[PATCH] Re: frozenset() without arguments should return a singleton

[Stefan Behnel]

Terry Reedy schrieb:
>>frozenset() called without arguments (or on empty sequences)
>>should always return a singleton object.
> 
> If we interpret 'should' as 'preferably by me', ok.
> 
> It will take some programmer's time to add the special case check and run 
> the test suite, and check in the changes.  Yours?  And perhaps some 
> execution time for each frozenset call.  Since frozenset is not much used, 
> and multiple empty frozensets very rare, and the difference mostly 
> invisible, the frozenset implementor probably went on to other things.

I read 'not much used' and 'very rare' as 'I rarely use them'. Others may.

It does not take much additional execution time, I just checked, a partial 
test is already in there that should be enough for the 'frozenset()' case. 
Additional tests for empty iterables would however be more costly and 
therefore not necessarily worth doing. Up to the maintainers.

I don't have any experience in writing extension modules for the standard 
library and 'running the test suite'. Implementing the check is trivial, 
though. Could anyone please 'run the test suite' ?

I tested it a bit, though, seems to work, including subclassing.

Stefan

--- Objects/setobject.c.ORIG    2005-02-12 14:04:54.000000000 +0100
+++ Objects/setobject.c 2005-02-12 14:41:04.000000000 +0100
@@ -76,6 +76,8 @@
         return (PyObject *)so;
  }

+PyObject *frozen_empty_set = NULL;
+
  static PyObject *
  frozenset_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
  {
@@ -83,7 +85,14 @@

         if (!PyArg_UnpackTuple(args, type->tp_name, 0, 1, &iterable))
                 return NULL;
-       if (iterable != NULL && PyFrozenSet_CheckExact(iterable)) {
+       if (iterable == NULL) {
+               if (type == &PyFrozenSet_Type) {
+                       if (frozen_empty_set == NULL)
+                               frozen_empty_set = make_new_set(type, NULL);
+                       Py_INCREF(frozen_empty_set);
+                       return frozen_empty_set;
+               }
+       } else if (PyFrozenSet_CheckExact(iterable)) {
                 Py_INCREF(iterable);
                 return iterable;
         }
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