[Python-Dev] buffer interface for C extensions (original) (raw)

M.-A. Lemburg mal at egenix.com
Thu May 22 23:10:09 CEST 2008

On 2008-05-19 00:59, Dan Lenski wrote:

Hi all,

I've written a small C extension to submit commands to SCSI devices via Linux's sgio driver (for a camera hacking project). The extension is just a wrapper around a couple ioctl()'s with Pythonic exception handling thrown in. One of my extension methods is called like this from python: sg.write(fd, command[, data, timeout) Both command and data are binary strings. I would like to be able to use either a regular Python string or an array('B', ...) for these read-only arguments. So I tried to use the "t#" argument specifier to indicate that these arguments could be either strings or objects that implement the read- only buffer interface: if (!PyArgParseTuple(args, "it#|t#i:write", &sgfd, &cmd, &cmdLen, &buf, &bufLen, &timeout)) return NULL; Now, this works fine with strings, but when I call it with an array I get a TypeError: TypeError: write() argument 2 must be string or read-only character buffer, not array.array So, I then tried changing "t#" to "w#" to indicate that the arguments must implement the /read-write/ buffer interface. Now the array objects work, but when I try a string argument, I naturally get this error: TypeError: Cannot use string as modifiable buffer So here's what I don't understand. Why doesn't the "t#" argument specifier support read-write buffers as well as read-only buffers? Aren't read-write buffers a superset of read-only buffers?? Is there something I'm doing wrong or a quick fix to get this to work appropriately?

You should probably ask such questions on the capi-sig list.

To answer your question:

t# requires support for the read-only 8-bit character buffer interface s# can use the read buffer interface w# requires support for the write buffer interface

Those are two different buffer interface slots, so whether a particular object works with t# or w# depends on whether it implements the slots in question.

You should probably try s#, as this will also work with objects that implement the getreadbuffer slot.

The details can be found in Python/getargs.c

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