On-topic: alternate Python implementations

[Stefan Behnel]

Paul Rubin, 04.08.2012 22:43:
> Stefan Behnel writes:
>>> Calling CPython hardly counts as compiling Python into C.
>> CPython is written in C, though. So anything that CPython does can be
>> done in C. It's not like the CPython project used a completely unusual
>> way of writing C code.
> 
> CPython is a relatively simple interpreter, and executing code 
> by invoking such an interpreter IMHO doesn't count as "compiling" it in
> any meaningful way.

Oh, CPython is substantially more than an interpreter. The eval loop is
only *one* way to use the runtime environment. Remember that it has many
builtin types and functions as well as a huge standard library. Much of the
runtime environment is already written in C or can be compiled down to C.
If you compile Python code into C code that avoids the eval loop and only
uses the CPython runtime environment (which is what Cython does), I think
that qualifies as compiling Python code to C. It's definitely the most
practical and user friendly way to do it.


>> You will always need some kind of runtime infrastructure when you
>> "compile Python into C", so you can just as well use CPython for that
>> instead of reimplementing it completely from scratch. 
> 
> Maybe there's parts of Cpython you can re-use, but having the CPython
> interpreter be the execution engine for "compiled" Python generators
> again fails the seriousness test of what it means to compile code.  If
> you mean something other than that, you might explain more clearly.

See above.


>> Both Cython and Nuitka do exactly that, 
> 
> I didn't know about Nuitka; it looks interesting but (at least after a
> few minutes looking) I don't have much sense of how it works.

It's mostly like Cython but without the type system, i.e. without all the
stuff that makes it useful in real life. Just a bare
Python-to-C++-in-CPython compiler, without much of a way to make it do what
you want.


>> Last I heard, PyPy had a couple of GCs to choose from,
> 
> PyPy doesn't compile to C

RPython (usually) does, though, and my guess is that the memory management
part of the runtime is written in RPython.


> but I guess compiling to C doesn't preclude
> precise GC, as long as the generated C code carefully tracks what C
> objects can contain GC-able pointers, and follows some constraints about
> when the GC can run.  Some other compilers do this so it's not as big a
> deal as it sounded like at first.  OK.

Yep, C really becomes a lot nicer when you generate it.


>> Huh? LuaJIT is a reimplementation of Lua that uses an optimising JIT
>> compiler specifically for Lua code. How is that similar to the Jython
>> runtime that runs *on top of* the JVM with its generic byte code based
>> JIT compiler?
> 
> I thought LuaJIT compiles the existing Lua VM code, but I haven't
> looked at it closely or used it.

Ok. It obviously reuses code, but the VM part of it is really different
from standard Lua.

Stefan