Generalize dictionary() to accept a sequence of 2-sequences. At the

outer level, the iterator protocol is used for memory-efficiency (the
outer sequence may be very large if fully materialized); at the inner
level, PySequence_Fast() is used for time-efficiency (these should
always be sequences of length 2).

dictobject.c, new functions PyDict_{Merge,Update}FromSeq2.  These are
wholly analogous to PyDict_{Merge,Update}, but process a sequence-of-2-
sequences argument instead of a mapping object.  For now, I left these
functions file static, so no corresponding doc changes.  It's tempting
to change dict.update() to allow a sequence-of-2-seqs argument too.

Also changed the name of dictionary's keyword argument from "mapping"
to "x".  Got a better name?  "mapping_or_sequence_of_pairs" isn't
attractive, although more so than "mosop" <wink>.

abstract.h, abstract.tex:  Added new PySequence_Fast_GET_SIZE function,
much faster than going thru the all-purpose PySequence_Size.

libfuncs.tex:
- Document dictionary().
- Fiddle tuple() and list() to admit that their argument is optional.
- The long-winded repetitions of "a sequence, a container that supports
  iteration, or an iterator object" is getting to be a PITA.  Many
  months ago I suggested factoring this out into "iterable object",
  where the definition of that could include being explicit about
  generators too (as is, I'm not sure a reader outside of PythonLabs
  could guess that "an iterator object" includes a generator call).
- Please check my curly braces -- I'm going blind <0.9 wink>.

abstract.c, PySequence_Tuple():  When PyObject_GetIter() fails, leave
its error msg alone now (the msg it produces has improved since
PySequence_Tuple was generalized to accept iterable objects, and
PySequence_Tuple was also stomping on the msg in cases it shouldn't
have even before PyObject_GetIter grew a better msg).

Objects/dictobject.c

@@ -993,7 +993,89 @@ dict_update(PyObject *mp, PyObject *other)
 
 /* Update unconditionally replaces existing items.
    Merge has a 3rd argument 'override'; if set, it acts like Update,
-   otherwise it leaves existing items unchanged. */
+   otherwise it leaves existing items unchanged.
+
+   PyDict_{Update,Merge} update/merge from a mapping object.
+
+   PyDict_{Update,Merge}FromSeq2 update/merge from any iterable object
+   producing iterable objects of length 2.
+*/
+
+static int
+PyDict_MergeFromSeq2(PyObject *d, PyObject *seq2, int override)
+{
+	PyObject *it;	/* iter(seq2) */
+	int i;		/* index into seq2 of current element */
+	PyObject *item;	/* seq2[i] */
+	PyObject *fast;	/* item as a 2-tuple or 2-list */
+
+	assert(d != NULL);
+	assert(PyDict_Check(d));
+	assert(seq2 != NULL);
+
+	it = PyObject_GetIter(seq2);
+	if (it == NULL)
+		return -1;
+
+	for (i = 0; ; ++i) {
+		PyObject *key, *value;
+		int n;
+
+		fast = NULL;
+		item = PyIter_Next(it);
+		if (item == NULL) {
+			if (PyErr_Occurred())
+				goto Fail;
+			break;
+		}
+
+		/* Convert item to sequence, and verify length 2. */
+		fast = PySequence_Fast(item, "");
+		if (fast == NULL) {
+			if (PyErr_ExceptionMatches(PyExc_TypeError))
+				PyErr_Format(PyExc_TypeError,
+					"cannot convert dictionary update "
+					"sequence element #%d to a sequence",
+					i);
+			goto Fail;
+		}
+		n = PySequence_Fast_GET_SIZE(fast);
+		if (n != 2) {
+			PyErr_Format(PyExc_ValueError,
+				     "dictionary update sequence element #%d "
+				     "has length %d; 2 is required",
+				     i, n);
+			goto Fail;
+		}
+
+		/* Update/merge with this (key, value) pair. */
+		key = PySequence_Fast_GET_ITEM(fast, 0);
+		value = PySequence_Fast_GET_ITEM(fast, 1);
+		if (override || PyDict_GetItem(d, key) == NULL) {
+			int status = PyDict_SetItem(d, key, value);
+			if (status < 0)
+				goto Fail;
+		}
+		Py_DECREF(fast);
+		Py_DECREF(item);
+	}
+
+	i = 0;
+	goto Return;
+Fail:
+	Py_XDECREF(item);
+	Py_XDECREF(fast);
+	i = -1;
+Return:
+	Py_DECREF(it);
+	return i;
+}
+
+static int
+PyDict_UpdateFromSeq2(PyObject *d, PyObject *seq2)
+{
+	return PyDict_MergeFromSeq2(d, seq2, 1);
+}
 
 int
 PyDict_Update(PyObject *a, PyObject *b)
@@ -1699,23 +1781,20 @@ static int
 dict_init(PyObject *self, PyObject *args, PyObject *kwds)
 {
 	PyObject *arg = NULL;
-	static char *kwlist[] = {"mapping", 0};
+	static char *kwlist[] = {"x", 0};
+	int result = 0;
 
 	if (!PyArg_ParseTupleAndKeywords(args, kwds, "|O:dictionary",
 					 kwlist, &arg))
-		return -1;
-	if (arg != NULL) {
-		if (PyDict_Merge(self, arg, 1) < 0) {
-			/* An error like "AttributeError: keys" is too
-			   cryptic in this context. */
-			if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
-				PyErr_SetString(PyExc_TypeError,
-					"argument must be of a mapping type");
-			}
-			return -1;
-		}
+		result = -1;
+
+	else if (arg != NULL) {
+		if (PyObject_HasAttrString(arg, "keys"))
+			result = PyDict_Merge(self, arg, 1);
+		else
+			result = PyDict_MergeFromSeq2(self, arg, 1);
 	}
-	return 0;
+	return result;
 }
 
 static PyObject *
@@ -1725,8 +1804,15 @@ dict_iter(dictobject *dict)
 }
 
 static char dictionary_doc[] =
-"dictionary() -> new empty dictionary\n"
-"dictionary(mapping) -> new dict initialized from mapping's key+value pairs";
+"dictionary() -> new empty dictionary.\n"
+"dictionary(mapping) -> new dict initialized from a mapping object's\n"
+"    (key, value) pairs.\n"
+"dictionary(seq) -> new dict initialized from the 2-element elements of\n"
+"    a sequence; for example, from mapping.items().  seq must be an\n"
+"    iterable object, producing iterable objects each producing exactly\n"
+"    two objects, the first of which is used as a key and the second as\n"
+"    its value.  If a given key is seen more than once, the dict retains\n"
+"    the last value associated with it.";
 
 PyTypeObject PyDict_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)