Property-Based Testing

Instead of writing individual test cases with fixed inputs, you define properties (invariants) that should always be true for any valid input.

How it Works

1. Define a strategy for generating random data (e.g., "any list of integers").
2. Define a property (e.g., "sorting a list doesn't change its length").
3. The framework generates hundreds of inputs to find a counter-example.
4. If it finds one, it shrinks the input to the simplest possible case that fails.

Tools

• Python: Hypothesis
• JavaScript: fast-check
• Rust: proptest

Example (Python with Hypothesis)

from hypothesis import given, strategies as st