Property Based Testing

Property-Based Testing

Expert knowledge for property-based testing - automatically generating test cases to verify code properties rather than testing specific examples.

When to Use This Skill

Core Expertise

Property-Based Testing Concept

• Traditional testing: Test specific examples
• Property-based testing: Test properties that should hold for all inputs
• Generators: Automatically create diverse test inputs
• Shrinking: Minimize failing cases to simplest example
• Coverage: Explore edge cases humans might miss

When to Use Property-Based Testing

• Mathematical operations (commutative, associative properties)
• Encoders/decoders (roundtrip properties)
• Parsers and serializers
• Data transformations
• API contracts
• Invariants and constraints

TypeScript/JavaScript (fast-check)

Installation

# Using Bun
bun add -d fast-check

# Using npm
npm install -D fast-check

Basic Example

import { test } from 'vitest'
import * as fc from 'fast-check'

// Property-based test
test('reverse twice returns original - property based', () => {
  fc.assert(
    fc.property(
      fc.array(fc.integer()), // Generate random arrays of integers
      (arr) => {
        expect(reverse(reverse(arr))).toEqual(arr)
      }
    )
  )
})
// fast-check automatically generates 100s of test cases!

Key Generators (Quick Reference)

Common Properties to Test

Configuration

fc.assert(property, {
  numRuns: 1000,      // Run 1000 tests (default: 100)
  seed: 42,           // Reproducible tests
  endOnFailure: true, // Stop after first failure
})

Preconditions

fc.pre(b !== 0) // Skip cases where b is 0

Python (Hypothesis)

Installation

# Using uv
uv add --dev hypothesis pytest

# Optional extensions
uv add --dev hypothesis[numpy]   # NumPy strategies
uv add --dev hypothesis[django]  # Django model strategies

Configuration

# pyproject.toml
[tool.hypothesis]
max_examples = 200
deadline = 1000

[tool.hypothesis.profiles.dev]
max_examples = 50
deadline = 1000

[tool.hypothesis.profiles.ci]
max_examples = 500
deadline = 5000
verbosity = "verbose"

# Activate profile in conftest.py
from hypothesis import settings
settings.load_profile("ci")

Basic Example

from hypothesis import given, example, assume
import hypothesis.strategies as st

# Test a property
@given(st.integers(), st.integers())
def test_addition_commutative(a, b):
    assert a + b == b + a

# Add explicit edge cases with @example
@given(st.integers())
@example(0)
@example(-1)
@example(2**31 - 1)
def test_with_explicit_examples(x):
    assert process(x) is not None

Key Strategies (Quick Reference)

Configuration (Settings)

from hypothesis import given, settings, strategies as st

@settings(max_examples=1000, deadline=None)
@given(st.lists(st.integers()))
def test_with_custom_settings(arr):
    assert sort(arr) == sorted(arr)

Assumptions

from hypothesis import assume
assume(b != 0)  # Skip cases where b is 0

Stateful Testing

Hypothesis supports stateful testing via RuleBasedStateMachine for testing sequences of operations against invariants.

CI Integration

# .github/workflows/test.yml
- name: Run hypothesis tests
  run: |
    uv run pytest \
      --hypothesis-show-statistics \
      --hypothesis-profile=ci \
      --hypothesis-seed=${{ github.run_number }}

- name: Upload hypothesis database
  uses: actions/upload-artifact@v4
  if: failure()
  with:
    name: hypothesis-examples
    path: .hypothesis/

Quick Reference

# Core decorators
@given(strategy)              # Generate test inputs
@example(value)               # Add explicit test case
@settings(max_examples=500)   # Configure behavior

# Key helpers
assume(condition)             # Skip invalid inputs
note(message)                 # Add debug info to failure output
target(value)                 # Guide generation toward value

Agentic Optimizations

For detailed examples, advanced patterns, and best practices, see REFERENCE.md.

See Also

• vitest-testing - Unit testing framework
• python-testing - Python pytest testing
• test-quality-analysis - Detecting test smells
• mutation-testing - Validate test effectiveness

References

• fast-check: https://fast-check.dev/
• Hypothesis: https://hypothesis.readthedocs.io/
• Property-Based Testing: https://fsharpforfunandprofit.com/posts/property-based-testing/