Property-Based Testing Skill

Property-Based Testing

Overview
When to Use
Quick Start
Reference Guides
Best Practices

Overview

Property-based testing verifies that code satisfies general properties or invariants for a wide range of automatically generated inputs, rather than testing specific examples. This approach finds edge cases and bugs that example-based tests often miss.

When to Use

Testing algorithms with mathematical properties
Verifying invariants that should always hold
Finding edge cases automatically
Testing parsers and serializers (round-trip properties)
Validating data transformations
Testing sorting, searching, and data structure operations
Discovering unexpected input combinations

Quick Start

Minimal working example:

# test_string_operations.py
import pytest
from hypothesis import given, strategies as st, assume, example

def reverse_string(s: str) -> str:
    """Reverse a string."""
    return s[::-1]

class TestStringOperations:
    @given(st.text())
    def test_reverse_twice_returns_original(self, s):
        """Property: Reversing twice returns the original string."""
        assert reverse_string(reverse_string(s)) == s

    @given(st.text())
    def test_reverse_length_unchanged(self, s):
        """Property: Reverse doesn't change length."""
        assert len(reverse_string(s)) == len(s)

    @given(st.text(min_size=1))
    def test_reverse_first_becomes_last(self, s):
        """Property: First char becomes last after reverse."""
        reversed_s = reverse_string(s)
        assert s[0] == reversed_s[-1]
        assert s[-1] == reversed_s[0]
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the references/ directory:

| Guide | Contents | |---|---| | Hypothesis for Python | Hypothesis for Python | | fast-check for JavaScript/TypeScript | fast-check for JavaScript/TypeScript | | junit-quickcheck for Java | junit-quickcheck for Java |

Best Practices

✅ DO

Focus on general properties, not specific cases
Test mathematical properties (commutativity, associativity)
Verify round-trip encoding/decoding
Use shrinking to find minimal failing cases
Combine with example-based tests for known edge cases
Test invariants that should always hold
Generate realistic input distributions

❌ DON'T

Test properties that are tautologies
Over-constrain input generation
Ignore shrunk test failures
Replace all example tests with properties
Test implementation details
Generate invalid inputs without constraints
Forget to handle edge cases in generators

Agent Skills: Property-Based Testing

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