Hypothesis is a property-based library for Python. It's implemented as a standalone library and can be used with most Python testing frameworks.
Hypothesis adopts an approach called internal test-case reduction, which does shrinking by manipulating the behaviour of the generation process rather than attempting to modify the generated value.
This approach is described in an ECOOP paper about the Hypothesis reducer, and is a form of integrated shrinking which ensures by construction that all constraints from value generation also hold in shrinking.