how to write django test meant to fail?

Question

If you’re expecting Thing(name=”1234″) to raise an exception, there are two ways to deal with this.

One is to use Django’s assertRaises (actually from unittest/unittest2):

def mytest(self):
    self.assertRaises(FooException, Thing, name="1234")

This fails unless Thing(name=”1234″) raises a FooException error. Another way is to catch the expected exception and raise one if it doesn’t happen, like this:

def mytest(self):
    try:
        thing = Thing(name="1234")
        self.fail("your message here")
    except FooException:
        pass

Obviously, replace the FooException with the one you expect to get from creating the object with too long a string. ValidationError?

A third option (as of Python 2.7) is to use assertRaises as a context manager, which makes for cleaner, more readable code:

def mytest(self):
    with self.assertRaises(FooException):
        thing = Thing(name="1234")

Sadly, this doesn’t allow for custom test failure messages, so document your tests well. See https://hg.python.org/cpython/file/2.7/Lib/unittest/case.py#l97 for more details.

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