>>> with patch('__main__.open', mock_open(read_data='bibble')) as m:
...     with open('foo') as h:
...         result = h.read()
...
>>> m.assert_called_once_with('foo')
>>> assert result == 'bibble'

 from mock import patch, mock_open
from StringIO import StringIO
 
filedata = """This is line1
and this is line2
and here is line3
"""
filedata_for_line = StringIO(filedata)
 
with patch('__main__.open', mock_open(read_data=filedata_for_line)) as m:
    with open('foo') as f:
        for line in f:
            print line
with patch('__main__.open', mock_open(read_data=filedata.split('\n'))) as m:
    with open('foo') as f:
        for line in f:
            print line

 for line in f

 for line in f.readlines()

 def create_file_mock(data):                                                     
     mock_obj = mock.mock_open(read_data=data)                                   
     mock_obj.return_value.__iter__.return_value = data.splitlines()             
     return mock_obj
 
with mock.patch('__builtin__.open', create_file_mock(filedata)) as m:
     with open('foo') as f:                                              
           for line in f:                                                  
                 print line