When defining behaviours or verifying interactions for methods, functions or item access matchers can be used instead of fixed arguments.

Matchers can be used in the following locations.

when(my_mock).method(matcher1, key=matcher2)
when(my_mock)(matcher1, key=matcher2)

verify(my_mock).method(matcher1, key=matcher2)
verify(my_mock)(matcher1, key=matcher2)

Note that methods and functions can accepts an arbitrary amount of positional argument matchers and keyword argument matchers, however item access can only use a single matcher.

Built-in matchers


Matches any positional argument or keyword argument depending on where it is used.

when(my_mock).a_method('first', any_value).then_return(1)
my_mock.a_method('first', 'python')  # 1
my_mock.a_method('second', None)  # not matched

when(my_mock).b_method('first', season=any_value).then_return(2)
my_mock.b_method('first', season='winter')  # 2
my_mock.b_method('first')  # not matched


Matches any positional arguments sent in, regardless of how many there are

my_mock.method('a', 'b')  # True
my_mock.method()  # True
my_mock.method('a', b='b')  # not matched


Matches all keyword arguments, regardless of how many there are, what their keys are and what their values.

my_mock.method(this='that')  # True
my_mock.method()  # True
my_mock.method(1)  # not matched


Effectively a combination of any_args and any_kwargs.

my_mock.method('put', anything='and everything')  # True
my_mock.method()  # True


The ExactValueMatcher matches a value exactly. When building behaviours or verifying interactions and passing in a value that is not matcher, Pock will convert that value into an ExactValueMatcher internally.

# Equivalent

Custom matchers

In order for Pock to efficiently determine whether a set of given arguments match a given set of matches, the magic methods __eq__, __ne__ and __hash__ should be implemented according to the following two principals:

  1. Two matchers are equal when they always match for the same set of values and also fail to match for the same set of values.
  2. If two matchers are equal they should have the same hash

See Python’s documentation on Special method names for more info on these magic methods.

Simple matchers

Here is an example of matcher that only matches even numbers.

from pock import Matcher

class EvenMatcher(Matcher):
    def matches(other):
        return hasattr(other, '__mod__') and
          other % 2 == 0

    def __eq__(self, other):
        return isinstance(other, EvenMatcher)

    def __ne__(self, other):
        return not isinstance(other, EvenMatcher)

    def __hash__(self):
        return hash(EvenMatcher)

even = EvenMatcher()

For the EvenMatcher, __eq__ is implemented to return True when compared to any other EvenMatcher since they’ll always match for the same values.

Because all EvenMatcher s are equal to each other, the hash value is taken from the hash value of the class.

Parametrised matchers

A more complicated matcher is one that can take parameters, consider a matcher that matches any number if it is divisible by n.

from pock import matcher

class DivisibleByN(Matcher):
    def __init__(self, n):
        self.n = n

    def matches(other):
        return (hasattr(other, '__mod__') and
                other % self.n == 0)

    def __eq__(self, other):
        return (isinstance(other, DivisibleByN) and
                self.n == other.n)

    def __ne__(self, other):
        return (not isinstance(other, EvenMatcher) or
                self.n != other.n)

    def __hash__(self):
        return (hash(DivisibleByN) ^
                hash(self.n) ^
                hash((DivisibleByN, self.n)))

divisible_by_7 = DivisibleByN(7)

Here the DivisibleByN matcher needs to also compare the value of n in __eq__ and __ne__ and likewise, the value of n is included in the hash function. This ensures that all DivisibleByN matchers of the same n are equal by comparison and hash but matchers with different n are not.