[Python-Dev] frozenset C API? (original) (raw)

Raymond Hettinger python at rcn.com
Tue Sep 4 21:14:17 CEST 2007

You can create a frozenset from any iterable using PyFrozenSet_New().

If you don't have an iterable and want to build-up the frozenset one element at a time, the approach is to create a regular set (or some other mutable container), add to it, then convert it to a frozenset when you're done:

s = PySet_New(NULL); PySet_Add(s, obj1); PySet_Add(s, obj2); PySet_Add(s, obj3); f = PyFrozenSet_New(s); Py_DECREF(s);

That approach is similar to what you do with tuples in pure python. You either build them from an iterable "t = tuple(iterable)" or your build-up a mutable object one element at a time and convert it all at once:

s = [] s.append(obj1) s.append(obj2) t = tuple(s)

The API you propose doesn't work because sets and frozensets are not indexed like tuples and lists. Accordingly, sets and frozensets have a C API that is more like dictionaries. Since dictionaries are not indexable, they also cannot have an API like the one you propose:

 PyDict_NEW(int) => PySetObject *
 PyDict_SET_ITEM(s, index, key, value)

If you find all this really annoying and need to fashion a small frozenset with a few known objects, consider using the abstract API:

f = PyObject_CallFunction(&PyFrozenSet_Type, "(OOO)", obj1, obj2, obj3);

That will roll the whole process up into one line.

Hope this was helpful,

Raymond

Bill Janssen <janssen at parc.com>Add To Address Book|This is Spam Subject:[Python-Dev] frozenset C API? To:python-dev at python.org

I'm looking at building a "frozenset" instance as a return value from a C function, and the C API seems ridiculously clumsy. Maybe I'm misunderstanding it. Apparently, I need to create a list object, then pass that to PyFrozenSet_New(), then decref the list object.

Is that correct?

What I'd like is something more like

 PyFrozenSet_NEW(int) => PySetObject *
 PyFrozenSet_SET_ITEM(s, i, v)

Any idea why these aren't part of the API?

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