealed" which once set to True the behaviour of
automatically creating mocks is blocked, as well as for all its "submocks".
See sealedmock
<https://github.com/mariocj89/sealedmock/blob/master/README.md>. Don't
focus on the implementation, it is ugly, it would be much simpler within
*mock.py*.

*Rationale*
Inspired by GMock
<https://github.com/google/googletest/tree/master/googlemock> RestrictedMock,
SealedMock aims to allow the developer to define a narrow interface to the
mock that defines what the mocks allows to be called on.
The feature of mocks returning mocks by default is extremely useful but not
always desired. Quite often you rely on it only at the time you are writing
the test but you want it to be disabled at the time the mock is passed into
your code, that is what SealedMock aims to address.

This solution also prevents user errors when mocking incorrect paths or
having typos when calling attributes/methods of the mock.
We have tried it internally in our company and it gives quite a nicer user
experience for many use cases, specially for new users of mock as it helps
out when you mock the wrong path.

*Alternatives*

   - Using auto_spec/spec is a possible solution but removes flexibility
   and is rather painful to write for each of the mocks and submocks being
   used.
   - Leaving it outside of the mock.py as it is not interesting enough. I
   am fine with it :) just proposing it in case you think otherwise.
   - Make it part of the standard Mock base class. Works for me, but I'd
   concerned on how can we do it in a backward compatible way. (Say someone is
   mocking something that has a "sealed" attribute already).

Let me know what you think, happy to open a enhancement in
https://bugs.python.org/ and send a PR.

Regards,
Mario
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