[Pythonmac-SIG] [SciPy-user] Best/Safest way to parse NumPy array in C extension?

[Christopher Barker]

Lou,

This type of question is best posed on the numpy list:

http://projects.scipy.org/mailman/listinfo/numpy-discussion

Lou Pecora wrote:

>>   PyArrayObject *mat;
>>   PyArg_ParseTuple(args, "O!", &PyArray_Type, &mat);

If you do this, you then need to check and see if the array that got 
passed in has the properties you want: size, shape, type.

>>   PyObject *Pymat;
>>   PyArrayObject *mat;
>>   PyArg_ParseTuple(args, "O", &Pymat);
>>   mat=PyArray_ContiguousFromObject(objin,NPY_DOUBLE,
>> 2,2);

I've always used PyArray_ContiguousFromObject, because then it does all 
the checking for me, and it allows my code to accept anything that can 
by turned into a numpy array.

>> The latter appears to have the constraint that the
>> array be contiguous.  Or is that an illusion and
>> it's
>> saying it _expects_ a Python object that has
>> contiguous data?

What it does is convert whatever you pass in to a Contiguous array. If 
it already is a contiguous array, then it returns itself (I think -- 
check the docs). There is probably something like:

PyArray_FromObject

That does not guarantee contiguous, if you don't need that.

By the way, I think these are the old Numeric names, which numpy has a 
compatibility layer for, but you may want to use the new, numpy ones. 
There are a lot, but most are special cases of:

Py_Array_FromAny

This is well documented in the Numpy book, I'm not sure about what's online.


>> I've done both.  Pointing C arrays to the resulting
>> PyArrays' data works fine, but I fear one way or the
>> other might be courting disaster.
>>
>> BTW, I do other checks on dimension, type, etc., but
>> I left those out here for clarity.  (I hope.)

If you do all those checks, you're OK, but do make sure you are not 
assuming contiguous, if you haven't forced it -- that can slip through 
testing.

-Chris
-- 
Christopher Barker, Ph.D.
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