[Python-checkins] r79486 - python/branches/release30-maint/Doc/library/itertools.rst

Sun Mar 28 20:32:17 CEST 2010

Author: raymond.hettinger
Date: Sun Mar 28 20:32:17 2010
New Revision: 79486
Log:
Update itertools recipes.
Modified:
 python/branches/release30-maint/Doc/library/itertools.rst
Modified: python/branches/release30-maint/Doc/library/itertools.rst
==============================================================================

--- python/branches/release30-maint/Doc/library/itertools.rst	(original)
+++ python/branches/release30-maint/Doc/library/itertools.rst	Sun Mar 28 20:32:17 2010
@@ -510,16 +510,19 @@
 "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"
@@ -611,10 +614,9 @@
 seen = set()
 seen_add = seen.add
 if key is None:
- for element in iterable:
- if element not in seen:
- seen_add(element)
- yield element
+ for element in filterfalse(seen.__contains__, iterable):
+ seen_add(element)
+ yield element
 else:
 for element in iterable:
 k = key(element)
@@ -627,3 +629,33 @@
 # 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))
