Duck Typing and **kwds

[Luis Zarrabeitia]

Hi there.

I just tried this test:

====
def f(**kwds):
print kwds

import UserDict
d = UserDict.UserDict(hello="world")
f(**d)
====

And it fails with a TypeError exception ("f() argument after ** must be a
dictionary"). I find that weird, as UserDict should support all protocols that
dict supports, yet it doesn't seem to support ** unpacking. If instead of
UserDict I use a derivate class from dict (class mydict(dict):pass), the **
operator works as expected. It also works if I execute f(**dict(d)) instead.

Is that behavior expected? Is there any reason (performance, perhaps?) to break
duck-typing in this situation?

Regards,

--
Luis Zarrabeitia
Facultad de Matemática y Computación, UH
http://profesores.matcom.uh.cu/~kyrie


--
"Al mundo nuevo corresponde la Universidad nueva"
UNIVERSIDAD DE LA HABANA
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[Kay Schluehr]

On 11 Okt., 06:05, Luis Zarrabeitia <[e-mail]:

Is that behavior expected? Is there any reason (performance, perhaps?) to break
duck-typing in this situation?

I guess it wasn't considered to be relevant writing a coercion
function since there aren't too many dict like types that are not
dicts. You can examine what happens in the relevant code fragment in
ceval.c

if (flags & CALL_FLAG_KW) {
kwdict = EXT_POP(*pp_stack);
if (!(kwdict && PyDict_Check(kwdict))) {
PyErr_Format(PyExc_TypeError,
"%s%s argument after ** "
"must be a dictionary",
PyEval_GetFuncName(func),
PyEval_GetFuncDesc(func));
goto ext_call_fail;
}
}
if (flags & CALL_FLAG_VAR) {
stararg = EXT_POP(*pp_stack);
if (!PyTuple_Check(stararg)) {
PyObject *t = NULL;
t = PySequence_Tuple(stararg);
if (t == NULL) {
if (PyErr_ExceptionMatches(PyExc_TypeError)) {
PyErr_Format(PyExc_TypeError,
"%s%s argument after * "
"must be a sequence",
PyEval_GetFuncName(func),
PyEval_GetFuncDesc(func));
}
goto ext_call_fail;
I paralleled this with the subsequent clause for tuples where some
coercion function is applied ( PySequence_Tuple ).

It would be cleaner to implement a simple conversion function
PyDictlike_Dict

which seeks for a __dictionary__ method providing the following
interface

__dictionary__(self) -dict

and is called when being available. Maybe you can suggest this at
python-dev? It might not require a PEP.