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Reformulate answer, and remove off-topic discussion of bytecode in a question about compiling to C.

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Antoine Pitrou 2011-12-03 22:56:02 +01:00
parent cc809a286a
commit 17bd792cd3
1 changed files with 17 additions and 32 deletions
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Doc/faq/design.rst
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@ -380,11 +380,24 @@ is exactly the same type of object that a lambda form yields) is assigned!
Can Python be compiled to machine code, C or some other language?
-----------------------------------------------------------------
Not easily. Python's high level data types, dynamic typing of objects and
Practical answer:
`Cython <http://cython.org/>`_ and `Pyrex <http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/>`_
compile a modified version of Python with optional annotations into C
extensions. `Weave <http://www.scipy.org/Weave>`_ makes it easy to
intermingle Python and C code in various ways to increase performance.
`Nuitka <http://www.nuitka.net/>`_ is an up-and-coming compiler of Python
into C++ code, aiming to support the full Python language.
Theoretical answer:
.. XXX not sure what to make of this
Not trivially. Python's high level data types, dynamic typing of objects and
run-time invocation of the interpreter (using :func:`eval` or :func:`exec`)
together mean that a "compiled" Python program would probably consist mostly of
calls into the Python run-time system, even for seemingly simple operations like
``x+1``.
together mean that a naïvely "compiled" Python program would probably consist
mostly of calls into the Python run-time system, even for seemingly simple
operations like ``x+1``.
Several projects described in the Python newsgroup or at past `Python
conferences <http://python.org/community/workshops/>`_ have shown that this
@ -395,34 +408,6 @@ speedups of 1000x are feasible for small demo programs. See the proceedings
from the `1997 Python conference
<http://python.org/workshops/1997-10/proceedings/>`_ for more information.)
Internally, Python source code is always translated into a bytecode
representation, and this bytecode is then executed by the Python virtual
machine. In order to avoid the overhead of repeatedly parsing and translating
modules that rarely change, this byte code is written into a file whose name
ends in ".pyc" whenever a module is parsed. When the corresponding .py file is
changed, it is parsed and translated again and the .pyc file is rewritten.
There is no performance difference once the .pyc file has been loaded, as the
bytecode read from the .pyc file is exactly the same as the bytecode created by
direct translation. The only difference is that loading code from a .pyc file
is faster than parsing and translating a .py file, so the presence of
precompiled .pyc files improves the start-up time of Python scripts. If
desired, the Lib/compileall.py module can be used to create valid .pyc files for
a given set of modules.
Note that the main script executed by Python, even if its filename ends in .py,
is not compiled to a .pyc file. It is compiled to bytecode, but the bytecode is
not saved to a file. Usually main scripts are quite short, so this doesn't cost
much speed.
.. XXX check which of these projects are still alive
There are also several programs which make it easier to intermingle Python and C
code in various ways to increase performance. See, for example, `Cython
<http://cython.org/>`_, `Pyrex
<http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/>`_ and `Weave
<http://www.scipy.org/Weave>`_.
How does Python manage memory?
------------------------------