f2pypy-0.0 (experimental)



http://www.dalkescientific.com/writings/diary/archive/2011/11/09/f2pypy.html



This is an experimental/prototype version of NumPy's f2py which reads
a pyf definition file to create a ctypes-based Python interface.

The goal is to explore what needs to be done to get f2py support for
pypy. As of 9 November 2011 "numpypy" is NOT mature enough to handle
this task. Specifically, I can't get the array data location to pass
it to a ctypes function.

If you only use scalar values then this interface will work. With some
luck. I've only tested it on one pyf file!

I am not an f2py user nor am I a numpy user. For a proper
implementation I would need to know more about how both f2py and numpy
work. What I have done is only enough to say that conversion is
possible. You are more than welcome to take on the task!

  ====== Getting started ========

Here's how I run the cblas code on my machine:

  $PYTHON -m f2pypy tests/fblas.pyf -l vecLib --skip cdotu,zdotu,cdotc,zdotc
  (works with pypy and with python2.7)

This generates "fblas.py", which works with python2.7 (only one test
failure with "test_fblas.py"). It does not work on pypy because pypy
doesn't implement enough of the numpy API.

PyPy's version of numpy arrays needs (at least):
  - a way to pass a numpy array into a ctypes function
  - support for the 'copy' keyword parameter to numpy.array()

For now you can test it with cpython. Even in that environment, the
limited tests don't all pass. There are many things about f2py which I
don't understand.

  ====== How does it work? ========

The f2py submodule 'crackfortan' reads a pyf file and creates an AST.
The subdirectory f2pypy/f2py/ contains an exact copy of the numpy/f2py
from the 1.6 distribution. Copyright 2000-2011 by Pearu
Peterson. Thank him for all the hard work in parsing Fortran program,
developing the interface file definition, and making code which was
very easy to repurpose for this project!

Because it uses features of numpy which don't yet exist in numpypy, I
created the "pypy_shim" module to monkey-patch those features.

This module uses crackfortan to get the AST, then generates the
corresponding Python code. The Python code includes:

  - lookup code to get the shared library functions
  - ctypes definitions for those functions
  - equivalent Python interface (including default values)
  - checks on the input values and ranges

Parts of the pyf file embed C code, which is inserted into the CPython
extension module. I used Fredrik Lundh description of the Pratt's
"Top-down Operator Precendence" to parse the C expression into another
AST. After a bit of manipulation, I convert that AST into Python code.

  ====== Thanks ========

Special thanks goes to Pearu Peterson for the f2py code, and to
Fredrik Lundh for his essay on how to parse expressions using simple
Python code.

  ====== Copyright ========

Copyright to f2py module and "test_blas.py" is held by others. I
hereby irrevocably donate the remaining code to the public domain.

                                Andrew Dalke
                                Gothenburg, Sweden
                                PyPy sprint, November 2011