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Module 08 / Project 04 — Property-Based Testing

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Focus

  • The Hypothesis library for property-based testing
  • @given decorator to generate random test inputs
  • Strategies: st.text, st.integers, st.lists, st.dictionaries
  • Testing properties (invariants) instead of specific examples

Why this project exists

Normal tests check specific examples: "sort [3,1,2] and expect [1,2,3]". Property-based tests check general truths: "sorting any list should produce a list where every element is less than or equal to the next". Hypothesis generates hundreds of random inputs — including edge cases you would never think of — and checks that your properties hold for all of them. This is how you find the bugs that hide in unusual inputs like empty strings, negative numbers, and Unicode characters.

Run

cd projects/modules/08-testing-advanced/04-property-based
pytest tests/test_properties.py -v

Expected output

tests/test_properties.py::test_sort_is_idempotent PASSED
tests/test_properties.py::test_sort_preserves_length PASSED
tests/test_properties.py::test_sort_elements_are_ordered PASSED
tests/test_properties.py::test_reverse_twice_is_original PASSED
tests/test_properties.py::test_reverse_preserves_length PASSED
tests/test_properties.py::test_encode_decode_roundtrip PASSED
tests/test_properties.py::test_merge_dicts_contains_all_keys PASSED
tests/test_properties.py::test_merge_dicts_second_wins_on_conflict PASSED

Hypothesis runs each test with many different random inputs. If any input causes a failure, Hypothesis shrinks it to the simplest failing example and shows it to you.

Alter it

  1. Add a property test for sort_list that checks: every element in the sorted output was present in the original input (no elements added or removed).
  2. Write a property test for reverse_string using only alphabetic text (st.text(alphabet=st.characters(whitelist_categories=("L",)))) and verify that reversing preserves the set of characters.
  3. Add a property test for merge_dicts with st.dictionaries(st.text(min_size=1), st.integers()) and verify the result length is at most the sum of the input lengths.

Break it

  1. Introduce a subtle bug in sort_list: make it drop duplicate elements (use list(set(lst)) before sorting). Run the property tests and see which one catches it.
  2. Break encode_decode_json by changing the encode step to strip whitespace. Hypothesis will find an input where the roundtrip fails.
  3. Remove the @given decorator from one test and run it. It becomes a normal test with no inputs — it will either error or do nothing.

Fix it

  1. Revert the bugs you introduced. Run the tests to confirm they all pass.
  2. If Hypothesis found a failing case you did not expect, add it as an explicit @example decorator so it is always tested.
  3. Think about whether your property is actually correct, or if you need to weaken it.

Explain it

  1. What is the difference between example-based testing and property-based testing?
  2. What does Hypothesis do when it finds a failing input? (Hint: "shrinking")
  3. Why is "sort twice equals sort once" (idempotency) a useful property to test?
  4. What kinds of bugs does property-based testing find that example-based testing misses?

Mastery check

You can move on when you can:

  • Write a @given decorator with appropriate strategies from memory.
  • Identify at least two properties for any function you are given.
  • Explain what shrinking means and why it helps debugging.
  • Describe when property-based testing is more valuable than example-based testing.

Next

Project 05 — Integration Testing