Why you should use PyHamcrest in testing

Table of Contents

Motivation

I’ve written about Software Testing Concepts in general and also about pytest in particular.

Now I want to introduce you to a nice little add-on which makes your testing -hopefully- easier.

Hamcrest

Hamcrest is around for a couple of years. I’ve used it together with JUnit back in the days. It had a bit of an on-off-relationship with JUnit, because it was first independent, than in 2012 included into JUnit 4 and finally excluded again from JUnit 5.

But nevertheless: the library became so popular that it was ported to Python in 2010.

fun fact: ‘Hamcrest’ is an anagram of ‘matchers’

PyHamcrest

Installation

pip install pyhamcrest

Now we can import the module:

from hamcrest import *

Replace assertEqual with matcher

If you have a test case like this

def test_digit(self):
    pattern = Pattern()
    pattern.starts_with(DIGIT)
    p = re.compile(pattern.pattern)
    self.assertEqual(p.findall('1'), ['1'])

you can replace the assertEqual with assert_that and a matcher equal_to

def test_digit(self):
    pattern = Pattern()
    pattern.starts_with(DIGIT)
    p = re.compile(pattern.pattern)
    assert_that(p.findall('1'), equal_to(['1']))

This isn’t that impressive, is it?

More Complex Matchers

The fun begins, when we want to compare e.g. lists

def test_compare_list_of_items(self):
    actual_list_of_items = ['apple', 'banana', 'cherry']
    expected_list_of_items = ['apple']

    assert_that(actual_list_of_items, has_items(*expected_list_of_items))

This runs smoothly. If we add beans to the list

expected_list_of_items = ['apple', 'beans']

we get the AssertionError:

> assert_that(actual_list_of_items, has_items(*expected_list_of_items))
E AssertionError: 
E Expected: (a sequence containing 'apple' and a sequence containing 'beans')
E but: a sequence containing 'beans' was <['apple', 'banana', 'cherry']>

This output is very readable.