What does PyPy have to offer over CPython, Jython, and IronPython?

Question

From what I have seen and read on blogs, PyPy is a very ambitious project. What are some advantages it will bring to the table over its siblings (CPython, Jython, and IronPython)? Is it speed, cross-platform compatibility (including mobile platforms), the ability to use c-extensions without the GIL, or is this more of a technical exercise on what can be done?

Answer 1

PyPy is really two projects:

• An interpreter compiler toolchain allowing you to write interpreters in RPython (a static subset of Python) and have cross-platform interpreters compiled standalone, for the JVM, for .NET (etc)
• An implementation of Python in RPython

These two projects allow for many things.

• Maintaining Python in Python is much easier than maintaining it in C
• From a single codebase you can generate Python interpreters that run on the JVM, .NET and standalone - rather than having multiple slightly incompatible implementations
• Part of the compiler toolchain includes an experimental JIT generator (now in its fifth incarnation and starting to work really well) - the goal is for a JITed PyPy to run much faster than CPython
• It is much easier to experiment with fundamental language features - like removing the GIL, better garbage collection, integrating stackless and so on

So there are really a lot of reasons for PyPy to be exciting, and it is finally starting to live up to all its promises.