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From Kevin Sweeney <kswee...@twitter.com.INVALID>
Subject Re: [DISCUSS] Deprecate use of mock.patch?
Date Thu, 20 Nov 2014 18:11:28 GMT
Brian and Bill, do you have any thoughts here?

On Wednesday, November 19, 2014, Joshua Cohen <jcohen@twopensource.com>
wrote:

> That's a fairly contrived example though, as most Java classes don't expose
> a mechanism for injecting mocks.
>
> I think points #3 and #4 make the strongest case for why we'd want to avoid
> this (though I don't believe we currently run tests in parallel so #4 is
> more of a nice-to-have). If it's generally limited to additional method
> args (and the review pointed at here is an outlier due to the way
> @app.command works) I'm on board.
>
> On Wed, Nov 19, 2014 at 4:59 PM, Kevin Sweeney
> <ksweeney@twitter.com.invalid
> > wrote:
>
> > I don't think this a dynamic vs static language thing - if this were Java
> > we could just as easily do
> >
> > public class MyTest {
> >   private PrintStream oldSystemOut;
> >
> >   @Before
> >   public void setUp() {
> >     oldSystemOut = System.out;
> >     System.setOut(mockPrintStream);
> >   }
> >
> >   @After
> >   public void tearDown() {
> >     System.setOut(oldSystemOut);
> >   }
> > }
> >
> > in our tests but that's mutable global state and makes our code brittle
> for
> > exactly the same 4 reasons as above.
> >
> > I don't think there's anything about Python that makes mutable global
> state
> > an inherently better idea.
> >
> > On Wed, Nov 19, 2014 at 4:33 PM, Joshua Cohen <jcohen@twopensource.com
> <javascript:;>>
> > wrote:
> >
> > > I'm actually waffling on my stance. I tried to frame it mentally in the
> > > context of how I'd handle the same use case in javascript (a language
> I'm
> > > much more comfortable with than Python), and I'd have a hard time
> arguing
> > > in favor of a similar mechanism there (e.g. in node.js patching require
> > to
> > > globally inject a mock, ugh). I think my objection in the case of this
> > > review is more due @app.command forcing us to delegate the injection to
> > an
> > > extracted method.
> > >
> > > I tried to get a feel for what was more "pythonic" by searching for
> uses
> > of
> > > @mock.patch versus an injected mock from create_autospec on GitHub. The
> > > former was definitely more common, but there's plenty of cases of the
> > > latter, and they looked cleaned enough to me. I'm leaning towards
> lifting
> > > my objection, though I'd love to hear thoughts from folks who have more
> > > python experience (e.g. Brian, Joe) as well.
> > >
> > > On Wed, Nov 19, 2014 at 4:20 PM, Maxim Khutornenko <maxim@apache.org
> <javascript:;>>
> > > wrote:
> > >
> > > > I am with Joshua on this. The increased complexity and indirection is
> > > > not the tradeoff I would fight for. The lack of coverage is a bigger
> > > > problem in my opinion. Requiring patch-less unit tests may just
> > > > encourage a proliferation of un-pythonic patterns and more obstacles
> > > > on the way to improving our python code coverage.
> > > >
> > > > On Wed, Nov 19, 2014 at 4:02 PM, Joshua Cohen <
> jcohen@twopensource.com <javascript:;>
> > >
> > > > wrote:
> > > > > As I mentioned in that review, I'm not sold on the idea. I feel
> that
> > it
> > > > > leads to a fair amount of extra code that exists solely to support
> > > > testing.
> > > > > One of the nice things about dynamic languages is they allow you
to
> > > avoid
> > > > > this sort of boilerplate. The main problem in that review is just
> > that
> > > > the
> > > > > wrong thing was being patched (instead of patching build_properties
> > > > > directly we should have patched from_pex). That being said, I can't
> > > > > actually argue against your points, they're all valid, I'm just not
> > > > > convinced that they're worth the tradeoff ;).
> > > > >
> > > > > On Wed, Nov 19, 2014 at 3:38 PM, Kevin Sweeney <kevints@apache.org
