Agent Skills: Property-Based Testing

Property-Based Testing

Overview

Property-based testing (PBT) generates random inputs and verifies that properties hold for all of them. Instead of testing specific examples, you test invariants.

When PBT beats example-based tests:

• Serialization pairs (encode/decode)
• Pure functions with clear contracts
• Validators and normalizers
• Data structure operations

Property Catalog

| Property | Formula | When to Use | |----------|---------|-------------| | Roundtrip | decode(encode(x)) == x | Serialization, conversion pairs | | Idempotence | f(f(x)) == f(x) | Normalization, formatting, sorting | | Invariant | Property holds before/after | Any transformation | | Commutativity | f(a, b) == f(b, a) | Binary/set operations | | Associativity | f(f(a,b), c) == f(a, f(b,c)) | Combining operations | | Identity | f(x, identity) == x | Operations with neutral element | | Inverse | f(g(x)) == x | encrypt/decrypt, compress/decompress | | Oracle | new_impl(x) == reference(x) | Optimization, refactoring | | Easy to Verify | is_sorted(sort(x)) | Complex algorithms | | No Exception | No crash on valid input | Baseline (weakest) |

Strength hierarchy (weakest to strongest):

No Exception -> Type Preservation -> Invariant -> Idempotence -> Roundtrip

Always aim for the strongest property that applies.

Pattern Detection

Use PBT when you see:

| Pattern | Property | Priority | |---------|----------|----------| | encode/decode, serialize/deserialize | Roundtrip | HIGH | | toJSON/fromJSON, pack/unpack | Roundtrip | HIGH | | Pure functions with clear contracts | Multiple | HIGH | | normalize, sanitize, canonicalize | Idempotence | MEDIUM | | is_valid, validate with normalizers | Valid after normalize | MEDIUM | | Sorting, ordering, comparators | Idempotence + ordering | MEDIUM | | Custom collections (add/remove/get) | Invariants | MEDIUM | | Builder/factory patterns | Output invariants | LOW |

When NOT to Use

• Simple CRUD without transformation logic
• UI/presentation logic
• Integration tests requiring complex external setup
• Code with side effects that cannot be isolated
• Prototyping where requirements are fluid
• Tests where specific examples suffice and edge cases are understood

Library Quick Reference

| Language | Library | Import | |----------|---------|--------| | Python | Hypothesis | from hypothesis import given, strategies as st | | TypeScript/JS | fast-check | import fc from 'fast-check' | | Rust | proptest | use proptest::prelude::* | | Go | rapid | import "pgregory.net/rapid" | | Java | jqwik | @Property annotations | | Haskell | QuickCheck | import Test.QuickCheck |

For library-specific syntax and patterns: Use @ed3d-research-agents:internet-researcher to get current documentation.

Input Strategy Best Practices

1. Constrain early: Build constraints INTO the strategy, not via assume()

   # GOOD
   st.integers(min_value=1, max_value=100)
   
   # BAD - high rejection rate
   st.integers().filter(lambda x: 1 <= x <= 100)
   

2. Size limits: Prevent slow tests

   st.lists(st.integers(), max_size=100)
   st.text(max_size=1000)
   

3. Realistic data: Match real-world constraints

   st.integers(min_value=0, max_value=150)  # Real ages, not arbitrary ints
   

4. Reuse strategies: Define once, use across tests

   valid_users = st.builds(User, ...)
   
   @given(valid_users)
   def test_one(user): ...
   
   @given(valid_users)
   def test_two(user): ...
   

Settings Guide

# Development (fast feedback)
@settings(max_examples=10)

# CI (thorough)
@settings(max_examples=200)

# Nightly/Release (exhaustive)
@settings(max_examples=1000, deadline=None)

Quality Checklist

Before committing PBT tests:

• [ ] Not tautological (assertion doesn't compare same expression)
• [ ] Strong assertion (not just "no crash")
• [ ] Not vacuous (inputs not over-filtered by assume())
• [ ] Edge cases covered with explicit examples (@example)
• [ ] No reimplementation of function logic in assertion
• [ ] Strategy constraints are realistic
• [ ] Settings appropriate for context

Red Flags

• Tautological: assert sorted(xs) == sorted(xs) tests nothing
• Only "no crash": Always look for stronger properties
• Vacuous: Multiple assume() calls filter out most inputs
• Reimplementation: assert add(a, b) == a + b if that's how add is implemented
• Missing edge cases: No @example([]), @example([1]) decorators
• Overly constrained: Many assume() calls means redesign the strategy

Common Mistakes

| Mistake | Fix | |---------|-----| | Testing mock behavior | Test real behavior | | Reimplementing function in test | Use algebraic properties | | Filtering with assume() | Build constraints into strategy | | No edge case examples | Add @example decorators | | One property only | Add multiple properties (length, ordering, etc.) |