Mock file read in binary mode in Python 2.7 - python-2.7

Question

I'm trying to unit test the file reading in binary in Python 2.7. but failing miserably.
test_my_module:
def test_read_binary(self):
d = {
"offset": [1, 4, 5, 7],
"bytes" : [3, 1, 2, 3]
}
spec = pd.DataFrame(d)
input_file = 'input.bin'
fake_file = StringIO.StringIO('foo\nbar\n')
with mock.patch('my_module.open', return_value=fake_file, create=True):
full_spec = self._tt.read_binary(spec, input_file)
print full_spec
my_module:
def read_binary(self, spec, input_file):
with open(input_file, 'rb') as infile:
def read_data(row):
print 'ROW:', row
infile.seek(row['offset'], 0)
value = infile.read(row['bytes'])
print value
return value
spec['raw'] = spec.apply(read_data, axis=1)
return spec
This produces the following exception:
AttributeError: StringIO instance has no attribute 'exit'
Any suggestions how to go about it?

Answer 1

For reference, I solved the problem using the following function:
def mock_open(self, mock_o=None, data=None):
file_spec = file
if mock_o is None:
mock_o = mock.MagicMock(spec=file_spec)
handle = mock.MagicMock(spec=file_spec)
handle.write.return_value = None
if data is None:
handle.__enter__.return_value = handle
else:
handle.__enter__.return_value = data
mock_o.return_value = handle
return mock_o
def test_read_binary(self):
data = b'\x68\x65\x6c\x6c\x6f\x77\x6f\x72\x6c\x64' # helloworld
d = {
'offset': [0, 4, 5, 7],
'bytes': [3, 1, 2, 3]
}
spec = pd.DataFrame(d)
input_file = 'input.bin'
expected_list = [b'\x68\x65\x6c', b'\x6f', b'\x77\x6f', b'\x72\x6c\x64']
m = self.mock_open(data=StringIO.StringIO(data))
with mock.patch('__builtin__.open', m, create=True):
full_spec = self._tt.read_binary(spec, input_file)
self.assertListEqual(expected_list, full_spec['raw'].tolist())