QuantLib

QuantLib is the open-source pricing library the industry quietly standardized on: a C++ core with Python bindings that covers instruments, pricing engines, term structures of yield and volatility, calendars, and day-count conventions. If you need to bootstrap a yield curve, calibrate a Hull-White model, or value a swaption with Monte Carlo, QuantLib already implements it, battle-tested across two decades of production use. The trade-off is weight: its object hierarchy mirrors a derivatives desk's vocabulary, so the learning curve is real, and it prices instruments rather than simulating strategies. Reach for it when valuation is the task. Strategy simulation belongs to the backtesters covered next.