A lean and efficient Python implementation for microcontrollers and constrained systems.


  • highly configurable due to many compile-time configuration options
  • support for many architectures (x86, x86-64, ARM, ARM Thumb, Xtensa)
  • extensive test suite with over 590 tests, and more than 18,500 individual testcases
  • code coverage at 98.4% for the core and at 96.3% for the core plus extended modules
  • fast start-up time from boot to loading of first script (150 microseconds to get to boot.py, on PYBv1.1 running at 168MHz)
  • a simple, fast and robust mark-sweep garbage collector for heap memory
  • a MemoryError exception is raised if the heap is exhausted
  • a RuntimeError exception is raised if the stack limit is reached
  • support for running Python code on a hard interrupt with minimal latency
  • errors have a backtrace and report the line number of the source code
  • constant folding in the parser/compiler
  • pointer tagging to fit small integers, strings and objects in a machine word
  • transparent transition from small integers to big integers
  • support for 64-bit NaN boxing object model
  • support for 30-bit stuffed floats, which don't require heap memory
  • a cross-compiler and frozen bytecode, to have pre-compiled scripts that don't take any RAM (except for any dynamic objects they create)
  • multithreading via the "_thread" module, with an optional global-interpreter-lock (still work in progress, only available on selected ports)
  • a native emitter that targets machine code directly rather than the bytecode virtual machine
  • inline assembler (currently Thumb and Xtensa instruction sets only)