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.)