Update itertools recipes.

Doc/library/itertools.rst

@@ -580,16 +580,19 @@ which incur interpreter overhead.
        "Return first n items of the iterable as a list"
        return list(islice(iterable, n))
 
-   def enumerate(iterable, start=0):
-       return zip(count(start), iterable)
-
    def tabulate(function, start=0):
        "Return function(0), function(1), ..."
        return map(function, count(start))
 
    def consume(iterator, n):
        "Advance the iterator n-steps ahead. If n is none, consume entirely."
-       collections.deque(islice(iterator, n), maxlen=0)
+       # Use functions that consume iterators at C speed.
+       if n is None:
+           # feed the entire iterator into a zero-length deque
+           collections.deque(iterator, maxlen=0)
+       else:
+           # advance to the emtpy slice starting at position n
+           next(islice(iterator, n, n), None)
 
    def nth(iterable, n, default=None):
        "Returns the nth item or a default value"
@@ -661,8 +664,7 @@ which incur interpreter overhead.
        seen = set()
        seen_add = seen.add
        if key is None:
-           for element in iterable:
+           for element in filterfalse(seen.__contains__, iterable):
-               if element not in seen:
                seen_add(element)
                yield element
        else:
@@ -677,3 +679,33 @@ which incur interpreter overhead.
        # unique_justseen('AAAABBBCCDAABBB') --> A B C D A B
        # unique_justseen('ABBCcAD', str.lower) --> A B C A D
        return map(next, map(itemgetter(1), groupby(iterable, key)))
+
+   def iter_except(func, exception, first=None):
+       """ Call a function repeatedly until an exception is raised.
+
+       Converts a call-until-exception interface to an iterator interface.
+       Like __builtin__.iter(func, sentinel) but uses an exception instead
+       of a sentinel to end the loop.
+
+       Examples:
+           iter_except(functools.partial(heappop, h), IndexError)   # priority queue iterator
+           iter_except(d.popitem, KeyError)                         # non-blocking dict iterator
+           iter_except(d.popleft, IndexError)                       # non-blocking deque iterator
+           iter_except(q.get_nowait, Queue.Empty)                   # loop over a producer Queue
+           iter_except(s.pop, KeyError)                             # non-blocking set iterator
+
+       """
+       try:
+           if first is not None:
+               yield first()            # For database APIs needing an initial cast to db.first()
+           while 1:
+               yield func()
+       except exception:
+           pass
+
+Note, many of the above recipes can be optimized by replacing global lookups
+with local variables defined as default values.  For example, the
+*dotproduct* recipe can be written as::
+
+   def dotproduct(vec1, vec2, sum=sum, map=map, mul=operator.mul):
+       return sum(map(mul, vec1, vec2))