python-peps / pep-0417.txt

PEP: 417
Title: Including mock in the Standard Library
Version: $Revision$
Last-Modified: $Date$
Author: Michael Foord <>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Mar-2012
Python-Version: 3.3
Post-History: 12-Mar-2012


This PEP proposes adding the mock [1]_ testing library
to the Python standard library as ``unittest.mock``.


Creating mock objects for testing is a common need in Python.
Many developers create ad-hoc mocks, as needed, in their test
suites. This is currently what we do in the Python test suite,
where a standardised mock object library would be helpful.

There are many mock object libraries available for Python [2]_.
Of these, mock is overwhelmingly the most popular, with as many
downloads on PyPI as the other mocking libraries combined.

An advantage of mock is that it is a mocking library and not a
framework. It provides a configurable and flexible mock object,
without being opinionated about how you write your tests. The
mock api is now well battle-tested and stable.

mock also handles safely monkeypatching and unmonkeypatching
objects during the scope of a test. This is hard to do safely
and many developers / projects mimic this functionality
(often incorrectly). A standardised way to do this, handling
the complexity of patching in the presence of the descriptor
protocol (etc) is useful. People are asking for a "patch" [3]_
feature to unittest. Doing this via mock.patch is preferable
to re-implementing part of this functionality in unittest.


Addition of mock to the Python standard library was discussed
and agreed to at the Python Language Summit 2012.

Open Issues

As of release 0.8, which is current at the time of writing,
mock is compatible with Python 2.4-3.2. Moving into the Python
standard library will allow for the removal of some Python 2
specific "compatibility hacks".

mock 0.8 introduced a new feature, "auto-speccing", obsoletes
an older mock feature called "mocksignature". The
"mocksignature" functionality can be removed from mock
altogether prior to inclusion.


.. [1] `mock library on PyPI <>`_
.. [2]
.. [3]


This document has been placed in the public domain.

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