PEP 424: update from Alex.

pep-0424.txt

@@ -27,7 +27,7 @@ Specification
 This PEP formally documents ``__length_hint__`` for other
 interpreters and non-standard-library Python modules to implement.
 
-``__length_hint__`` must return an integer (else a TypeError is raised) or
+``__length_hint__`` must return an integer (else a ``TypeError`` is raised) or
 ``NotImplemented``, and is not required to be accurate. It may return a value
 that is either larger or smaller than the actual size of the container. A
 return value of ``NotImplemented`` indicates that there is no finite length
@@ -47,29 +47,25 @@ be used)::
         exact. Otherwise, it may over- or under-estimate by an
         arbitrary amount. The result will be an integer >= 0.
         """
-        if <obj has a __len__ method>:
+        try:
             return len(obj)
-        else:
+        except TypeError:
             try:
-                get_hint = obj.__length_hint__
+                get_hint = type(obj).__length_hint__
             except AttributeError:
                 return default
-            hint = get_hint()
+            try:
+                hint = get_hint(obj)
+            except TypeError:
+                return default
             if hint is NotImplemented:
                 return default
             if not isinstance(hint, int):
                 raise TypeError("Length hint must be an integer, not %r" %
                                 type(hint))
-            return max(hint, 0)
-
-Note: there is no good way to spell "obj has a __len__ method" in pure
-Python. In CPython, this comes down to checking for a ``sq_length``
-slot. Other implementations presumably have their own way of
-checking. Calling ``len(obj)`` and catching TypeError is not quite
-correct (as it would assume no __len__ method exists when in fact one
-exists but calling it raises TypeError); checking ``hasattr(obj,
-'__len__')`` likewise is incorrect if obj is a class defining a
-``__len__`` method for its instances.
+            if hint < 0:
+                raise ValueError("__length_hint__() should return >= 0")
+            return hint
 
 
 Rationale