Matchers

Lemoncheesecake comes with support of matchers, a feature inspired by Hamcrest / PyHamcrest.

The matchers

The following stock matchers are available:

  • Values:
    • equal_to(expected): check if actual == expected
    • not_equal_to(expected): check if actual != expected
    • greater_than(expected): check if actual > expected
    • greater_than_or_equal_to(expected): check if actual >= expected
    • less_than(expected): check if actual < expected
    • less_than_or_equal_to(expected): check if actual <= expected
    • is_between(min, max): check if actual is between min and max
    • is_none(): check if actual == None
    • is_not_none(): check if actual != None
    • has_length(expected): check if value has expected length (expected can be a value or a Matcher object)
    • is_true(): check if value is a boolean true
    • is_false(): check if value is a boolean false
    • is_json(expected): check is the actual JSON equals expected, if not, the unified diff of actual vs expected is displayed
  • Character strings:
    • starts_with(expected): check if the actual string starts with expected
    • ends_with(expected): check if the actual string ends with expected
    • contains_string(expected): check if the actual contains expected
    • match_pattern(expected): check if the actual string match expected regexp (expected can be a raw string or an object returned by re.compile())
    • is_text(expected): check is the actual (multi-lined) text equals expected, if not, the unified diff of actual vs expected is displayed
  • Types (expected is optional and can be a value or a matcher object):
    • is_integer([expected]): check if actual is of type int
    • is_float([expected]): check if actual is of type float
    • is_bool([expected]): check if actual is of type bool
    • is_str([expected]): check if actual is of type str (or unicode if Python 2.7)
    • is_list([expected]): check if actual is of type list or tuple
    • is_dict([expected]): check if actual is of type dict
  • Iterable:
    • has_item(expected): check if the actual iterable has an item that matches expected (expected can be a value or a Matcher)
    • has_items(expected): check if the actual iterable contains at least the expected items (raw values)
    • has_only_items(expected): check if the actual iterable only contains the expected items (raw values)
    • has_all_items(expected): check if all items of the actual iterable match the expected matcher
    • is_in(expected): check if actual is among the expected items
  • Dict:
    • has_entry(expected_key [,expected_value]): check if actual dict has expected_key and (optionally) the expected associated value expected_value (which can be a value or a matcher)
  • Logical:
    • is_(expected): return the matcher if expected is a matcher, otherwise wraps expected in the equal_to matcher
    • is_not(expected), not_(expected): make the negation of the expected matcher (or equal_to if the argument is not a matcher)
    • all_of(matcher1, [matcher2, [...]]): check if all the matchers succeed (logical AND between all the matchers)
    • any_of(matcher1, [matcher2, [...]]): check if any of the matchers succeed (logical OR between all the matchers)
    • anything(), something(), existing(), present(): these matchers always succeed whatever the actual value is (only the matcher description changes to fit the matcher’s name)

The matching operations

Those matcher are used by a matching function:

  • check_that(hint, actual, matcher, quiet=False): run the matcher, log the result and return the matching result as a boolean
  • require_that(hint, actual, matcher, quiet=False): run the matcher, log the result and raise an AbortTest exception in case of a match failure
  • assert_that(hint, actual, matcher, quiet=False): run the match, in case of a match failure (and only in this case) log the result and raise an AbortTest exception

The quiet flag can be set to True to hide the matching result details in the report.

The lemoncheesecake.matching module also provides helper functions to ease operations on dict object:

The code:

data = {"foo": 1, "bar": 2}
check_that("data", data, has_entry("foo", equal_to(1)))
check_that("data", data, has_entry("bar", equal_to(2)))

Can be shortened like this:

check_that_in(
    {"foo": 1, "bar": 2},
    "foo", equal_to(1),
    "bar", equal_to(2)
)

Nested dicts can be checked easily by expressing the nested keys as a tuple:

check_that_in(
    {"foo": {"bar": 1}},
    ("foo", "bar"), equal_to(1),
)

Nested lists are also supported:

check_that_in(
    {"foo": [{"bar": 1}]},
    ("foo", 0, "bar"), equal_to(1),
)

The base_key keyword-argument can also be used when checking nested dicts:

check_that_in(
    {"foo": {"bar": 1, "baz": 2}},
    "bar", equal_to(1),
    "baz", equal_to(2),
    base_key=("foo",)
)

The same dict helper counterparts are available for:

  • require_that => require_that_in
  • assert_that => assert_that_in

Like their *_that counterpart, the *_that_in functions can also take a quiet keyword argument.

If one match fails in a test, this test will be marked as failed.

Creating custom matchers

A custom matcher example:

from lemoncheesecake.matching.matcher import Matcher, MatchResult

class MultipleOf(Matcher):
    def __init__(self, value):
        self.value = value

    def build_description(self, transformation):
        return transformation("to be a multiple of %s" % self.value)

    def matches(self, actual):
        return MatchResult(actual % self.value == 0, "got %s" % actual)

def multiple_of(value):
    return MultipleOf(value)

And how to use it:

check_that("value", 42, is_(multiple_of(2))

A matcher must inherit the Matcher class and implements two methods: build_description and matches.

  • the build_description method will build the description part of the matcher in the check description using the instance of MatcherDescriptionTransformer passed as argument. This callable will do a transformation of the description such as conjugating the verb or turn it into its negative form depending on the calling context. The former example will produce this description for instance: Expect value to be a multiple of 2.

    Here are two examples of transformations depending on the context:

    check_that("value", 42, is_(not_(multiple_of(2)))
    # => "Expect value to not be a multiple of 2"
    
    check_that("value", 42, is_integer(multiple_of(2)))
    # => "Expect value to be an integer that is a multiple of 2"
    
  • the matches method tests if passed argument fulfills the matcher requirements. The method must return an instance of MatchResult that will indicate whether or not the match succeed and an optional match description.