How to assert output with nosetest/unittest in python?

by

I use this context manager to capture output. It ultimately uses the same technique as some of the other answers by temporarily replacing sys.stdout. I prefer the context manager because it wraps all the bookkeeping into a single function, so I don’t have to re-write any try-finally code, and I don’t have to write setup and teardown functions just for this.

import sys
from contextlib import contextmanager
from StringIO import StringIO

@contextmanager
def captured_output():
    new_out, new_err = StringIO(), StringIO()
    old_out, old_err = sys.stdout, sys.stderr
    try:
        sys.stdout, sys.stderr = new_out, new_err
        yield sys.stdout, sys.stderr
    finally:
        sys.stdout, sys.stderr = old_out, old_err

Use it like this:

with captured_output() as (out, err):
    foo()
# This can go inside or outside the `with` block
output = out.getvalue().strip()
self.assertEqual(output, 'hello world!')

Furthermore, since the original output state is restored upon exiting the with block, we can set up a second capture block in the same function as the first one, which isn’t possible using setup and teardown functions, and gets wordy when writing try-finally blocks manually. That ability came in handy when the goal of a test was to compare the results of two functions relative to each other rather than to some precomputed value.

More Related Contents:

  1. Why doesn’t Python run my bash code?
  2. PATH issue with pytest ‘ImportError: No module named YadaYadaYada’
  3. python mocking raw input in unittests
  4. Run / Debug a Django application’s UnitTests from the mouse right click context menu in PyCharm Community Edition?
  5. Can I patch a Python decorator before it wraps a function?
  6. Running a single test from unittest.TestCase via the command line
  7. Python unit test with base and sub class
  8. Python unittest passing arguments
  9. Getting Python’s unittest results in a tearDown() method
  10. Why python mock patch doesn’t work?
  11. Python Mock object with method called multiple times
  12. How can I create a ramdisk in Python?
  13. How to test Python 3.4 asyncio code?
  14. Continuing in Python’s unittest when an assertion fails
  15. How can I disable logging while running unit tests in Python Django?
  16. Mock Python’s built in print function
  17. Can Python’s unittest test in parallel, like nose can?
  18. Mocking Functions Using Python Mock
  19. Python mock Patch os.environ and return value
  20. Customizing unittest.mock.mock_open for iteration
  21. Running a specific test case in Django when your app has a tests directory
  22. What is the difference between setUp() and setUpClass() in Python unittest?
  23. Outputting data from unit test in Python
  24. Unit testing a python app that uses the requests library
  25. Python Unit Testing: Automatically Running the Debugger when a test fails
  26. Pass a Python unittest if an exception isn’t raised
  27. pydev breakpoints not working
  28. How can I specify a database for Django Tests to use instead of having it build it every time?
  29. Can I “fake” a package (or at least a module) in python for testing purposes?
  30. How to mock a readonly property with mock?

Leave a Comment