x-python 1.0.1

Python cross-version byte-code interpeter

x-python

This is a C Python interpreter written Python.

You can use this to:

• Learn about how the internals of CPython works since this models that

• Use as a sandboxed environment inside a debugger for trying pieces of execution

• Have one Python program that runs multiple versions of Python bytecode. For example running Python 2.5 or 2.6 bytecode from Python 3.7. No need to install Python 2.6!

The sandboxed environment in a debugger I find interesting. Since there is a separate execution, and traceback stack, inside a debugger you can try things out in the middle of a debug session without effecting the real execution. On the other hand if a sequence of executions works out, it is possible to copy this (under certain circumstances) back into CPython’s execution stack.

Going the other way, I may at some point hook in my debugger into this interpreter and then you’ll have a conventional pdb/gdb like debugger also with the ability to step bytecode instructions.

Status:

Currently only Python 2.5 - 2.7, and 3.2 - 3.5 bytecode is well understood. Other versions will start to appear with the help of xdis.

Whereas Byterun was a bit loose in accepting bytecode opcodes that is invalid for particular Python but may be valid for another; x-python is more stringent. This has pros and cons. On the plus side Byterun might run certain Python 3.4 bytecode because the opcode sets are similar. However starting with Python 3.5 and beyond the likelihood gets much less because, while the underlying opcode names may be the same, the semantics of the operation may change subtely. See for example https://github.com/nedbat/byterun/issues/34.

Internally Byterun needs the kind of overhaul we have here to be able to scale to support bytecode for more Pythons, and to be able to run bytecode across different versions of Python. Specifically, you can’t rely on Python’s dis module if you expect to expect to run a bytecode other than the bytecode that the interpreter is running.

In x-python there is a clear distinction between the version being interpreted and the version of Python that is running. There is tighter control of opcodes and an opcode’s implementation is kept for each Python version. So we’ll warn early when something is invalid. You can run 3.3 bytecode using Python 3.7 (largely).

The “largely” part is because the interpreter has always made use of Python builtins. When a Python version running the interperter matches a supported bytecode close enough, the interpreter can (and does) make use interpreter internals. For example, built-in functions like range() are supported this way.

However running 2.7 bytecode on 3.x is often not feasible since the runtime and internal libraries used like inspect are too different.

I would say this is more than a a simple toy interpreter, but it will never be as complete as CPython or PyPy.

History

This is a fork of Byterun. which is a pure-Python implementation of a Python bytecode execution virtual machine. Net Batchelder started it (based on work from Paul Swartz) to get a better understanding of bytecodes so he could fix branch coverage bugs in coverage.py.