Testing a Small Query Language in Python with Hypothesis

An experimental setup with three beakers, illustration from a book

This entry was intended to be cross-posted to the CERN Databases blog, but is currently pending review. Consider it a pre-release version.

Hypothesis is an implementation of Property-based testing for Python, similar to QuickCheck in Haskell/Erlang and test.check in Clojure (among others). Basically, it allows the programmer to formulate invariants about their programs, and have an automated system attempt to generate counter-examples that invalidates them.

A Small Query Language

During my internship at CERN, I am developing a small (partial) two-way monitoring system to propagate alerts from filers to CERN’s incident management system. In the course of developing this monitor, I decided to invent a very minimal query/filtering language for logging events. It maps directly against Python objects using regular expressions (basically: “does object x have a property y matching regex z”?). The following is its grammar (written for the Grako parser-generator):

start = expression ;

        '(' expression ')' binary_operator '(' expression ')'
        | unary_operator '(' expression ')'
        | statement

binary_operator = 'AND' | 'OR';
unary_operator = 'NOT';
statement = field ':' "'" regex "'";
field =

    = /([^'])*/

An example (from a test configuration file) could be event_type:'disk.failed' (disk failures) or (source_type:'(?i)Aggregate') AND (NOT(source_name:'aggr0')) (log events from aggregates, but not aggr0).

The following invariants should hold, where q is any valid query:

In addition, the following properties should also hold:

Generating Examples

There are several types of inputs we need to generate to test the system. Let’s break them down:

Let’s start from the top. As Python is a dynamic language, we can do crazy things, like dynamically generating objects from dictionaries. The following is a fairly common hack:

class objectview(object):
    def __init__(self, d):
        self.__dict__ = d

    def __repr__(self):
        return str(self.__dict__)

    def __str__(self):
        return str(self.__dict__)

This allows us to instantiate an object with (almost) arbitrary fields:

cat = objectview({'colour': 'red', 'fav_food_barcode': '1941230190'})
>>> cat.colour
>>> cat.fav_food_barcode

Given this, we can just generate valid objects using the @composite decorator in Hypothesis:

def objects(draw):
    ds = draw(dictionaries(keys=valid_properties,

    return objectview(ds)

Generating valid values is much simpler:

valid_values = text()

Any text string is a valid string value. Of course! Properties are a bit trickier though:

valid_properties = (characters(max_codepoint=91,
                               whitelist_categories=["Ll", "Lu", "Nd"])
                    .filter(lambda s: not s[0].isdigit()))

Variable names can’t start with a number, and has to be basically mostly ASCII, so we slightly modify and filter the characters strategy.

Statements can be generated in much the same way, using composite strategies:

def statements(draw):
    # any valid key followed by a valid regex
    key = draw(valid_properties)
    regex = draw(regexes)

    return u"{key}:'{regex}'".format(key=key, regex=regex)

However, how do we produce regular expressions? Let’s start with some valid candidates:

regex_string_candidates = characters(blacklist_characters=[u'?', u'\\', u"'"])

Then we can generate regular expressions using Hypothesis’ back-tracking functionality through assume(), which causes it to discard bad examples (in this instance is_valid_regex() simply tries to compile the string as a Python regular expression, and returns False if it fails):

def regex_strings(draw):
    maybe_regex = draw(regex_string_candidates)
    return maybe_regex

But we can also use recursive generation strategies to produce more complex regular expressions:

regexes = recursive(regex_strings(), lambda subexps:
                    # match one or more
                    subexps.map(lambda re: u"({re})+".format(re=re)) |

                    # match zero or more
                    subexps.map(lambda re: u"({re})*".format(re=re)) |

                    # Append "match any following"
                    subexps.map(lambda re: u"{re}.*".format(re=re)) |

                    # Prepend "match any following"
                    subexps.map(lambda re: u".*{re}".format(re=re)) |

                    # Prepend start of string
                    subexps.map(lambda re: u"^{re}".format(re=re)) |

                    # Append end of string
                    subexps.map(lambda re: u"{re}$".format(re=re)) |

                    # Append escaped backslash
                    subexps.map(lambda re: u"{re}\\\\".format(re=re)) |

                    # Append escaped parenthesis
                    subexps.map(lambda re: u"{re}\(".format(re=re)) |

                    # Append dot
                    subexps.map(lambda re: u"{re}.".format(re=re)) |

                    # Match zero or one
                    subexps.map(lambda re: u"({re})?".format(re=re)) |

                    # Match five to six occurrences
                    subexps.map(lambda re: (u"({re})"
                                            .format(re=re)) + u"{5,6}") |

                    # concatenate two regexes
                    tuples(subexps, subexps).map(lambda res: u"%s%s" % res) |

                    # OR two regexes
                    tuples(subexps, subexps).map(lambda res: u"%s|%s" % res))

The same strategy also works for the highly recursive structure of the query language:

queries = recursive(statements(),
                    lambda subqueries:
                    subqueries.map(negated_query) |
                    tuples(subqueries, subqueries).map(ored_queries) |
                    tuples(subqueries, subqueries).map(anded_queries))

Read as: “a valid query is any statement, or a any valid query negated, or two valid queries AND:ed or OR:ed”.

Making Assertions

To finally assert properties, we assert things similarly to how we would in normal unit tests. For example, let’s verify that the empty regular expression matches anything:

@given(target=objects(), key=valid_properties)
def test_query_for_empty_regex_always_matches(target, key):
    q = "{key}:''".format(key=key)
    assert query.matches_object(q, target)

Hypothesis immediately finds a counter-example: ``` > assert query.matches_object(q, target) E assert False E + where False = <function matches_object at 0x7fa76dc6f5f0>(“A:’’”, {u’B’: u’’}) E + where <function matches_object at 0x7fa76dc6f5f0> = query.matches_object

key = ‘A’ q = “A:’’” target = {u’B’: u’’}

syncd/eql/test/test_hypothesis.py:188: AssertionError ———- Hypothesis ——— Falsifying example: test_query_for_empty_regex_always_matches(target={u’B’: u’’}, key=u’A’) ```

An object which doesn’t have the specified property will not match the query, even if the query is looking for the empty string. Ok, so that’s a bad example depending on how we want to treat this edge-case. If we really did want the empty regular expression to match even objects which does not have their keys, this would have been a proper bug in the implementation. However, it makes more sense to require the object to have the property checked for, and so this is a bad counter-example. We can exclude it by adding assume(hasattr(target, key)) to the test, causing it to back-track on any examples where the target object does not have the key:

@given(target=objects(), key=valid_properties)
def test_query_for_empty_regex_always_matches(target, key):
    assume(hasattr(target, key))

    q = "{key}:''".format(key=key)

    assert query.matches_object(q, target)

And now, the test passes.

The image is from “Chemistry: general, medical and pharmaceutical…” from 1894, courtesy of the Internet Archive Book Images