AttributeError: __exit__ when I try to mock out build in functions

Question

I'm currently trying to mock out the open() built in method in Python for a test. However, I always end up getting a crash and this resulting message:

   File "/opt/home/venv/lib/python2.7/site-packages/nose-1.3.0-py2.7.egg/nose/result.py", line 187, in _exc_info_to_string
return _TextTestResult._exc_info_to_string(self, err, test)
 File "/opt/python-2.7.3/lib/python2.7/unittest/result.py", line 164, in _exc_info_to_string
msgLines = traceback.format_exception(exctype, value, tb)
 File "/opt/python-2.7.3/lib/python2.7/traceback.py", line 141, in format_exception
list = list + format_tb(tb, limit)
 File "/opt/python-2.7.3/lib/python2.7/traceback.py", line 76, in format_tb
return format_list(extract_tb(tb, limit))
  File "/opt/python-2.7.3/lib/python2.7/traceback.py", line 101, in extract_tb
line = linecache.getline(filename, lineno, f.f_globals)
  File "/opt/home/venv/lib/python2.7/linecache.py", line 14, in getline
lines = getlines(filename, module_globals)
 File "/opt/home/venv/lib/python2.7/linecache.py", line 40, in getlines
return updatecache(filename, module_globals)
 File "/opt/home/venv/lib/python2.7/linecache.py", line 127, in updatecache
with open(fullname, 'rU') as fp:
AttributeError: __exit__

Here is my test code:

m = mox.Mox()
m.StubOutWithMock(__builtin__, 'open')
mock_file = m.CreateMock(__builtin__.file)

open(mox.IgnoreArg(), mox.IgnoreArg()).AndReturn(mock_file)
mock_file.write(mox.IgnoreArg()).MultipleTimes()
mock_file.close()

write_file_method()

Answer 1

__exit__ is the method that gets called when you try to close a file. Your mock file does not handle mock_file.close(), just open(). You'll need to mock the close method too.

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Edit:

On second though, why do you want to mock open? AFAIK you shouldn't be doing that method. The method under test should take an open stream (instead of a filename, for instance). In production code, clients are responsible for opening a file (e.g. pickle.dump). In your tests, you pass in a StringIO, or a mock object that supports writing.

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Edit 2: I would split your method in two and test each bit separately.

• creating a file: check that prior to calling this method the file does not exist, and it does after that. One might argue such a one-line method isn't worth testing.
• writing to a file: see above. Create a StringIO and write to that, so your tests can then verify the correct thing has been written.