Property-Based Testing

When to use: parsers, serializers, data transformers, encoders/decoders, validators — anything where "for all valid inputs X, property Y holds."

The principle: Instead of testing specific examples, describe properties of the output and let the framework generate hundreds of random inputs to try to falsify them.

Platform tools:

At least one property test MUST be written per data transformation function. Examples:

• encode(decode(x)) == x (round-trip)
• sort(xs).length == xs.length (preservation)
• parse(serialize(obj)).fields == obj.fields (fidelity)

from hypothesis import given
import hypothesis.strategies as st

@given(st.text())
def test_encode_decode_roundtrip(s):
    assert decode(encode(s)) == s

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