Mock the open() builtin as a context manager.

mock_file_open.py

# http://www.voidspace.org.uk/python/mock/

from mock import MagicMock

open_name = '%s.open' % __name__
with patch(open_name, create=True) as mock_open:
    mock_file = MagicMock(spec=file)
    mock_file.read.return_value = 'hello'
    mock_open.return_value.__enter__.return_value = mock_file
    with open('/some/path', 'rU') as f:
        print f.read()