[Python-Dev] frozenset C API?

Bill Janssen janssen at parc.com
Tue Sep 4 21:31:09 CEST 2007

Raymond, thanks for the note.

> 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);

This is essentially the same thing I mentioned, except using a set
instead of a list as the iterable.

I'm just a tad annoyed at the fact that I know at set creation time
exactly how many elements it's going to have, and this procedure
strikes me as a somewhat inefficient way to create that set.  Just
tickles my "C inefficiency" funnybone a bit :-).

> 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)

Didn't really mean to propose "PyDict_SET_ITEM(s, index, key, value)",
should have been

       PyDict_SET_ITEM(s, index, value)

But your point is still well taken.  How about this one, though:

     PyDict_NEW(int) => PySetObject *
     PyDict_ADD(s, value)

ADD would just stick value in the next empty slot (and steal its


More information about the Python-Dev mailing list