Custom Matchers

Extending Should-DSL with custom matchers is very easy. It is possible to add matchers through functions and classes, for simple and complex behaviors.

Simple Custom Matchers through Functions

For extending Should-DSL with simple matchers a simple decorated function is enough. The function name must be the name of the matcher. The function must have no parameters and it must return a tuple containing two elements.

The first tuple item is the function (or lambda), receiving two parameters, to be run for the comparison, and the second is the failure message. The failure message must have three string formatting operators (such as %s, %r, %d, etc) placeholders. The first and the third will be used for the actual and expected values, respectively. The second operator will be the placeholder for a ‘not ‘ string for failed expectations or an empty string for succeded expectation - and the opossite if should_not is used.

>>> from should_dsl import matcher, should, should_not

>>> @matcher
... def be_the_square_root_of():
...     import math
...     return (lambda x, y: x == math.sqrt(y), "%s is %sthe square root of %s")

>>> 3 |should| be_the_square_root_of(9)

>>> 4 |should| be_the_square_root_of(9)
Traceback (most recent call last):
ShouldNotSatisfied: 4 is not the square root of 9

>>> 4 |should_not| be_the_square_root_of(16)
Traceback (most recent call last):
ShouldNotSatisfied: 4 is the square root of 16

Not so Simple Matchers through Classes

If your custom matcher has a more complex behaviour, or if both should and should_not messages differ, you can create custom matchers as classes. In fact, classes as matchers are the preferred way to create matchers, being function matchers only a convenience for simple cases.

Below is an example of the square root matcher defined as a class:

>>> import math
>>> class SquareRoot(object):
...     name = 'be_the_square_root_of'
...     def __call__(self, radicand):
...         self._radicand = radicand
...         return self
...     def match(self, actual):
...         self._actual = actual
...         self._expected = math.sqrt(self._radicand)
...         return self._actual == self._expected
...     def message_for_failed_should(self):
...         return 'expected %s to be the square root of %s, got %s' % (
...             self._actual, self._radicand, self._expected)
...     def message_for_failed_should_not(self):
...         return 'expected %s not to be the square root of %s' % (
...             self._actual, self._radicand)
>>> matcher(SquareRoot)
<class ...SquareRoot...>
>>> 3 |should| be_the_square_root_of(9)
>>> 4 |should| be_the_square_root_of(9)
Traceback (most recent call last):
ShouldNotSatisfied: expected 4 to be the square root of 9, got 3.0
>>> 2 |should_not| be_the_square_root_of(4.1)
>>> 2 |should_not| be_the_square_root_of(4)
Traceback (most recent call last):
ShouldNotSatisfied: expected 2 not to be the square root of 4


If you are using Python 2.6 or greater you can use the class decorator feature (just a cool syntax sugar):

class SquareRoot(object):
    """same body here"""

# instead of:

class SquareRoot(object):
    """same body here"""


A matcher class must fill the following requirements:

  • A class attribute called name containing the desired name for the matcher;
  • A match(actual) method receiving the actual value of the expectation as a parameter (e.g., in 2 |should| equal_to(3) the actual is 2 and the expected is 3). This method should return the boolean result of the desired comparison;
  • Two methods, called message_for_failed_should and message_for_failed_should_not for returning the failure messages for, respectively, should and should_not.

The most common way the expected value is injected to the matcher is through making the matcher callable. Thus, the matcher call can get the expected value and any other necessary or optional information. As example, the close_to matcher’s __call__() method receives 2 parameters: the expected value and a delta. Once a matcher is a regular Python object, any Python statement can be used. In close_to, delta can be used as a named parameter for readability purposes.

should or should_not?

For most of the matchers, should is the exact opposite to should_not. For the same expected and actual values, if should_not fails, should will pass; in the same way, if should fails, should_not passes. However, this is not true for all matchers. Depending on your matcher semantics, the same expected and actual values can fail or pass both should and should_not. A good example is the matcher include_keys. The calls shown below will fail:

>>> {'a': 1, 'b': 2, 'c': 3} |should| include_keys('a', 'd')
Traceback (most recent call last):
ShouldNotSatisfied: expected target to include key 'd'

>>> {'a': 1, 'b': 2, 'c': 3} |should_not| include_keys('a', 'd')
Traceback (most recent call last):
ShouldNotSatisfied: expected target to not include key 'a'

In order to make possible to implement matchers like include_keys, Should-DSL injects, into all matchers, information about what kind of should is being run: should or should_not. The matcher can access this information in the attribute “run_with_negate”. So, within your matcher you can have:

if self.run_with_negate:
    # do what you want for a should_not
    # this matcher was run with should

With this information, the matcher can act in accordance to the way it is being run.

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