[Pyobjc-dev] PyBuffer* vs. array.array()

[Bill B. <bb...@co...>]

I'm in the process of bridging various methods in PyObjC that take  
(void *) types as arguments or return (void *) buffer references.

On the C side of the fence, I'm using PyBuffer_FromReadWriteMemory() or  
PyBuffer_FromMemory() as appropriate to create the buffer.  On the  
Python side, I use array.array('B') to create the buffer that is passed  
across the bridge and mapped to the (void *) method arguments.

However, I noticed that the two types are not the same.  They both work  
as byte buffers just fine, but the result of PyBuffer*() acts as,  
basically, a big <str> on the Python side whereas array.array('B') is a  
buffer of specifically typed unsigned chars.

In writing the unit tests, I came across a problematic situation that  
could easily arise in code (feel free to comment on the silliness of  
this code, if any... and note that I'm using the comprehension style  
even after that long rant I posted earlier :-):

         singlePlane = array.array('B')
         singlePlane.fromlist([0 for x in  range(0, width*height*3)] )
         for i in range(0, 256*256):
             si = i * 3
             singlePlane[si] = rPlane[i]
             singlePlane[si+1] = gPlane[i]
             singlePlane[si+2] = bPlane[i]

	    i2 = ... create i2 using data in singlePlane ...
....  then ...

         bitmapData = i2.bitmapData()
         self.assertEquals(len(bitmapData), len(singlePlane))
         for i in range(0,100):
             self.assertEquals(bitmapData[i], singlePlane[i],  
"bitmapData and singlePlane differ at byte %d" % i)

The contents of singlePlane and bitmapData are identical, but the unit  
test fails:

<type 'str'> -- type of element 0 of bitmapData
<type 'int'> -- type of element 0 of singlePlane
F.
======================================================================
FAIL: testImageData (__main__.TestNSBitmapImageRep)
----------------------------------------------------------------------
Traceback (most recent call last):
   File "Lib/AppKit/test/test_nsbitmapimagerep.py", line 74, in  
testImageData
     self.assertEquals(bitmapData[i], singlePlane[i], "bitmapData and  
singlePlane differ at byte %d" % i)
   File  
"/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/ 
unittest.py", line 292, in failUnlessEqual
     raise self.failureException, \
AssertionError: bitmapData and singlePlane differ at byte 0

----------------------------------------------------------------------
Ran 2 tests in 1.564s

FAILED (failures=1)

--

The real problem is that something created via array.array() comes back  
as an object that behaves differently.   Clearly, a bug in my bridge  
code, yes, but raises a question:

What should I be using?

 From the python side, it appears to only be possible to create buffer  
style objects-- big byte bags-- via the array module [yes, a <str> will  
work, but a byte-array seems so much more correct].  On the C side, it  
seems that the only way to create a byte buffer like object is to use  
the PyBuffer* API.

Advice, please?

BTW:  All of this lays the foundation for creating a PILImage object in  
Cocoa that is a seamless part of the NSImage family of classes.  I.e.  
PIL would then be fully integrated into any Cocoa app that can  
dynamically load the python interpreter (another problem I'm working  
through).

Thanks.

b.bum


b.bum
We gladly feast on those who would subdue us.