How to create a writeable buffer?

[Steve Chaplin]

I'm trying to create an object (as a C extension) to support the buffer
interface and was getting "TypeError: buffer is read-only" errors. So I
had a look at the array object and it has the same problem:

$ python

>>import array
a=array.array('c', ['a', 'b', 'c'])
a

array('c', 'abc')

>>a[0]

'a'

>>a[0]='d'
a

array('c', 'dbc')

>>buf=buffer(a)
buf[0]

'd'

>>buf[0]='e'

TypeError: buffer is read-only

The array is a mutable (writable) object, and the array implements the
buffer interface and provides a 'getwritebuf' method.
(Python-2.4.4/Modules/arraymodule.c has an array_buffer_getwritebuf())
so why does it give "TypeError: buffer is read-only"?

Steve