On 25 Jul, 02:25 pm, jamesbroadh...@gmail.com wrote:
Hey all -
I've recently started working with the 'mock' library in our trial
tests,
and am looking for some best-practice advice. I'm really just starting
to
get used to the library, so it might well have functionality that I'm
unaware of or am misusing.
I very quickly ran into a problem where I mistakenly returned a Mock()
in
the place of a deferred, causing the asserts in callbacks to not be
called,
and for the test to spuriously pass.
To address this problem, I suggest you get into the habit of watching
your unit tests fail in the expected way before you make the necessary
implementation changes to make them pass.
This is only one of an unlimited number of ways your unit tests can be
buggy. It might be tempting to try to fix the test runner to prevent
you from ever falling into this trap again - and who knows, it might
even be a good idea.
However, if you run your tests and see them fail in the way you expected
them to fail before you write the code that makes them pass, then you
will be sure to avoid the many, many, many *other* pitfalls that have
nothing to do with accidentally returning the wrong object.
This is just one of the attractions of test-driven development for me.
Jean-Paul
A basic example:
def test_foo():
d = Mock()
def check_result(res):
self.assertEqual(res.code, expected) # never called
d.addCallback(check_result)
return d # Mock is truthy, test passes
This occurred where I was mocking some internals of the class under
test;
something like the below
A slightly more believable example:
== myclass.py ==
def some_function(...):
d = self.authenticate()
d.addCallback(foo) # foo never called
d.addErrback(bar) # bar never called
return d
== test_myclass.py ==
def setUp(self):
self.resource.authenticate = Mock(return_value=Mock())
def test_foo():
d = self.resource.some_function
def check_result(res): # never called
self.assertEqual(res.code, expected)
d.addCallback(check_result)
return d # test passes
Currently, I'm experimenting with wrapping Mock instantiations by
defining
common deferred methods on them in advance; this approach would
eventually
lead to extending Mock itself with this functionality.
def nonDeferredMock():
m = Mock()
def notimpl(*args, **kwargs):
raise NotImplementedError('You treated a Mock like a Deferred!')
m.addCallback = notimpl
m.addErrback = notimpl
m.addBoth = notimpl
m.addCallbacks = notimpl
return m
Another approach might be extending TestCase to check that return
values
are always not Mock objects.
Does anyone on the list have experience with this? Obviously, this only
happens when mistakes are made when writing tests, but I'd rather have
confidence that when my tests pass, that they've passed for the right
reasons.
Another antipattern that I've come across has been:
resource.mymethod = Mock(return_value=defer.succeed(None))
which works fine for tests in which mymethod() is called once, but
always
returns the same deferred object if multiple calls are made. What would
be
a better approach?
Cheers-
James
_______________________________________________
Twisted-Python mailing list
Twisted-Python@twistedmatrix.com
http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
