tutorial.rst

PyHamcrest Tutorial

Introduction

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~unittest.TestCase.assertEqual method, we'll use PyHamcrest's :py~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__':
    unittest.main()

The :py~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~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~unittest.TestCase.assert_ method of :pyunittest.TestCase, but because it's a standalone function, it offers greater flexibility in test writing.

A tour of common matchers

PyHamcrest comes with a library of useful matchers. Here are some of the most important ones.

• Core
  • :py~hamcrest.core.core.isanything.anything - always matches, useful if you don't care what the object under test is
  • :py~hamcrest.core.core.described_as.described_as - decorator to adding custom failure description
  • :py~hamcrest.core.core.is_.is_ - decorator to improve readability -see sugar, below
• Logical
  • :py~hamcrest.core.core.allof.all_of - matches if all matchers match, short circuits (like Python and)
  • :py~hamcrest.core.core.anyof.any_of - matches if any matchers match, short circuits (like Python or)
  • :py~hamcrest.core.core.isnot.is_not - matches if the wrapped matcher doesn't match and vice versa
• Object
  • :py~hamcrest.core.core.isequal.equal_to - tests object equality using ==
  • :py~hamcrest.library.object.haslength.has_length - tests whether len(item) satisfies a given matcher
  • :py~hamcrest.library.object.hasstring.has_string - tests whether str(item) satisfies another matcher
  • :py~hamcrest.core.core.isinstanceof.instance_of - tests type
  • :py~hamcrest.core.core.isnone.none, :py~hamcrest.core.core.isnone.not_none - tests for None
  • :py~hamcrest.core.core.issame.same_instance - tests object identity
• Collection
  • :py~hamcrest.library.collection.isdict_containing.has_entry, :py~hamcrest.library.collection.isdict_containingkey.has_key, :py~hamcrest.library.collection.isdict_containingvalue.has_value -tests that a dictionary contains an entry, key or value
  • :py~hamcrest.library.collection.issequence_containing.has_item, :py~hamcrest.library.collection.issequence_containing.has_items -tests that a sequence contains elements
• Number
  • :py~hamcrest.library.number.iscloseto.close_to - tests that numeric values are close to a given value
  • :py~hamcrest.library.number.ordering_comparison.greater_than, :py~hamcrest.library.number.ordering_comparison.greater_than_or_equal_to, :py~hamcrest.library.number.ordering_comparison.less_than, :py~hamcrest.library.number.ordering_comparison.less_than_or_equal_to
    • tests ordering
• Text
  • :py~hamcrest.library.text.isequal_ignoring_case.equal_to_ignoring_case
    • tests string equality ignoring case
  • :py~hamcrest.library.text.isequal_ignoring_whitespace.equal_to_ignoring_whitespace
    • test strings equality ignoring differences in runs of whitespace
  • :py~hamcrest.library.text.stringcontains.contains_string, :py~hamcrest.library.text.stringendswith.ends_with, :py~hamcrest.library.text.stringstartswith.starts_with - tests string matching

Syntactic sugar

PyHamcrest strives to make your tests as readable as possible. For example, the :py~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~hamcrest.core.core.isequal.equal_to. But if the argument is a type, it is wrapped with :py~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))