Update bltinmodule.c

Fix incorrect specialized deallocation of float subclasses in sum()

In the complex-number fast path of builtin_sum_impl ( Python/bltinmodule.c ), items are tested with PyFloat_Check() but then released with _Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc) . PyFloat_Check() also matches float subclasses, whereas _PyFloat_ExactDealloc is only valid for exact float objects. As a result, when summing instances of a float subclass into a complex start value, the subclass instances are deallocated as exact floats, bypassing their real tp_dealloc (instance __dict__ , weakref, and GC cleanup). This can leak memory and leave the GC in an inconsistent state.

The adjacent float fast path already guards the same specialized dealloc with PyFloat_CheckExact() ; this change brings the complex path in line by using PyFloat_CheckExact() as well. Subclass instances then fall through to the generic PyNumber_Add path with a normal Py_DECREF , preserving numeric behavior while fixing the deallocation.

Reproducer:

class F(float):
    pass

sum([F(1.0), F(2.0)], 0j)  # subclass instances were freed as exact floats

Change: in the complex branch of builtin_sum_impl , replace PyFloat_Check(item) with PyFloat_CheckExact(item) (one line).

Author: opale-c

Python/bltinmodule.c

@@ -3046,7 +3046,7 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
                     return NULL;
                 }
             }
-            if (PyFloat_Check(item)) {
+            if (PyFloat_CheckExact(item)) {
                 double value = PyFloat_AS_DOUBLE(item);
                 re_sum = cs_add(re_sum, value);
                 _Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc);