commit 0e0e332b466a9784620c483faa374067381e96ce
Author: Panu Matilainen <pmatilai@redhat.com>
Date:   Wed May 19 10:12:43 2010 +0300

    Handle non-existent dependency sets in python (RhBug:593553)
    - rpmdsNew() returns NULL if the requested dependency type doesn't
      exist in the header. The C-side API can handle NULL to all rpmds
      "methods" and this is how librpm deals with non-existent sets rather
      than waste memory on for empty ds structures. However the python side
      wasn't expecting NULL for legal requests (but not setting error either)
      and thus blowing up with SystemError exception.
    - Raise TypeError on illegal arguments to rpm.ds constructor, and present
      non-existent dependency sets as empty rpm.ds objects to python. This
      lets python callers use iteration over ds items regardless of whether
      the dependency actually exists or not. The alternative of returning
      None (or raising exceptions) would break existing code for no
      particularly good reason.

diff --git a/python/rpmds-py.c b/python/rpmds-py.c
index 771cd06..4587201 100644
--- a/python/rpmds-py.c
+++ b/python/rpmds-py.c
@@ -288,10 +288,11 @@ static PyObject * rpmds_new(PyTypeObject * subtype, PyObject *args, PyObject *kw
 	} else {
 	    ds = rpmdsNew(h, tagN, 0);
 	}
+    } else {
+	PyErr_SetString(PyExc_TypeError, "header or tuple expected");
+	return NULL;
     }
     
-    if (ds == NULL) return NULL;
-	
     return rpmds_Wrap(subtype, ds);
 }