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A tour of common matchers
Hamcrest comes with a library of useful matchers. Here are some of the most important ones.
anything - always matches, useful if you don't care what the object under test is
described_as - decorator to adding custom failure description
is_ - decorator to improve readability - see "Syntactic sugar", below
all_of - matches if all matchers match, short circuits (like Python &&)
any_of - matches if any matchers match, short circuits (like Python ||)
is_not - matches if the wrapped matcher doesn't match and vice versa
equal_to - test object equality using ==
has_length - test whether len(item) satisfies another matcher
has_string - test whether str(item) satisfies another matcher
instance_of - test type
none, not_none - test for None
same_instance - test object identity
has_entry, has_key, has_value - test a dictionary contains an entry, key or value
has_item, has_items - test a sequence contains elements
close_to - test numeric values are close to a given value
greater_than, greater_than_or_equal_to, less_than, less_than_or_equal_to - test ordering
equal_to_ignoring_case - test string equality ignoring case
equal_to_ignoring_whitespace - test string equality ignoring differences in runs of whitespace
contains_string, ends_with, starts_with - test string matching
Syntactic sugar
Hamcrest strives to make your tests as readable as possible. For example, the is_ matcher is a wrapper that doesn't
add any extra behavior to the underlying matcher. The following assertions are all equivalent:
assert_that(theBiscuit, equal_to(myBiscuit))
assert_that(theBiscuit, is_(equal_to(myBiscuit)))
assert_that(theBiscuit, is_(myBiscuit))
The last form is allowed since is_(value) wraps non-matcher arguments with equal_to.
Writing custom matchers
Hamcrest 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 string is comprised of only digits. This is the test we want to write:
def testStringIsOnlyDigits(self):
assert_that('17', is_(only_digits()))
And here's the implementation:
from hamcrest.core.base_matcher import BaseMatcher
from hamcrest.core.internal.hasmethod import hasmethod
class IsStringOfDigits(BaseMatcher):
def _matches(self, item):
if not hasmethod(item, 'isdigit'):
return False
return item.isdigit()
def describe_to(self, description):
description.append_text('string of digits')
only_digits = IsStringOfDigits
For our Matcher implementation we implement the _matches method - which calls the isdigit string method after confirming
that the argument (which may not be a string) 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('17b', is_(only_digits()))
fails with the message
Expected: is string of digits
got: '17b'
The factory function only_digits is defined as an alias because no further logic is needed, but other matchers may work best
with an actual factory. We import only_digits to use the matcher in our test:
from hamcrest import *
import unittest
from hamcrest_local.isstringofdigits import only_digits
class StringTest(unittest.TestCase):
def testStringIsOnlyDigits(self):
assert_that('17', is_(only_digits()))
if __name__ == '__main__':
Even though the only_digits 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.