How We Ported Python’s natsort to Rust — And the 133,000 Fuzz Inputs That Proved Us Wrong

A post-mortem on building natsort-rs, fixing a 45x performance bug, and catching subtle edge cases with a custom Pytest monkeypatch bridge.

Why natsort?

Porting Python code to Rust using the "Astral playbook" sounds straightforward on paper: rewrite the hot path, compile to native binary, profit. We chose Seth M. Morton’s popular natsort library—a ubiquitous utility for natural sorting (item2 before item10).

In Python, natsort relies heavily on dynamic typing, tuple-based comparisons, and fallback numeric parsing (try_int, try_float). Translating Python’s dynamic Union[str, int, float] return types into Rust required introducing explicit tagged enums (ParsedComponent) and a hand-crafted Ord trait implementation in key.rs.

Zero unsafe blocks. Pure memory safety. But getting it to build was only 10% of the battle.

How We Ported Python’s natsort to Rust — And the 133,000 Fuzz Inputs That Proved Us Wrong

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