In the examples section, show how zip() is reversible.

Patches to the reference implementation:

    __getitem__() raises IndexError immediately if no sequences were
    given.

    __len__() returns 0 if no sequences were given.

    __cmp__() new method

Added a little more explanation to raise-a-TypeError-for-zip(a)

Added `fold' as one of the alternative names proposed.

pep-0201.txt

@@ -172,6 +172,26 @@ Examples
     >>> map(None, a, b, c, d)
     [(1, 5, 9, 12), (2, 6, 10, 13), (3, 7, 11, None), (4, 8, None, None)]
 
+    Note that when the sequences are of the same length, zip() is
+    reversible:
+
+    >>> a = (1, 2, 3)
+    >>> b = (4, 5, 6)
+    >>> x = zip(a, b)
+    >>> y = zip(*x) # alternatively, apply(zip, x)
+    >>> z = zip(*y) # alternatively, apply(zip, y)
+    >>> x
+    [(1, 4), (2, 5), (3, 6)]
+    >>> y
+    [(1, 2, 3), (4, 5, 6)]
+    >>> z
+    [(1, 4), (2, 5), (3, 6)]
+    >>> x == z
+    1
+
+    It is not possible to reverse zip this way when the sequences are
+    not all the same length.
+
 
 
 Reference Implementation
@@ -195,6 +215,8 @@ Reference Implementation
             self.__seqlen = len(args)
 
         def __getitem__(self, i):
+            if not self.__sequences:
+                raise IndexError
             ret = []
             exhausted = 0
             for s in self.__sequences:
@@ -218,6 +240,8 @@ Reference Implementation
                     slen = len(s)
                     if shortest < 0 or slen < shortest:
                         shortest = slen
+                if shortest < 0:
+                    return 0
                 return shortest
             longest = 0
             for s in self.__sequences:
@@ -226,6 +250,30 @@ Reference Implementation
                     longest = slen
             return longest
 
+        def __cmp__(self, other):
+            i = 0
+            smore = 1
+            omore = 1
+            while 1:
+                try:
+                    si = self[i]
+                except IndexError:
+                    smore = 0
+                try:
+                    oi = other[i]
+                except IndexError:
+                    omore = 0
+                if not smore and not omore:
+                    return 0
+                elif not smore:
+                    return -1
+                elif not omore:
+                    return 1
+                test = cmp(si, oi)
+                if test:
+                    return test
+                i = i + 1
+
         def __str__(self):
             ret = []
             i = 0
@@ -335,7 +383,8 @@ Open Issues
 
       3) Raises TypeError
 
-         Pros: None
+         Pros: zip(a) doesn't make much sense and could be confusing
+         to explain.
 
          Cons: needless restriction
 
@@ -345,9 +394,9 @@ Open Issues
       with the zip compression algorithm.  Other suggestions include
       (but are not limited to!): marry, weave, parallel, lace, braid,
       interlace, permute, furl, tuples, lists, stitch, collate, knit,
-      plait, and with.  All have disadvantages, and there is no clear
-      unanimous choice, therefore the decision was made to go with
-      `zip' because the same functionality is available in other
+      plait, fold, and with.  All have disadvantages, and there is no
+      clear unanimous choice, therefore the decision was made to go
+      with `zip' because the same functionality is available in other
       languages (e.g. Haskell) under the name `zip'[2].