pyhamcrest-toolbox 0.0.3

A library that makes writing pyhamcrest matchers easier and more fun.

PyHamcrest Toolbox

Important note

This project is at the stage of initial setup, it is to go live very soon. Don’t use it just yet.

PyHamcrest is a great thing for writing reusable tests, but sometimes writing your own matchers may be a bit of a chore. This project aims at making this task easy, fast and even fun (for want of a better word).

To reiterate the obvious, a test should always do all the checks it has to do, and even if some of them fail, the rest should still be run. That way you will get a better idea of what’s gone wrong with your code, and you will waste less time fixing the first of the checks only to find the the second one is still failing, and that means that you should have fixed the first one in a different way.

So, instead of this:

def test_the_holy_grail():
    the_holy_grail = seek_the_holy_grail()
    assert_that(the_holy_grail.is_holy(), is_(True))
    assert_that(the_holy_grail.depth, greater_than(5))
    assert_that(the_holy_grail.width, greater_than(6))
    assert_that(the_holy_grail.height, greater_than(7))

this should be written:

def test_the_holy_grail():
    the_holy_grail = seek_the_holy_grail()
    assert_that(
        the_holy_grail,
        is_holy()\
            .with_depth(greater_than(5)).\
            .with_width(greater_than(6)).\
            .with_height(greater_than(7))
    )

The second one, however, requires writing your own matchers. With this toolbox, it is easy.

All you have to do is to write your is_holy matcher that inherits from the MultisegmentMatcher as the backbone. Then you write individual matchers for each of the holy grail properties enhancing them with the MatcherPluginMixin, and you register them with the is_holy matcher.

So, this is your is_holy matcher:

class IsHolyMatcher(MultisegmentMatcher):
    def __init__(self):
        super().__init__()

def is_holy():
    return IsHolyMatcher()

And that’s it. You don’t have to override the usual matcher methods. Everything will be done by the parent class. However, it doesn’t do any matching yet, so we need to write the plugins. Let’s start with the actual holyness:

class HolynessMatcher(BaseMatcher, MatcherPluginMixin):
    def __init__(is_holy=True):
        super().__init__()
        self.is_holy = is_holy

    def component_matches(self, item):
        # the item will be a grail
        return self.is_holy == item.is_holy()

    def describe_to(self, description):
        description.append_text(
            "A grail which is {}holy".format("" if self.is_holy else "not "))

    def describe_component_mismatch(self, item, mismatch_description):
        mismatch_description.append_text(
            "The grail was {}holy".format("" if item.is_holy() else "not "))

And then you register it with the main matcher:

class IsHolyMatcher(MultisegmentMatcher):
    def __init__(self, is_holy):
        super().__init__()
        self.register(HolynessMatcher(is_holy))

def holy(is_holy):
    return IsHolyMatcher(is_holy)

Of course, you could write that HolynessMatcher logic in your IsHolyMatcher, but if we have the power of plugins, then why not use it?

For now, we only have this bit: assert_that(the_grail, is_holy()), and not the .with_width(...) stuff. So let’s write it. I won’t go through the process of writing the plugin for the width as it is rather straightforward, but here’s how you register it with the main matcher:

class IsHolyMatcher(MultisegmentMatcher):
    def __init__(self, is_holy):
        super().__init__()
        self.register(HolynessMatcher(is_holy))

    def with_width(self, value):
        self.register(GrailWidthMatcher(value))
        return self

def holy(is_holy):
    return IsHolyMatcher(is_holy)

Now you can do the is_holy().with_width(greater_than(5)) stuff. Note that you have to return ``self`` from the plugin registering methods, as (a) you might want to chain them, and (b) the result of the chain still needs to be a matcher.