x-python 1.3.1

Python cross-version byte-code interpeter

x-python

This is a CPython bytecode interpreter written Python.

You can use this to:

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

• Experiment with additional opcodes, or ways to change the run-time environment

• Use as a sandboxed environment for trying pieces of execution

• Have one Python program that runs multiple versions of Python bytecode.

• Use in a dynamic fuzzer or in coholic execution for analysis

The ability to run Python bytecode as far back as 2.5 from Python 3.7 I find pretty neat. (Even more could easily be added).

Also, 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 have hooked in trepan3k into this interpreter so you have a pdb/gdb like debugger also with the ability to step bytecode instructions.

I may also experiment with faster ways to support trace callbacks such as those used in a debugger. In particular I may add a BREAKPOINT instruction to support fast breakpoints and breakpointing on a particular instruction that doesn’t happen to be on a line boundary.

Although this is far in the future, suppose you to add a race detector? It might be easier to prototype it in Python here. (This assumes the interpreter supports threading well, I suspect it doesn’t)

Another unexplored avenue implied above is mixing interpretation and direct CPython execution. In fact, there are bugs so this happens right now, but it will be turned into a feature. Some functions or classes you may want to not run under a slow interpreter while others you do want to run under the interpreter.

Examples:

What to know instructions get run when you write some simple code? Try this:

$ xpython -vc "x, y = 2, 3; x **= y"
INFO:xpython.vm:L. 1   @  0: LOAD_CONST (2, 3)
INFO:xpython.vm:       @  2: UNPACK_SEQUENCE 2
INFO:xpython.vm:       @  4: STORE_NAME (2) x
INFO:xpython.vm:       @  6: STORE_NAME (3) y
INFO:xpython.vm:L. 1   @  8: LOAD_NAME x
INFO:xpython.vm:       @ 10: LOAD_NAME y
INFO:xpython.vm:       @ 12: INPLACE_POWER (2, 3)
INFO:xpython.vm:       @ 14: STORE_NAME (8) x
INFO:xpython.vm:       @ 16: LOAD_CONST None
INFO:xpython.vm:       @ 18: RETURN_VALUE (None)

Option -c is the same as Python’s flag (program passed in as string) and -v is also analogus Python’s flag. Here, it shows the bytecode instructions run.

Note that the disassembly above in the dynamic trace above gives a little more than what you’d see from a static disassembler from Python’s dis module. In particular, the STORE_NAME instructions show the value that is store, e.g. “2” at instruction offset 4 into name x. Similarly INPLACE_POWER shows the operands, 2 and 3, which is how the value 8 is derived as the operand of the next instruction, STORE_NAME.

Want more like the execution stack stack and block stack in addition? Add another v:

$ xpython -vvc "x, y = 2, 3; x **= y"

DEBUG:xpython.vm:make_frame: code=<code object <module> at 0x7f8018507db0, file "<string x, y = 2, 3; x **= y>", line 1>, callargs={}, f_globals=(<class 'dict'>, 140188140947488), f_locals=(<class 'NoneType'>, 93856967704000)
DEBUG:xpython.vm:<Frame at 0x7f80184c1e50: '<string x, y = 2, 3; x **= y>':1 @-1>
DEBUG:xpython.vm:  frame.stack: []
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:L. 1   @  0: LOAD_CONST (2, 3) <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [(2, 3)]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @  2: UNPACK_SEQUENCE 2 <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [3, 2]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @  4: STORE_NAME (2) x <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [3]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @  6: STORE_NAME (3) y <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: []
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:L. 1   @  8: LOAD_NAME x <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [2]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @ 10: LOAD_NAME y <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [2, 3]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @ 12: INPLACE_POWER (2, 3)  <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [8]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @ 14: STORE_NAME (8) x <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: []
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @ 16: LOAD_CONST None <module> in <string x, y = 2, 3; x **= y>:1
DEBUG:xpython.vm:  frame.stack: [None]
DEBUG:xpython.vm:  blocks     : []
INFO:xpython.vm:       @ 18: RETURN_VALUE (None)  <module> in <string x, y = 2, 3; x **= y>:1

Want to see this colorized in a terminal? Use this via trepan-xpy -x:

Suppose you have Python 2.5 bytecode (or some other bytecode) for this, but you are running Python 3.7?

