pytest-mock

This plugin installs a mocker fixture which is a thin-wrapper around the patching API provided by the excellent mock package, but with the benefit of not having to worry about undoing patches at the end of a test:

def test_unix_fs(mocker):
    mocker.patch('os.remove')
    UnixFS.rm('file')
    os.remove.assert_called_once_with('file')

Usage

The mocker fixture has the same API as mock.patch, supporting the same arguments:

def test_foo(mocker):
    # all valid calls
    mocker.patch('os.remove')
    mocker.patch.object(os, 'listdir', autospec=True)
    mocked_isfile = mocker.patch('os.path.isfile')

The supported methods are:

• mocker.patch: see http://www.voidspace.org.uk/python/mock/patch.html#patch.
• mocker.patch.object: see http://www.voidspace.org.uk/python/mock/patch.html#patch-object.
• mocker.patch.multiple: see http://www.voidspace.org.uk/python/mock/patch.html#patch-multiple.
• mocker.patch.dict: see http://www.voidspace.org.uk/python/mock/patch.html#patch-dict.
• mocker.stopall(): stops all active patches at this point.

You can also access Mock and MagicMock directly using from mocker fixture:

def test_feature(mocker):
    ret = [mocker.Mock(return_value=True), mocker.Mock(return_value=True)]
    mocker.patch('mylib.func', side_effect=ret)

New in version 0.5

Spy

New in version 0.6

The spy acts exactly like the original method in all cases, except it allows use of mock features with it, like retrieving call count.

def test_spy(mocker):
    class Foo(object):
        def bar(self):
            return 42

    foo = Foo()
    mocker.spy(foo, 'bar')
    assert foo.bar() == 42
    assert foo.bar.call_count == 1

Stub

New in version 0.6

The stub is a mock object that accepts any arguments and is useful to test callbacks, for instance.

def test_stub(mocker):
    def foo(on_something):
        on_something('foo', 'bar')

    stub = mocker.stub()

    foo(stub)
    stub.assert_called_once_with('foo', 'bar')

Note

Prior to version 0.4.0, the mocker fixture was named mock. This was changed because naming the fixture mock conflicts with the actual mock module, which made using it awkward when access to both the module and the plugin were required within a test.

The old fixture mock still works, but its use is discouraged and will be removed in version 1.0.

Requirements

• Python 2.6+, Python 3.2+
• pytest
• mock (for Python < 3.3)

Install

Install using pip:

$ pip install pytest-mock

Changelog

Please consult releases.

Why bother with a plugin?

There are a number of different patch usages in the standard mock API, but IMHO they don't scale very well when you have more than one or two patches to apply.

It may lead to an excessive nesting of with statements, breaking the flow of the test:

import mock

def test_unix_fs():
    with mock.patch('os.remove'):
        UnixFS.rm('file')
        os.remove.assert_called_once_with('file')

        with mock.patch('os.listdir'):
            assert UnixFS.ls('dir') == expected
            # ...

    with mock.patch('shutil.copy'):
        UnixFS.cp('src', 'dst')
        # ...

One can use patch as a decorator to improve the flow of the test:

@mock.patch('os.remove')
@mock.patch('os.listdir')
@mock.patch('shutil.copy')
def test_unix_fs(mocked_copy, mocked_listdir, mocked_remove):
    UnixFS.rm('file')
    os.remove.assert_called_once_with('file')

    assert UnixFS.ls('dir') == expected
    # ...

    UnixFS.cp('src', 'dst')
    # ...

But this poses a few disadvantages:

• test functions must receive the mock objects as parameter, even if you don't plan to access them directly; also, order depends on the order of the decorated patch functions;
• receiving the mocks as parameters doesn't mix nicely with pytest's approach of naming fixtures as parameters, or pytest.mark.parametrize;
• you can't easily undo the mocking during the test execution;