import mock

mock_func = mock.Mock()
mock_func.return_value = 42
print mock_func(6, 9)

mock_func.assert_called_with(6, 9)

mock_func.assert_called_with(6, 7)

mock_class = mock.NonCallableMock()
mock_class.some_method.return_value = 42
print mock_class.some_method(6, 9)

mock_class.some_method.assert_called_once_with(6, 9)

mock_func_w_side_effect = mock.Mock()
mock_func_w_side_effect.side_effect = ValueError('Wrong!')
mock_func_w_side_effect()

mock_func_w_side_effect.side_effect = lambda x, y: x + y
mock_func_w_side_effect('spam', 'SPAM')

import json
def func_with_json(d):
    return json.dumps(d)

d = {'a': 1, 'b': [2, 3]}  # a simple input for func_with_json

with mock.patch('json.dumps') as mock_dumps:
    mock_dumps.return_value = 'JSON'
    r = func_with_json(d)
    assert r == 'JSON'
    mock_dumps.assert_called_once_with(d)

print json.dumps(d)

@mock.patch('json.dumps')
def test_func_with_json(mock_dumps):
    mock_dumps.return_value = 'JSON'
    r = func_with_json({'c': {'d': [4]}})
    assert r == 'JSON'
    mock_dumps.assert_called_once_with(d)  # whoops, we didn't pass in d, this should fail.
test_func_with_json()