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In Python 3.3.0b1, if the number of calls to a mock is, say, zero, and you call assert_called_once_with() on that mock, it will throw an exception which says "Expected to be called once. Called 0 times."
This is nice, but it would be nicer if the output message actually told the end user *what* it expected to be called once. I've supplied a patch and documentation update that makes this change by adding _mock_name to the output message using the variable interpolation method already being used in that message to output the call count. 
The result looks like this (for the case in the documentation): 
Python 3.3.0b1 (default:2ecdda96f970+, Jul 10 2012, 22:45:18) 
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from unittest.mock import Mock
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>>> mock = Mock(return_value=None)
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>>> mock('foo', bar='baz')
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>>> mock.assert_called_once_with('foo', bar='baz')
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>>> mock('foo', bar='baz')
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>>> mock.assert_called_once_with('foo', bar='baz')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/Users/brianj/Dropbox/Source/Python/cpython/Lib/unittest/mock.py", line 736, in assert_called_once_with
 raise AssertionError(msg)
AssertionError: Expected 'mock' to be called once. Called 2 times.
[103758 refs]
In the above case, the mock was never given a name, and wasn't instantiated with any particular target. Here's an example using a patch on 'time.time' to show the effect: 
Python 3.3.0b1 (default:2ecdda96f970+, Jul 10 2012, 22:45:18) 
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from unittest.mock import patch
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>>> with patch('time.time') as foo:
... foo.assert_called_once_with()
... 
Traceback (most recent call last):
 File "<stdin>", line 2, in <module>
 File "/Users/brianj/Dropbox/Source/Python/cpython/Lib/unittest/mock.py", line 736, in assert_called_once_with
 raise AssertionError(msg)
AssertionError: Expected 'time' to be called once. Called 0 times.
[106621 refs]

I have a minor suggestion. I would suggest--
+ msg = ("Expected %s to be called once. Called %s times." %
+ (repr(self._mock_name) or 'mock', self.call_count))
so that one can distinguish between self._mock_name not being defined and self._mock_name being equal to 'mock'.

Looks good, thanks.

Nice idea! A small question: Why print 'time' in the message and not 'time.time'?

Added a test to the patch supplied.

New changeset 70d43fedb2d7 by Michael Foord in branch 'default':
Closes issue 15323. Improve failure message of Mock.assert_called_once_with
http://hg.python.org/cpython/rev/70d43fedb2d7 

Cool change. I assume when you use mock.patch as decorator the mock name is set to the mocked object?

Eric: yes, patch sets the name on the mock objects it creates. A mock only knows its direct name, not its full 'dotted' name (i.e. a mock knows it is called 'time' but not that it is 'time.time'). It is possible (but harder) to deduce that name (follow the chain of parents), so for the moment this change will do.