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A Python QuickCheck implementation
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Hey! PayCheck!

PayCheck is a half-baked implementation of ScalaCheck, which itself is an implementation of QuickCheck for Haskell. PayCheck is useful for defining a specification of what a function should do, rather than testing its results for a given input.

Thanks to gcross for some of the more recent changes

Installing PayCheck

sudo easy_install paycheck

That’s it. Get going.

A Quick Example

Let’s steal an example right from ScalaCheck. Here are the string functions ported to PayCheck. See what’s going on? We’re defining the types of the parameters in with_checker, then values of that type are getting passed to the function.

import unittest
from paycheck import with_checker

class TestStrings(unittest.TestCase):
    More-or-less a direct port of the string testing example from the ScalaCheck
    doc at:

    @with_checker(str, str)
    def test_starts_with(self, a, b):

    @with_checker(str, str)
    def test_ends_with(self, a, b):

    # Is this really always true?
    @with_checker(str, str)
    def test_concat(self, a, b):
        self.assertTrue(len(a+b) > len(a))
        self.assertTrue(len(a+b) > len(b))

    @with_checker(str, str)
    def test_substring2(self, a, b):
        self.assertEquals( (a+b)[len(a):], b )

    @with_checker(str, str, str)
    def test_substring3(self, a, b, c):
        self.assertEquals((a+b+c)[len(a):len(a)+len(b)], b)

if __name__ == '__main__':

Then give the ol’ test a run. You’ll likely see a problem:

$ python
FAIL: test_concat (__main__.TestStrings)
Traceback (most recent call last):
  File "paycheck/", line 11, in wrapper
    test_func(self, *v)
  File "", line 20, in test_concat
    self.assertTrue(len(a+b) > len(a))
AssertionError: Failed for input ('UGzo2LP<(9Gl_*o*GH$H<+{wPiNk?', '')

Ran 5 tests in 0.051s

FAILED (failures=1)

As predicted, test_concat has bombed; note that PayCheck is nice enough to tell you which inputs caused the problem. In this case, we see that propety test_concat fails for the empty string, which is caused by the fact that we used “>” instead of “>=”.

Nested and More Complex Types

Container and nested types are specified to PayCheck “by analogy”. That is, unlike scalar types, which are specified by giving the type that you want, container types are specified by creating a non-empty container of the type you want which contains a specification of the type that you want generated containers to contain; note that the contained element is itself allowed to be a container, allowing for arbitrarily nested types. PayCheck will infer from your containers the type of container and contained elements that you want to generate. This includes dictionaries: it will look at the first key/value mapping that it sees and infer from it both the key type and the value type. Containers will be generated with between 0 and paycheck.generator.LIST_LEN elements.

The following examples illustrate how this works:

import unittest
from paycheck import with_checker

class TestTypes(unittest.TestCase):

    def test_int(self, i):
        self.assertTrue(isinstance(i, int))

    def test_get_list(self, list_of_ints):
        self.assertTrue(isinstance(list_of_ints, list))
        for i in list_of_ints:
            self.assertTrue(isinstance(i, int))

    @with_checker([{str: int}])
    def test_list_of_dict_of_int_string(self, list_of_dict_of_int_string):
        self.assertTrue(isinstance(list_of_dict_of_int_string, list))

        for dict_of_int_string in list_of_dict_of_int_string:
            self.assertTrue(isinstance(dict_of_int_string, dict))

            for key, value in dict_of_int_string.items():
                self.assertTrue(isinstance(key, str))
                self.assertTrue(isinstance(value, int))

if __name__ == '__main__':
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