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PyHamcrest Tutorial


PyHamcrest is a framework for writing matcher objects, allowing you to declaratively define "match" rules. There are a number of situations where matchers are invaluable, such as UI validation, or data filtering, but it is in the area of writing flexible tests that matchers are most commonly used. This tutorial shows you how to use PyHamcrest for unit testing.

When writing tests it is sometimes difficult to get the balance right between overspecifying the test (and making it brittle to changes), and not specifying enough (making the test less valuable since it continues to pass even when the thing being tested is broken). Having a tool that allows you to pick out precisely the aspect under test and describe the values it should have, to a controlled level of precision, helps greatly in writing tests that are "just right." Such tests fail when the behavior of the aspect under test deviates from the expected behavior, yet continue to pass when minor, unrelated changes to the behaviour are made.

My first PyHamcrest test

We'll start by writing a very simple PyUnit test, but instead of using PyUnit's :py:meth:`~unittest.TestCase.assertEqual` method, we'll use PyHamcrest's :py:func:`~hamcrest.core.matcher_assert.assert_that` construct and the standard set of matchers:

from hamcrest import *
import unittest

class BiscuitTest(unittest.TestCase):
    def testEquals(self):
        theBiscuit = Biscuit('Ginger')
        myBiscuit = Biscuit('Ginger')
        assert_that(theBiscuit, equal_to(myBiscuit))

if __name__ == '__main__':

The :py:func:`~hamcrest.core.matcher_assert.assert_that` function is a stylized sentence for making a test assertion. In this example, the subject of the assertion is the object theBiscuit, which is the first method parameter. The second method parameter is a matcher for Biscuit objects, here a matcher that checks one object is equal to another using the Python == operator. The test passes since the Biscuit class defines an __eq__ method.

If you have more than one assertion in your test you can include an identifier for the tested value in the assertion:

assert_that(theBiscuit.getChocolateChipCount(), equal_to(10), 'chocolate chips')
assert_that(theBiscuit.getHazelnutCount(), equal_to(3), 'hazelnuts')

As a convenience, :py:func:`~hamcrest.core.matcher_assert.assert_that` can also be used to verify a boolean condition:

assert_that(theBiscuit.isCooked(), 'cooked')

This is equivalent to the :py:meth:`~unittest.TestCase.assert_` method of :py:class:`unittest.TestCase`, but because it's a standalone function, it offers greater flexibility in test writing.

Asserting Exceptions

There's a utility function and matcher available to help you test that your code has the expected behavior in situations where it should raise an exception. The :py:func:`~hamcrest.core.core.raises.calling` function wraps a callable, and then allows you to set arguments to be used in a call to the wrapped callable. This, together with the :py:func:`~hamcrest.core.core.raises.raises` matcher lets you assert that calling a method with certain arguments causes an exception to be thrown. It is also possible to provide a regular expression pattern to the :py:func:`~hamcrest.core.core.raises.raises` matcher allowing you assure that the right issue was found:

assert_that(calling(parse, bad_data), raises(ValueError))

assert_that(calling(translate).with_(curse_words), raises(LanguageError, "\w+very naughty"))

assert_that(broken_function, raises(Exception))

# This will fail and complain that 23 is not callable
# assert_that(23, raises(IOError))

Predefined matchers

PyHamcrest comes with a library of useful matchers:

The arguments for many of these matchers accept not just a matching value, but another matcher, so matchers can be composed for greater flexibility. For example, only_contains(less_than(5)) will match any sequence where every item is less than 5.

Syntactic sugar

PyHamcrest strives to make your tests as readable as possible. For example, the :py:func:`~hamcrest.core.core.is_.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 most non-matcher arguments with :py:func:`~hamcrest.core.core.isequal.equal_to`. But if the argument is a type, it is wrapped with :py:func:`~hamcrest.core.core.isinstanceof.instance_of`, so the following are also equivalent:

assert_that(theBiscuit, instance_of(Biscuit))
assert_that(theBiscuit, is_(instance_of(Biscuit)))
assert_that(theBiscuit, is_(Biscuit))

(Note that PyHamcrest's is_ matcher is unrelated to Python's is operator. The matcher for object identity is :py:func:`~hamcrest.core.core.issame.same_instance`.)