> <javascript:;>>
> > > > wrote:
> > > > >
> > > > >> Hi folks,
> > > > >>
> > > > >> I wanted to have a discussion about the usage of mock.patch in
our
> > > unit
> > > > >> tests. In my opinion its use is a code smell versus writing
> > production
> > > > code
> > > > >> to have explicit injection points for test dependencies.
> > > > >>
> > > > >> The review at https://reviews.apache.org/r/28250/ is a good
> example
> > > of
> > > > why
> > > > >> I think the patch approach is brittle: in this case the test
code
> > > > patched
> > > > >> out
> > > > >>
> > > > >>   @patch('twitter.common.python.pex.PexInfo.build_properties',
> > > > >> new_callable=PropertyMock)
> > > > >>
> > > > >>
> > > > >> but the production code didn't actually call
> > PexInfo.build_properties
> > > -
> > > > >> rather it called PexInfo.from_pex, which usually returns a PexInfo
> > > > >> instance, which has a build_properties property. So this test
only
> > > > worked
> > > > >> when PexInfo.from_pex(sys.argv[0]) actually returned a valid
> > PexInfo,
> > > > but
> > > > >> due to the way patching works there's no way to ensure that the
> > > > >> function-under-test was the one calling the mocked method.
> > > > >>
> > > > >> In my opinion an explicit injection approach is preferable, via
> the
> > > use
> > > > of
> > > > >> defaulted private method parameters like:
> > > > >>
> > > > >> def production_function(arg1, arg2, ..., _print=print,
> > > > >> _from_pex=PexInfo.from_pex):
> > > > >>    # use _print, _from_pex
> > > > >>
> > > > >> or
> > > > >>
> > > > >> class ProductionClass(object):
> > > > >>   def __init__(self, arg1, arg2, ..., _print=print,
> > > > >> _from_pex=PexInfo.from_pex):
> > > > >>     self._print = _print
> > > > >>     self._from_pex = _from_pex
> > > > >>
> > > > >>   def method(self):
> > > > >>     # Use self._print, self._from_pex, etc
> > > > >>
> > > > >> Then tests can explicitly replace the dependencies of the
> > > > unit-under-test
> > > > >> with mocks:
> > > > >>
> > > > >> def test_production_function():
> > > > >>   mock_print = create_autospec(print, spec_set=True)
> > > > >>   mock_pex_info = create_autospec(PexInfo, instance=True,
> > > spec_set=True)
> > > > >>   mock_from_pex = create_autospec(PexInfo.from_pex, spec_set=True,
> > > > >> return_value=mock_pex_info)
> > > > >>   mock_pex_info.build_properties = {}
> > > > >>
> > > > >>
> > > > >>   production_function(arg1, arg2, ..., _print=mock_print,
> > > > >> _from_pex=mock_from_pex)
> > > > >>   # or
> > > > >>   prod = ProductionClass(arg1, arg2, ..., _print=mock_print,
> > > > >> _from_pex=mock_from_pex)
> > > > >>   prod.method()
> > > > >>
> > > > >>   mock_print.assert_called_once_with("Some string")
> > > > >>   # other assertions about what the class-under-test did with
the
> > > mocked
> > > > >> deps
> > > > >>
> > > > >> There are a good number of properties that this allows:
> > > > >>
> > > > >> 1. No unused dependencies - if a parameter is unused the linter
> will
> > > > still
> > > > >> complain
> > > > >> 2. Can't mock out something the unit-under-test isn't actually
> > using -
> > > > if
> > > > >> you give a kwarg parameter that isn't defined the test will raise
> a
> > > > >> TypeError
> > > > >> 3. No action-at-a-distance - you can't mock the
> > > > dependency-of-a-dependency
> > > > >> (in this case PexInfo.build_properties instead of
> PexInfo.from_pex)
> > > > >> 4. Thread-safety - patch is not thread-safe as it's temporarily
> > > > replacing
> > > > >> global state for the duration of the test.
> > > > >>
> > > > >> I'd like to propose that we consider use of mock.patch in our
> tests
> > to
> > > > be a
> > > > >> code smell that should be refactored to use explicit injection
at
> > our
> > > > >> earliest convenience
> > > > >>
> > > >
> > >
> >
> >
> >
> > --
> > Kevin Sweeney
> > @kts
> >
>


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