Matt Bachmann - Better Testing With Less Code: Property Based Testing With Python

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Property-Based Testing In Python Matt Bachmann @mattbachmann

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Testing is Important Refactoring Design Regression Protection Faster Development

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Testing is Hard Code Isolation Fixtures Indirect Value

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Capture the Important Cases Minimize The Coding Overhead

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Sorting a list of integers

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def test_sort_empty(xs): assert quicksort([]) == [] def test_sorted(xs): assert quicksort([1, 2, 3]) == [1, 2, 3] def test_sort_unsorted(xs): assert quicksort([5, 4]) == [4, 5]

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Property Based Testing ● Describe the arguments ● Describe the result ● Have the computer try to prove your code wrong

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● The Arguments ○ List of Integers ● Properties of the Result ○ List ○ All elements preserved ○ Results are in ascending order

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Hypothesis Inspired by Haskell’s QuickCheck Fantastic Docs Offers training/contracting http://hypothesis.works/

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@given(st.lists(st.integers())) def test_sort(xs): sorted_xs = quicksort(xs) assert isinstance(sorted_xs, list) assert Counter(xs) == Counter(sorted_xs) assert all( x <= y for x, y in zip(sorted_xs, sorted_xs[1:]) )

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@given(st.lists(st.integers())) test_sort([]) test_sort([0]) test_sort([93932932923, 82883982983838]) test_sort([9999,77,2,3,3,100,3,39,3993])

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[-3223, -99999999999, 32]

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[-3223, -99999999999, 32] [-3223, -99999999999]

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[-3223, -99999999999, 32] [-3223, -99999999999] [1, 0]

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@given

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Strategies booleans, floats, strings, complex_numbers

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Strategies booleans, floats, strings, complex_numbers dictionaries, tuples, lists, sets, builds

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Strategies booleans, floats, strings, complex_numbers dictionaries, tuples, lists, sets, builds one_of, sampled_from, recursive

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@given( st.builds( Dog, breed=st.text(), name=st.text(), height=st.floats(), weight=st.floats() ) )

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@given( st.builds( Dog, breed=st.sampled_from(KNOWN_BREEDS), name=st.text(), height=st.floats(), weight=st.floats() ) )

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@given( st.builds( Dog, breed=st.sampled_from(KNOWN_BREEDS), name=st.text(min_size=5), height=st.floats(), weight=st.floats() ) )

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@given( st.builds( Dog, breed=st.sampled_from(KNOWN_BREEDS), name=st.text(min_size=5), height=st.floats( min_value=1, max_value=6, allow_nan=False, allow_infinity=False ), weight=st.floats( min_value=1, max_value=300, allow_nan=False, allow_infinity=False ), ) )

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Potentially infinite cases Nothing hardcoded Very little code

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https://www.flickr.com/photos/t0fugurl/2507049701 Picture (without crossout) by Leanne Poon https://creativecommons.org/licenses/by-nc-nd/2.0/

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Pattern 1: The code should not explode

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https://upload.wikimedia. org/wikipedia/commons/3/39/Chip-pan- fire.jpg

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/batputer/criminals/aliases/{id}? sort={sort}&max_results={max} ● JSON response ● Expected response codes ○ 200 - OK ○ 401 - Forbidden ○ 400 - Invalid data ○ 404 - Not Found

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✓

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https://upload.wikimedia. org/wikipedia/commons/3/39/Chip-pan- fire.jpg

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@given(st.integers(), st.text(), st.integers())) def test_no_explosion(id, sort, max): response = requests.get( BATPUTER_URL.format(id, sort, max) ) assert response and response.json() assert (response.status_code in [200, 401, 400, 404])

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Pattern 2: Reversible Operations

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Encoding Undo Operations Serialization

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Encoding Undo Operations Serialization

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class HistoricalEvent(object): def __init__(self, id, desc, time): self.id = id self.desc = desc self.time = time

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class EventEncoder(json.JSONEncoder): def default(self, obj): if isinstance(obj, HistoricalEvent): return { "id": obj.id "description": obj.description "event_time":obj.event_time.isoformat() } return json.JSONEncoder.default(self, obj)

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def fromJson(json_str): dct = json.loads(json_str) return HistoricalEvent( dct['id'], dct['description'], dateutil.parser.parse( dct['event_time'] ) )

