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List:       python-capi-sig
Subject:    [capi-sig]Call protocol: tuple/dict subclasses?
From:       Jeroen Demeyer <J.Demeyer () UGent ! be>
Date:       2019-05-09 14:08:02
Message-ID: 5CD43442.3070409 () UGent ! be
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Dear C API lovers,

I have a question about the usual args/kwargs call protocol (as used by 
PyObject_Call and tp_call): is the "args" tuple supposed to be of type 
exactly tuple or are tuple subclasses allowed? Same question for 
"kwargs" and dict.

The documentation is not clear about this.

I do know that calls from Python are done with an exact tuple and exact 
dict, but in C this is not so clear. For example, the implementation of 
_PyErr_CreateException uses PyTuple_Check(), not PyTuple_CheckExact():

static PyObject*
_PyErr_CreateException(PyObject *exception, PyObject *value)
{
     if (value == NULL || value == Py_None) {
         return _PyObject_CallNoArg(exception);
     }
     else if (PyTuple_Check(value)) {
         return PyObject_Call(exception, value, NULL);
     }
     else {
         return PyObject_CallFunctionObjArgs(exception, value, NULL);
     }
}

Also, the implementation of the call protocol has assertions of both 
forms: you see both assert(PyTuple_Check(args)) and 
assert(PyTuple_CheckExact(args)) and similar for dict. It seems pretty 
random whether or not an exact type check is done.

I'm pretty sure that there are code paths that would result in an 
assertion failure because of this.

So basically my question is: what is the right behavior? Given the 
implementation of the Python bytecode interpreter, I would be inclined 
to say that type checks should be exact.

I'm noticing this because I'm working on an implementation of PEP 590.


Jeroen.
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