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16. Property-Based Testing with Hypothesis

Date: 2025-10-13

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Status

Accepted

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Category

Testing & Quality

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Context

Traditional unit tests check specific examples: 
assert calculate_discount(100, 0.1) == 90
Edge cases often missed:
  • Negative numbers
  • Zero values
  • Very large numbers
  • Unicode in strings

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Decision

Use Hypothesis for property-based testing to discover edge cases automatically.

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Example

from hypothesis import given
from hypothesis.strategies import integers, floats

@given(price=floats(min_value=0), discount=floats(min_value=0, max_value=1))
def test_discount_properties(price, discount):
    result = calculate_discount(price, discount)
    assert result >= 0  # Never negative
    assert result <= price  # Never more than original
Hypothesis generates 100+ test cases including edge cases.

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Consequences

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Positive

  • Edge Case Discovery: Finds bugs unit tests miss
  • Higher Confidence: Tests properties, not examples
  • Less Test Code: One property test replaces many examples
  • Regression Prevention: Generated cases saved on failure

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Negative

  • Slower Tests: 100 cases per test vs 1
  • Learning Curve: Property thinking vs example thinking
  • Flaky Tests: Random generation can cause intermittent failures

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Usage

27+ property tests covering:
  • LLM factory fallback logic
  • Authentication providers
  • Session management
  • Role mapping

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References

  • Tests: tests/property/
  • Dependency: pyproject.toml:66 - hypothesis>=6.100.0