>>> import unittest.mock
>>> o = unittest.mock.mock_open(read_data="fred")
>>> f = o()
>>> f.read()
'fred'
>>> f.read()
''
>>> f.readlines()
[]
>>> f.readline()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/robertc/work/cpython/Lib/unittest/mock.py", line 935, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/robertc/work/cpython/Lib/unittest/mock.py", line 994, in _mock_call
    result = next(effect)
StopIteration