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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 :py:meth:`~hamcrest.core.base_matcher.BaseMatcher._matches` method - which calls the weekday method after confirming that the argument (which may not be a date) has such a method - and the :py:func:`~hamcrest.core.selfdescribing.SelfDescribing.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.