Quick mock example showing how to simulate errors

gistfile1.py

>>> from unittest import mock
>>> import json
>>> m_o = mock.mock_open()
>>> m_o.side_effect = OSError('Insufficient voltage')
>>> json.load(m_o('foo.json', 'rb'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 952, in _mock_call
    raise effect
OSError: Insufficient voltage
>>> with mock.patch('__main__.open', m_o, create=True):
...     json.load(open('foo', 'rb'))
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 952, in _mock_call
    raise effect
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 952, in _mock_call
    raise effect
OSError: Insufficient voltage
>>> def read_the_json(filename):
...     with open(filename, 'r') as f:
...         return json.load(f)
... 
>>> read_the_json('foo.json')
[0, 1, 2]
>>> with patch('__main__.open', m_o, create=True):
...     read_the_json('foo.json')
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in read_the_json
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/unittest/mock.py", line 952, in _mock_call
    raise effect
OSError: Insufficient voltage