[issue16132] ctypes incorrectly encodes .format attribute of memory views

[David Beazley]

Change by David Beazley :


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[issue16132] ctypes incorrectly encodes .format attribute of memory views

[Mark Lawrence]

Changes by Mark Lawrence :


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[issue16132] ctypes incorrectly encodes .format attribute of memory views

[Martin Panter]

Martin Panter added the comment:

In Python 3.5 (since Issue 15944), you can now cast normal C-contiguous 
memoryviews (including ones from ctypes) to bytes:

>>> a = (ctypes.c_double * 3)(1,2,3)
>>> m = memoryview(a)
>>> m.format
'>> byteview = m.cast("B")
>>> byteview.format
'B'
>>> byteview[0]
0

Also, the format has changed at some point. See '

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[issue16132] ctypes incorrectly encodes .format attribute of memory views

[Mark Lawrence]

Mark Lawrence added the comment:

@David please accept our apologies for the delay in getting back to you.

Can someone else take a look please as I know nothing about ctypes, thanks.

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versions: +Python 3.4, Python 3.5 -Python 3.3

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[issue16132] ctypes incorrectly encodes .format attribute of memory views

[David Beazley]

New submission from David Beazley:

This is somewhat related to an earlier bug report concerning memory views, but 
as far as I can tell, ctypes is not encoding the '.format' attribute correctly 
in most cases.   Consider this example:

First, create a ctypes array:

 a = (ctypes.c_double * 3)(1,2,3)
 len(a)
3
 a[0]
1.0
 a[1]
2.0
 

Now, create a memory view for it:

 m = memoryview(a)
 len(m)
3
 m.itemsize
8
 m.ndim
1
 m.shape
(3,)
 

All looks well.  However, if you try to do anything with the .format or access 
the items, it's completely broken:

 m.format
'(3)d'
 m[0]
Traceback (most recent call last):
  File stdin, line 1, in module
NotImplementedError: memoryview: unsupported format (3)d
 

This is quite inconsistent with the behavior observed elsewhere. For example:

 import array
 b = array.array('d',[1,2,3])
 memoryview(b).format
'd'
 import numpy
 c = numpy.array([1,2,3],dtype='d')
 memoryview(c).format
'd'
 

As you can see, array libraries are using .format to encode the format of a 
single array item.  ctypes is encoding the format of the entire array (all 
items).  ctypes also includes endianness which presents additional difficulties.

This behavior affects both Python code that wants to use memoryviews, but also 
C extension code that wants to use the underlying buffer protocol to work with 
arrays in a generic way. Essentially, it cuts the use of ctypes off entirely 
unless you modify the underlying buffer handling code to special case it. 

Suggested fix:  Have ctypes only encode the format for a single item in the 
case of arrays.  Also, for items that are encoded using the native byte 
ordering, don't include an endianness modifier ('','', etc.).  Including the 
byte order just complicates all of the handling code because it has to be 
modified to a) know what the native byte ordering is and b) to check multiple 
cases such as for d and d.

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messages: 171965
nosy: dabeaz
priority: normal
severity: normal
status: open
title: ctypes incorrectly encodes .format attribute of memory views
type: behavior
versions: Python 3.3

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[issue16132] ctypes incorrectly encodes .format attribute of memory views

[Meador Inge]

Changes by Meador Inge mead...@gmail.com:


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