Customizing unittest.mock.mock_open for iteration

by

The mock_open() object does indeed not implement iteration.

If you are not using the file object as a context manager, you could use:

m = unittest.mock.MagicMock(name="open", spec=open)
m.return_value = iter(self.TEST_TEXT)

with unittest.mock.patch('builtins.open', m):

Now open() returns an iterator, something that can be directly iterated over just like a file object can be, and it’ll also work with next(). It can not, however, be used as a context manager.

You can combine this with mock_open() then provide a __iter__ and __next__ method on the return value, with the added benefit that mock_open() also adds the prerequisites for use as a context manager:

# Note: read_data must be a string!
m = unittest.mock.mock_open(read_data="".join(self.TEST_TEXT))
m.return_value.__iter__ = lambda self: self
m.return_value.__next__ = lambda self: next(iter(self.readline, ''))

The return value here is a MagicMock object specced from the file object (Python 2) or the in-memory file objects (Python 3), but only the read, write and __enter__ methods have been stubbed out.

The above doesn’t work in Python 2 because a) Python 2 expects next to exist, not __next__ and b) next is not treated as a special method in Mock (rightly so), so even if you renamed __next__ to next in the above example the type of the return value won’t have a next method. For most cases it would be enough to make the file object produced an iterable rather than an iterator with:

# Python 2!
m = mock.mock_open(read_data="".join(self.TEST_TEXT))
m.return_value.__iter__ = lambda self: iter(self.readline, '')

Any code that uses iter(fileobj) will then work (including a for loop).

There is a open issue in the Python tracker that aims to remedy this gap.

More Related Contents:

  1. Python mock multiple return values
  2. Python Mock object with method called multiple times
  3. Mock attributes in Python mock?
  4. python mocking raw input in unittests
  5. Can I patch a Python decorator before it wraps a function?
  6. Python Mocking a function from an imported module
  7. Why python mock patch doesn’t work?
  8. How can one mock/stub python module like urllib
  9. Mocking a class: Mock() or patch()?
  10. How can I mock user input (from “input” in 3.x, or “raw_input” in 2.x) for a unit test?
  11. Mock Python’s built in print function
  12. How to test or mock “if __name__ == ‘__main__'” contents
  13. Mocking Functions Using Python Mock
  14. Python mock Patch os.environ and return value
  15. Unit testing a python app that uses the requests library
  16. Django: How to create a model dynamically just for testing
  17. Can I “fake” a package (or at least a module) in python for testing purposes?
  18. How to mock a readonly property with mock?
  19. What is the fastest and pythonic way to generate a list [(0,0), (0,1), (0,2)…(0,100)]? [closed]
  20. List Element without iteration
  21. Is generator.next() visible in Python 3?
  22. How to test Python 3.4 asyncio code?
  23. Continuing in Python’s unittest when an assertion fails
  24. Mocking boto3 S3 client method Python
  25. What is your favorite Python mocking library? [closed]
  26. How can I collect the results of a repeated calculation in a list, dictionary etc. (make a copy of a list with each element modified)?
  27. What is the meaning of list[:] in this code? [duplicate]
  28. range(len(list)) or enumerate(list)? [duplicate]
  29. ValueError: no such test method in : runTest
  30. Do python’s variable length arguments (*args) expand a generator at function call time?

Leave a Comment