[Python-Dev] Is "t#" argument format meant to be char buffer, or just read-only?

Thomas Heller theller at python.net
Thu Jun 8 10:27:09 CEST 2006

Brett Cannon wrote:
> I fixed the crasher for ``int(buffer(array.array('c')))`` by making
> buffer objects operate properly.  Problem is that by doing so I broke
> the ctypes tests with a bunch of similar errors::

You have not yet committed this fix, right?

> ======================================================================
> ERROR: test_endian_double (ctypes.test.test_byteswap.Test)
> ----------------------------------------------------------------------
> Traceback (most recent call last):
>   File "/code/python/trunk/Lib/ctypes/test/test_byteswap.py", line
> 134, in test_endian_double
>     self.failUnlessEqual(bin(struct.pack("d", math.pi)), bin(s))
>   File "/code/python/trunk/Lib/ctypes/test/test_byteswap.py", line 7, in bin
>     return hexlify(buffer(s)).upper()
> TypeError: requested buffer type not available
> Turns out the test does the following::
>   binascii.hexlify(buffer(ctypes.c_double(math.pi)))
> This is a problem because binascii.hexlify() uses "t#" as its argument
> format string to PyArg_ParseTuple() and that fails now with a
> TypeError since ctypes.c_double (which subclasses ctypes._SimpleCData
> which defines the buffer interface) does not have a char buffer
> defined.
> Now this used to pass since buffer objects just used the read or write
> buffer in place of the char buffer, regardless if the wrapped object
> had a char buffer function defined.
> But in checking out what "t#" did, I found a slight ambiguity in the
> docs.  The docs say "read-only character buffer" for the short
> description, but "read-only buffer" for the longer description.  Which
> is it?

I am using binascii.hexlify(buffer(obj)) as a simple way to look at the bytes of
the memory block.

I think that hexlify should be able to use any buffer object that has
a readable memory block, not only those with charbuffers.

The docs say that the binascii methods are used to "convert between binary
and various ASCII-encoded binary representations".

> Plus, Thomas, you might want to change _SimpleCData if you want it to
> truly suport char buffers.

I did not implement that because the memory block contains binary data,
not text.


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