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
assertEqual
method, we’ll use PyHamcrest’s
assert_that
construct and the standard
set of matchers:
from hamcrest import assert_that, equal_to
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:
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:
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.
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 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
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 raises
matcher allowing you
assure that the right issue was found:
assert_that(calling(parse, bad_data), raises(ValueError))
assert_that(calling(translate).with_args(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))
Asserting exceptions from async methods¶
An async method does not directly return the result or raise an exception but
instead returns a Future-object that represent the async operation that can
later be resolved with the await
keyword. The
resolved
utility function can be used to
wait for a future to be done but without retrieving the value or raising the
exception. The future_raising
matcher can
be used with any future object but combined lets you assert that calling some
async method, and waiting for the result, causes an exception to be raised.
This is best used together with an async test runner like IsolatedAsyncioTestCase or pytest-asyncio:
async def parse(input: str):
...
class Test(unittest.IsolatedAsyncioTestCase):
async def testParse(self):
future = parse("some bad data")
assert_that(await resolved(future), future_raising(ValueError))
But it’s possible to use with an async unware runner by explicitly running the event loop in the test:
class Test(unittest.TestCase):
def test_parse(self):
async def test():
future = parse("some bad data")
assert_that(await resolved(future), future_raising(ValueError))
asyncio.get_event_loop().run_until_complete(test())
Predefined matchers¶
PyHamcrest comes with a library of useful matchers:
Object
equal_to
- match equal objecthas_length
- matchlen(item)
has_property
- match value of property with given namehas_properties
- match anobject that has all of the given properties.
has_string
- matchstr(item)
instance_of
- match object typesame_instance
- match same objectcalling
,raises
- wrap a method call and assert that it raises an exception
Number
close_to
- match number close to a given valuegreater_than
,greater_than_or_equal_to
,less_than
,less_than_or_equal_to
- match numeric ordering
Text
contains_string
- match part of a stringends_with
- match the end of a stringequal_to_ignoring_case
- match the complete string but ignore caseequal_to_ignoring_whitespace
- match the complete string but ignore extra whitespacestarts_with
- match the beginning of a stringstring_contains_in_order
- match parts of a string, in relative order
Logical
Sequence
contains
- exactly match the entire sequencecontains_inanyorder
- match the entire sequence, but in any orderhas_item
- match if given item appears in the sequencehas_items
- match if all given items appear in the list, in any orderis_in
- match if item appears in the given sequenceonly_contains
- match if sequence’s items appear in given listempty
- match if the sequence is empty
Dictionary
has_entries
- match dictionary with list of key-value pairshas_entry
- match dictionary containing a key-value pairhas_key
- match dictionary with a keyhas_value
- match dictionary with a value
Decorator
described_as
- give the matcher a custom failure descriptionis_
- 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:
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:
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
.)