[Numpy-discussion] Optionally using Numeric in another compiled extension package.

Todd Miller jmiller at stsci.edu
Wed Jan 15 10:57:02 EST 2003

Chris Barker wrote:

>Hi folks,
>I use Numeric an wxPython together a lot (of course I do, I use Numeric
>for everything!).
>Unfortunately, since wxPython is not Numeric aware, you lose some real
>potential performance advantages. For example, I'm now working on
>expanding the extensions to graphics device contexts (DCs) so that you
>can draw a whole bunch of objects with a single Python call. The idea is
>that the looping can be done in C++, rather than Python, saving a lot of
>overhead of the loop itself, as well as the Python-wxWindows translation
>For drawing thousands of points, the speed-up is substantial. It's less
>substantial on more complex objects (rectangles give a factor of two
>improvement for ~1000 objects), due to the longer time it takes to draw
>the object itself, rather than make the call. 
>Anyway, at the moment, Robin Dunn has the wrappers set up so that you
>can pass in a NumPy array (or, indeed, and sequence) rather than a list
>or tuple of coordinates, but it is faster to use a list than a NumPy
>array, because for arrays, it uses the generic PySequence_GetItem call.
>If we used the NumPy API directly, it should be faster than using a
>list, not slower! THis is how a representative section of the code looks
>bool      isFastSeq  = PyList_Check(pyPoints) ||
>                // Get the point coordinants
>                if (isFastSeq) {
>                    obj = PySequence_Fast_GET_ITEM(pyPoints, i);
>                }
>                else {
>                    obj = PySequence_GetItem(pyPoints, i);
>                }
>So you can see that if a NumPy array is passed in, PySequence_GetItem
>will be used.
>What I would like to do is have an isNumPyArray check, and then access
>the NumPy array directly in that case.
>The tricky part is that Robin does not want to have wxPython require
>Numeric. (Oh how I dream of the day that NumArray becomes part of the
>standard library!)
>How can I check if an Object is a NumPy array (and then use it as such),
>without including Numeric during compilation?
>I know one option is to have condition compilation, with a NumPy and
>non-Numpy version, but Robin is managing a whole lot of different
>version as it is, and I don't think he wants to deal with twice as many!
>Anyone have any ideas?
Use the Python C-API and string literals as the basis for the interface. 
 I think the steps are something like this:

1.  Import "Numeric". (PyImport_ImportModule)

2.  Get the module dictionary.    (PyModule_GetDict)

3.  Get "array" out of the dictionary.   (PyDict_GetItemString)

4.  Call "isinstance" on Numeric.array and the object.   


1. Import "numarray".

2. Get the module dictionary.

3. Get "NumArray" out of the dictionary

4. Call the C-API equivalent of "isinstance" on numarray.NumArray and 
the object.

The first 3 steps of both cases can be initialized once, I think, and 
stored in C static variables to avoid repeated fetches.
If any of the first 3 steps fail, then consider that case failed and 
returning False.
If it's not a Numeric array,  check to see if it's a numarray.

>By the way, you can substitute NumArray for NumPy in this, as it is the
>wave of the future, and particularly if it would be easier.

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