Testing in Django is usually done using the
unittest framework, which comes with Python. You can also test using
doctest, with a little bit of work.
One advantage of doctest is that it’s super-easy to test for an exception: you just expect the traceback (which can be trimmed using
\n ... \n).
unittest.TestCase, you can do a similar thing, but it’s a little more work.
Basically, you want to temporarily replace
sys.stderr) with a
StringIO instance, and set it back after the block you care about has finished.
Python has had a nice feature for some time called Context Managers. These enable you to ensure that cleanup code will be run, regardless of what happens in the block.
The syntax for running code within a context manager is:
with context_manager(thing) as other: # Code we want to run # Can use 'other' in here.
One place that you can see this syntax, in the context of testing using unittest is to check a specific exception is raised when a function that uses keyword arguments, or a statement that is not a callable is executed:
class FooTest(TestCase): def test_one_way(self): self.assertRaises(ExceptionType, callable, arg1, arg2) def test_another_way(self): with self.assertRaises(ExceptionType): callable(arg1, arg2) # Could also be: # callable(arg1, arg2=arg2) # Or even: # foo = bar + baz # Which are not possible in the test_one_way call.
So, we could come up with a similar way of calling our code that we want to capture the
class BarTest(TestCase): def test_and_capture(self): with capture(callable, *args, **kwargs) as output: self.assertEquals("Expected output", output)
And the context manager:
import sys from cStringIO import StringIO from contextlib import contextmanager @contextmanager def capture(command, *args, **kwargs): out, sys.stdout = sys.stdout, StringIO() try: command(*args, **kwargs) sys.stdout.seek(0) yield sys.stdout.read() finally: sys.stdout = out
It’s simple enough to do the same with
Update: thanks to Justin Patrin for pointing out that we should wrap the command in a