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
.