[TIP] New alpha of Mock

Michael Foord fuzzyman at voidspace.org.uk
Mon Apr 13 09:00:01 PDT 2009

Hello all,

I've done a bit more work on Mock and would appreciate some feedback 
before I do a new release.

Specifically I've implemented simple object wrapping which I would like 
feedback on. The wrapping *doesn't* wrap return values (my thinking is 
that you use it when you need the call to really go ahead and the code 
you are testing needs the return value). It does wrap attribute access, 
so that method calls are wrapped and recorded. If you need to track the 
use of the returned values then use a normal mock.

The updated version is available at:


I've built a PDF of docs for the new version available from:


I'm afraid this version *isn't* a drop-in replacement, there are a few 
API changes. The major changes are:

* patch and patch_object are now context managers and can be used with 
the with statement (not backwards incompatible)
* ``side_effect`` is now called with the same arguments as the mock is 
called with and
  if returns a non-DEFAULT value that is automatically set as the 
mock.return_value. (Probably will require changes to your side_effect 
* The methods keyword argument to Mock has been removed and merged with 
spec. The spec
  argument can now be a list of methods or an object to take the spec 
from. (Just change 'methods' to 'spec' if you were using it)
* ``wraps`` keyword argument used for wrapping objects (and passing 
calls through to the wrapped object).
* Mock.reset renamed to Mock.reset_mock, as reset is a common API name.

The full list of changes (in the changelog file in the docs) are:

* Made DEFAULT part of the public api.
* Documentation built with Sphinx.
* A new 'create' keyword argument to patch and patch_object that allows 
them to patch
  (and unpatch) attributes that don't exist. (Potentially unsafe to use 
- it can allow
  you to have tests that pass when they are testing an API that doesn't 
exist - use at
  your own risk!)
* Nested patches may now be applied in a different order (created mocks 
  in the opposite order). This is actually a bugfix.
* patch and patch_object now take a spec keyword arguments. If spec is
  passed in as 'True' then the Mock created will take the object it is 
  as its spec object. If the object being replaced is a class, then the 
  value for the mock will also use the class as a spec.

The version in SVN (and the docs) have some support for mocking magic 
methods. I'm really not sure about the technique I've chosen - it's very 
clean but I'm not sure how useful it is. I'll probably rip this out 
before doing the release and create a branch with the magic method 
support in it. Feedback on that also welcomed.

All the best,

Michael Foord


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