Writing Custom Matchers

PyHamcrest comes bundled with lots of useful matchers, but you’ll probably find that you need to create your own from time to time to fit your testing needs. This commonly occurs when you find a fragment of code that tests the same set of properties over and over again (and in different tests), and you want to bundle the fragment into a single assertion. By writing your own matcher you’ll eliminate code duplication and make your tests more readable!

Let’s write our own matcher for testing if a calendar date falls on a Saturday. This is the test we want to write:

def testDateIsOnASaturday(self):
    d = datetime.date(2008, 04, 26)
    assert_that(d, is_(on_a_saturday()))

And here’s the implementation:

from hamcrest.core.base_matcher import BaseMatcher
from hamcrest.core.helpers.hasmethod import hasmethod

class IsGivenDayOfWeek(BaseMatcher):

    def __init__(self, day):
        self.day = day  # Monday is 0, Sunday is 6

    def _matches(self, item):
        if not hasmethod(item, 'weekday'):
            return False
        return item.weekday() == self.day

    def describe_to(self, description):
        day_as_string = ['Monday', 'Tuesday', 'Wednesday', 'Thursday',
                         'Friday', 'Saturday', 'Sunday']
        description.append_text('calendar date falling on ')    \
                   .append_text(day_as_string[self.day])

def on_a_saturday():
    return IsGivenDayOfWeek(5)

For our Matcher implementation we implement the _matches method - which calls the weekday method after confirming that the argument (which may not be a date) has such a method - and the describe_to method - which is used to produce a failure message when a test fails. Here’s an example of how the failure message looks:

assert_that(datetime.date(2008, 04, 06), is_(on_a_saturday()))

fails with the message:

AssertionError:
Expected: is calendar date falling on Saturday
     got: <2008-04-06>

Let’s say this matcher is saved in a module named isgivendayofweek. We could use it in our test by importing the factory function on_a_saturday:

from hamcrest import *
import unittest
from isgivendayofweek import on_a_saturday

class DateTest(unittest.TestCase):
    def testDateIsOnASaturday(self):
        d = datetime.date(2008, 04, 26)
        assert_that(d, is_(on_a_saturday()))

if __name__ == '__main__':
    unittest.main()

Even though the on_a_saturday function creates a new matcher each time it is called, you should not assume this is the only usage pattern for your matcher. Therefore you should make sure your matcher is stateless, so a single instance can be reused between matches.

If you need your matcher to provide more details in case of a mismatch, you can override the describe_mismatch method. For example, if we added this describe_mismatch implementation to our IsGivenDayOfWeek matcher:

def describe_mismatch(self, item, mismatch_description):
    day_as_string = ['Monday', 'Tuesday', 'Wednesday', 'Thursday',
                     'Friday', 'Saturday', 'Sunday']
    mismatch_description.append_text('got ') \
                        .append_description_of(item) \
                        .append_text(' which is a ') \
                        .append_text(day_as_string[item.weekday()])

Our matcher would now give the message:

AssertionError:
Expected: is calendar date falling on Saturday
     got: <2008-04-06> which is a Sunday

Occasionally, you might also need your matcher to explain why it matched successfully. For example, if your matcher is wrapped by a is_not matcher, the is_not matcher can only explain its mismatches by understanding why your matcher succeeded. In this case, your matcher can implement describe_match.