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@given(st.integers(), st.text(), datetimes(timezones=['UTC'])) def test_to_from_json(id, desc, date): original_event = HistoricalEvent(id, desc, date) encoded_event = json.dumps(event, cls=EventEncoder) decoded_event = json.loads( encoded_event, object_hook=HistoricalEvent.fromJson ) assert decoded_event == original_event

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Pattern 3: Testing Oracle

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Optimizing Refactoring Emulating

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https://www.flickr.com/photos/versageek/493800514 versageek https://creativecommons.org/licenses/by-sa/2.0/ Leave It Alone

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https://www.flickr.com/photos/versageek/493800514 versageek https://creativecommons.org/licenses/by-sa/2.0/ https://upload.wikimedia. org/wikipedia/commons/3/36/Under_Floor_Cable_ Runs_Tee.jpg =

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@given(st.integers(), st.text()) def test_against_legacy(arg1, arg2): assert ( new_hotness(arg1, arg2) == legacy_system(arg1, arg2) )

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Compare Against Brute Force

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@given(st.lists(st.integers())) def test_against_brute_force(input): assert ( easy_but_inefficent(input) == optimized(input) )

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Pattern 4: Stateful Testing

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Stateful Tests ● Define a state ● What operations can happen in what conditions? ● How do operations affect the state? ● What must be true for each step?

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https://upload.wikimedia.org/wikipedia/commons/8/83/Jan_Fabre's_%22Searching_for_Utopia%22.jpg FIND. ME. BUGS.

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https://commons.wikimedia.org/wiki/File:Max-Heap.svg http://hypothesis.works/articles/rule-based-stateful-testing/

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http://hypothesis.works/articles/rule-based-stateful-testing/ __init__ push pop merge

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http://hypothesis.works/articles/rule-based-stateful-testing/ Integers Heaps

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http://hypothesis.works/articles/rule-based-stateful-testing/ __init__ Integers Heaps

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http://hypothesis.works/articles/rule-based-stateful-testing/ push Integers Heaps

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http://hypothesis.works/articles/rule-based-stateful-testing/ merge Integers Heaps

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http://hypothesis.works/articles/rule-based-stateful-testing/ merge Integers Heaps

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http://hypothesis.works/articles/rule-based-stateful-testing/ pop Integers Heaps assert result actually max

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http://hypothesis.works/articles/rule-based-stateful-testing/ class HeapMachine(RuleBasedStateMachine): Heaps = Bundle('heaps') @rule(target=Heaps) def new_heap(self): return Heap() @rule(heap=Heaps, value=integers()) def heap_push(self, heap, value): push(heap, value)

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class HeapMachine(RuleBasedStateMachine): Heaps = Bundle('heaps') @rule(target=Heaps) def new_heap(self): return Heap() @rule(heap=Heaps, value=integers()) def heap_push(self, heap, value): push(heap, value) http://hypothesis.works/articles/rule-based-stateful-testing/

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http://hypothesis.works/articles/rule-based-stateful-testing/ … @rule(target=Heaps, heap1=Heaps, heap2=Heaps) def merge(self, heap1, heap2): return heap_merge(heap1, heap2) @rule(heap=Heaps.filter(bool)) def pop(self, heap): correct = max(list(heap)) result = heap_pop(heap) assert correct == result

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https://upload.wikimedia.org/wikipedia/commons/8/83/Jan_Fabre's_%22Searching_for_Utopia%22.jpg FIND. ME. BUGS.

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http://hypothesis.works/articles/rule-based-stateful-testing/ @rule(heap=Heaps.filter(bool)) def pop(self, heap): correct = max(list(heap)) result = heap_pop(heap) > assert correct == result E AssertionError: assert 1 == 0

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http://hypothesis.works/articles/rule-based-stateful-testing/ v1 = new_heap() push(heap=v1, value=0) push(heap=v1, value=1) push(heap=v1, value=1) v2 = merge(heap2=v1, heap1=v1) pop(heap=v2) pop(heap=v2)

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Property Based Testing ● Describe the arguments ● Describe the result ● Have the computer try to prove your code wrong

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Your Turn ● Download Hypothesis ● Use it ● Share how you used it ● FIND MORE PATTERNS

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More about Hypothesis ● http://hypothesis.works/ More On Property Based Testing In General ● http://www.quviq.com/ ● https://fsharpforfunandprofit.com/posts/property-based-testing-2/ Testing Eventual Consistency with RIAK ● https://www.youtube.com/watch?v=x9mW54GJpG0

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Thanks! Matt Bachmann https://github.com/Bachmann1234 Twitter: @mattbachmann Slides: goo.gl/g0mGgU