Passing back an array from an extension module?

[Bob Greschke]

"This" is an array, but it doesn't look like I can either pass an
array object to the C extension module, and pass an array back to the
Python code, or create an array in C and pass it back to the Python
code.  I don't see anything in the Format Specifiers lists for
PtArg_Parse() or Py_BuildValue() functions to handle arrays.  However,
I do see C functions (Low-Level Functions on Built-in Types, Python
Essential Reference, pg 261) for creating tuples in a C function, and
then functions for creating long integers, and then functions for
adding items (_SetItem) to a tuple, and then the ability to pass back
an 'any object' in Py_BuildValue (the "O" format specifier).  Is that
a sensible thing to do?  It feels like it would chew up too much time
given the amount of data involved.


"Martin von Loewis" <loewis at informatik.hu-berlin.de> wrote in message
news:j4ae3iev04.fsf at informatik.hu-berlin.de...
> "Bob Greschke" <bob at passcal.nmt.edu> writes:
>
> > Then I need to pass that back to the Python side for graphing
> > the values.  I initially thought I could just create a tuple and
pass
> > that back, since nothing is going to be done with the values
except to
> > draw a graph, but there is the possibility of there being
16,000,000
> > data points...that makes for a slightly large statement, doesn't
it?
> > It doesn't look like I can do this with arrays in stock Python,
but it
> > does look like I can do it with the NumPy arrays.  Is that
correct?
>
> Not sure what 'this' is here. You certainly can create 'custom'
array
> types in Python, see Modules/arraymodule for an example of an array
> type that is implemented using a C memory buffer of primitive C
> values.
>
> I don't know whether you can use a NumPy array off-the-shelf; I
assume
> no since it probably manages its own memory.
>
> So you need to create a new type (see xxmodule.c for an example),
and
> implement its tp_as_sequence slot.
>
> > Now what about Py_DECREF, INCREF and all of that stuff in these
> > situations?  I read through a number of posts on dejagoogle that
were
> > talking about this, but they only confused me more. :-)
>
> I recommend you read the "Embedding and Extending
> tutorial". Basically, you need to implement an array accessor, which
> creates and returns a PyInt_FromLong every time you access the nth
> element of your array. When the reference counter of your Python
> wrapper object drops to zero, you should free(3) the memory of the
> very large array.
>
> Regards,
> Martin
>