Adjusted the wording to make __iter__() more strongly required on iterators.

See http://mail.python.org/pipermail/python-dev/2002-July/026668.html .

Added text describing the distinct purposes of __iter__() and next().
See http://mail.python.org/pipermail/python-dev/2002-July/026683.html .

pep-0234.txt

@@ -139,10 +139,26 @@ Python API Specification
 
     Classes can define how they are iterated over by defining an
     __iter__() method; this should take no additional arguments and
-    return a valid iterator object.  A class is a valid iterator
-    object when it defines a next() method that behaves as described
-    above.  A class that wants to be an iterator also ought to
-    implement __iter__() returning itself.
+    return a valid iterator object.  A class that wants to be an
+    iterator should implement two methods: a next() method that behaves
+    as described above, and an __iter__() method that returns self.
+
+    The two methods correspond to two distinct protocols:
+
+    1. An object can be iterated over with "for" if it implements
+       __iter__() or __getitem__().
+
+    2. An object can function as an iterator if it implements next().
+
+    Container-like objects usually support protocol 1.  Iterators are
+    currently required to support both protocols.  The semantics of
+    iteration come only from protocol 2; protocol 1 is present to make
+    iterators behave like sequences.  But the analogy is weak -- unlike
+    ordinary sequences, iterators are "sequences" that are destroyed
+    by the act of looking at their elements.
+
+    Consequently, whenever any Python programmer says "for x in y",
+    he or she must be sure of whether this is going to destroy y.
 
 
 Dictionary Iterators