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![PyHamcrest](http://hamcrest.org/images/logo.jpg)



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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.



Installation

============



Hamcrest can be installed using the usual Python packaging tools. It depends on

distribute, but as long as you have a network connection when you install, the

installation process will take care of that for you.



My first PyHamcrest test

========================



We'll start by writing a very simple PyUnit test, but instead of using PyUnit's

``assertEqual`` method, we'll use PyHamcrest's ``assert_that`` construct and

the standard set of matchers::



```python

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 ``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::



```python

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

assert_that(theBiscuit.getHazelnutCount(), equal_to(3), 'hazelnuts')

```



As a convenience, assert_that can also be used to verify a boolean condition::



```python

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

```



This is equivalent to the ``assert_`` method of unittest.TestCase, but because

it's a standalone function, it offers greater flexibility in test writing.





Predefined matchers

===================



PyHamcrest comes with a library of useful matchers:



* Object



* ``equal_to`` - match equal object

* ``has_length`` - match ``len()``

* ``has_property`` - match value of property with given name

* ``has_properties`` - match an object that has all of the given

properties.

* ``has_string`` - match ``str()``

* ``instance_of`` - match object type

* ``none``, ``not_none`` - match ``None``, or not ``None``

* ``same_instance`` - match same object



* Number



* ``close_to`` - match number close to a given value

* ``greater_than``, ``greater_than_or_equal_to``, ``less_than``,

``less_than_or_equal_to`` - match numeric ordering



* Text



* ``contains_string`` - match part of a string

* ``ends_with`` - match the end of a string

* ``equal_to_ignoring_case`` - match the complete string but ignore case

* ``equal_to_ignoring_whitespace`` - match the complete string but ignore

extra whitespace

* ``matches_regexp`` - match a regular expression in a string

* ``starts_with`` - match the beginning of a string

* ``string_contains_in_order`` - match parts of a string, in relative order



* Logical



* ``all_of`` - ``and`` together all matchers

* ``any_of`` - ``or`` together all matchers

* ``anything`` - match anything, useful in composite matchers when you don't

care about a particular value

* ``is_not`` - negate the matcher



* Sequence



* ``contains`` - exactly match the entire sequence

* ``contains_inanyorder`` - match the entire sequence, but in any order

* ``has_item`` - match if given item appears in the sequence

* ``has_items`` - match if all given items appear in the sequence, in any

order

* ``is_in`` - match if item appears in the given sequence

* ``only_contains`` - match if sequence's items appear in given list



* Dictionary



* ``has_entries`` - match dictionary with list of key-value pairs

* ``has_entry`` - match dictionary containing a key-value pair

* ``has_key`` - match dictionary with a key

* ``has_value`` - match dictionary with a value



* Decorator



* ``described_as`` - give the matcher a custom failure description

* ``is_`` - decorator to improve readability - see `Syntactic sugar` below



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

``is_`` matcher is a wrapper that doesn't add any extra behavior to the

underlying matcher. The following assertions are all equivalent::



```python

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 ``equal_to``. But if the argument is a type, it is wrapped with

``instance_of``, so the following are also equivalent::



```python

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 ``same_instance``.*





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::



```python

def testDateIsOnASaturday(self):

d = datetime.date(2008, 04, 26)

assert_that(d, is_(on_a_saturday()))

```



And here's the implementation::



```python

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::



```python

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``::



```python

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.





More resources

==============



* [Documentation](http://readthedocs.org/docs/pyhamcrest/en/V1.7/)

* [Package](http://pypi.python.org/pypi/PyHamcrest)

* [Sources](https://github.com/hamcrest/PyHamcrest)

* [Hamcrest](http://hamcrest.org)