$ xpython -v test/examples/assign-2.5.pyc
INFO:xpython.vm:L. 1   @  0: LOAD_CONST (2, 3)
INFO:xpython.vm:       @  3: UNPACK_SEQUENCE 2
INFO:xpython.vm:       @  6: STORE_NAME (2) x
INFO:xpython.vm:       @  9: STORE_NAME (3) y
INFO:xpython.vm:L. 2   @ 12: LOAD_NAME x
INFO:xpython.vm:       @ 15: LOAD_NAME y
INFO:xpython.vm:       @ 18: INPLACE_POWER (2, 3)
INFO:xpython.vm:       @ 19: STORE_NAME (8) x
INFO:xpython.vm:       @ 22: LOAD_CONST None
INFO:xpython.vm:       @ 25: RETURN_VALUE (None)

Not much has changed here, other then the fact that that in after 3.6 instructions are two bytes instead of 1- or 3-byte instructions.

The above examples show straight-line code, so you see all of the instructions. But don’t confuse this with a disassembler like pydisasm from xdis. The below example, with conditional branching example makes this more clear:

$ xpython -vc "x = 6 if __name__ != '__main__' else 10"
INFO:xpython.vm:L. 1   @  0: LOAD_NAME __name__
INFO:xpython.vm:       @  2: LOAD_CONST __main__
INFO:xpython.vm:       @  4: COMPARE_OP ('__main__', '__main__') !=
INFO:xpython.vm:       @  6: POP_JUMP_IF_FALSE 12
                                               ^^ Note jump below
INFO:xpython.vm:       @ 12: LOAD_CONST 10
INFO:xpython.vm:       @ 14: STORE_NAME (10) x
INFO:xpython.vm:       @ 16: LOAD_CONST None
INFO:xpython.vm:       @ 18: RETURN_VALUE (None)

Want even more status and control? See trepan-xpy.

Status:

Currently bytecode from Python versions 3.7 - 3.2, and 2.7 - 2.5 are supported. Extending to 3.8 and beyond is on hold until there is more interest, I get help, I need or there is or funding,

Whereas Byterun is loose in accepting bytecode opcodes that is invalid for particular Python but may be valid for another; x-python is more stringent. This has some pros but mostly 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 happening becomes vanishingly small. And 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.

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, or run newer “wordcode” bytecode on a “byte”-oriented byteocde, or vica versa.

In contrast, 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 bytecode back to Python 2.5 using Python 3.7 (largely), which is amazing give that 3.7’s native byte code is 2 bytes per instruction while 2.5’s is 1 or 3 bytes per instruction.

The “largely” part is because the interpreter has always made use of Python builtins and libraries, and for the most part these haven’t changed very much. Often, since many of the underlying builtins are the same, the interpreter can (and does) make use interpreter internals. For example, built-in functions like range() are supported this way.

So interpreting bytecode from a newer Python release than the release the Pyton interpreter is using, is often doable too. Even though Python 2.7 doesn’t support keyword-only arguments or format strings, it can still interpret bytecode created from using these constructs.

That’s possible here because these specific features are more syntactic sugar rather than extensions to the runtime. For example, format strings basically map down to using the format() function which is available on 2.7.

New features like asynchronous I/O and concurrency primatives are not in the older versions and need to be simulated. However that too is a possibility if there is interest or support.

You can run many of the tests that Python uses to test itself, (and I do!) and those work. Right now this program works best on Python up to 3.4 when life in Python was much simpler. It runs over 300 in Python’s test suite for itself without problems. For Python 3.6 the number drops down to about 237; Python 3.7 is worse still.

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.