bug skipping optional keyword arguments of type "w#"

[mdr0]

BPO	985713
Nosy	@birkenfeld
Files	Python-2.3.4-getargs-buffer-fix.diff: Python/getargs.c patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2005-06-01.18:24:29.000>
created_at = <Date 2004-07-06.04:37:06.000>
labels = ['interpreter-core']
title = 'bug skipping optional keyword arguments of type "w#"'
updated_at = <Date 2005-06-01.18:24:29.000>
user = 'https://bugs.python.org/mdr0'

bugs.python.org fields:

activity = <Date 2005-06-01.18:24:29.000>
actor = 'georg.brandl'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2004-07-06.04:37:06.000>
creator = 'mdr0'
dependencies = []
files = ['6076']
hgrepos = []
issue_num = 985713
keywords = ['patch']
message_count = 4.0
messages = ['46317', '46318', '46319', '46320']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'mdr0', 'mdehoon']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue985713'
versions = []

[mdr0]

When using PyArg_ParseTupleAndKeywords(), if you use
"w#" as an optional keyword argument, you cannot skip
it and still specify an optional argument that follows
it. For example, take the following function:

static PyObject *
dummy_func(PyObject *self, PyObject *args, PyObject *kwds)
{
        char *string;
        char *buf = NULL;
        int bufsize = 0;
        unsigned short ushort = 5;
        static char *kwlist[] = { "string", "buffer",
"ushort", NULL };

    if (! PyArg_ParseTupleAndKeywords(args, kwds,

"s|w#H", kwlist,
&string,
&buf,
&bufsize,
&ushort))
return NULL;

    printf("buf=%p, bufsize=%d, ushort=%hu\\n", buf,

bufsize, ushort);
snprintf(buf, bufsize, "you said \"%s\"\n",
hostname);

    return Py_BuildValue("s", hostname);

}

You can call this function successfully as follows:

# don't specify buffer or ushort
dummy_func("text")

Or like this:

# specify buffer
buf = array.array('c', [' ' for x in range(0, 1023)])
dummy_func("text", buffer=buf)

Or like this:

# specify buffer and ushort
buf = array.array('c', [' ' for x in range(0, 1023)])
dummy_func("text", buffer=buf, ushort=10)

However, the following does NOT work:

# specify ushort without first specifying buffer
dummy_func("text", ushort=10)

It fails with the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: argument 2 impossible<bad format char>

This is because the skipitem() function in
Python/getargs.c does not know how to skip arguments
for the "w#" format unit.
I've attached a patch that fixes this.

Note that skipitem() is also missing code for many
other legal format units, in addition to "w" and "w#".
In particular:

u, u# (see patch bpo-853890)
es, es#
et, et#
I (capital i)
k
K
D
S
U
t#
(tuple)

It might be a good idea to refactor the code so that
if/when new format units are added, the same code will
handle them for both skipitem() and convertsimple().

[mdr0]

Logged In: YES
user_id=994239

Whoops! You'll need to change "hostname" to "string" in
that example function. I missed a few occurances of that
variable when I renamed it to make the example more clear...
*sheepish grin*

[mdehoon]

Logged In: YES
user_id=488897

I applied your patch and ran the Python test suite, finding
no problems with this patch. I'll send a patch review to
python-dev.

[birkenfeld]

Logged In: YES
user_id=1188172

Closing; subsumed by bpo-1212928.