PyRPL is a Python library and GUI application that turns a Red Pitaya / STEMlab FPGA board into a multi-instrument real-time digital servo controller for quantum optics experiments. It provides hardware modules (PID, IIR filter, IQ modulator/demodulator, arbitrary signal generator, scope, trigger, PWM, etc.) and software modules (lockbox, network analyzer, spectrum analyzer, curve viewer). The system is used primarily for laser frequency stabilization (e.g., PDH locking) and phase-locked loops. Communication with the FPGA occurs over SSH and a custom low-latency TCP monitor server. A full Qt-based GUI allows live data display and module configuration. The repository also includes the complete FPGA firmware source (Verilog, Vivado TCL) for the Red Pitaya board, example Jupyter notebooks with experimental data, and CI pipelines for hardware-in-the-loop testing. While the core application is mature and actively used in research, the codebase carries legacy artifacts (old CI configs, Python 2 workarounds) and the FPGA design has unresolved timing violations.