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Diffstat (limited to 'Doc/library/itertools.rst')
-rw-r--r-- | Doc/library/itertools.rst | 111 |
1 files changed, 46 insertions, 65 deletions
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst index b76d59b..bc8986b 100644 --- a/Doc/library/itertools.rst +++ b/Doc/library/itertools.rst @@ -68,6 +68,7 @@ loops that truncate the stream. Equivalent to:: def chain(*iterables): + # chain('ABC', 'DEF') --> A B C D E F for it in iterables: for element in it: yield element @@ -80,6 +81,7 @@ loops that truncate the stream. @classmethod def from_iterable(iterables): + # chain.from_iterable(['ABC', 'DEF']) --> A B C D E F for it in iterables: for element in it: yield element @@ -105,7 +107,8 @@ loops that truncate the stream. Equivalent to:: def combinations(iterable, r): - 'combinations(range(4), 3) --> (0,1,2) (0,1,3) (0,2,3) (1,2,3)' + # combinations('ABCD', 2) --> AB AC AD BC BD CD + # combinations(range(4), 3) --> 012 013 023 123 pool = tuple(iterable) n = len(pool) indices = range(r) @@ -138,10 +141,11 @@ loops that truncate the stream. Make an iterator that returns consecutive integers starting with *n*. If not specified *n* defaults to zero. Often used as an argument to :func:`map` to - generate consecutive data points. Also, used with :func:`izip` to add sequence + generate consecutive data points. Also, used with :func:`zip` to add sequence numbers. Equivalent to:: def count(n=0): + # count(10) --> 10 11 12 13 14 ... while True: yield n n += 1 @@ -154,6 +158,7 @@ loops that truncate the stream. indefinitely. Equivalent to:: def cycle(iterable): + # cycle('ABCD') --> A B C D A B C D A B C D ... saved = [] for element in iterable: yield element @@ -174,6 +179,7 @@ loops that truncate the stream. start-up time. Equivalent to:: def dropwhile(predicate, iterable): + # dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1 iterable = iter(iterable) for x in iterable: if not predicate(x): @@ -212,6 +218,8 @@ loops that truncate the stream. :func:`groupby` is equivalent to:: class groupby(object): + # [k for k, g in groupby('AAAABBBCCDAABBB')] --> A B C D A B + # [(list(g)) for k, g in groupby('AAAABBBCCD')] --> AAAA BBB CC D def __init__(self, iterable, key=None): if key is None: key = lambda x: x @@ -240,6 +248,7 @@ loops that truncate the stream. that are false. Equivalent to:: def filterfalse(predicate, iterable): + # filterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8 if predicate is None: predicate = bool for x in iterable: @@ -260,6 +269,10 @@ loops that truncate the stream. multi-line report may list a name field on every third line). Equivalent to:: def islice(iterable, *args): + # islice('ABCDEFG', 2) --> A B + # islice('ABCDEFG', 2, 4) --> C D + # islice('ABCDEFG', 2, None) --> C D E F G + # islice('ABCDEFG', 0, None, 2) --> A C E G s = slice(*args) it = range(s.start or 0, s.stop or sys.maxsize, s.step or 1) nexti = next(it) @@ -272,29 +285,6 @@ loops that truncate the stream. then the step defaults to one. -.. function:: izip(*iterables) - - Make an iterator that aggregates elements from each of the iterables. Like - :func:`zip` except that it returns an iterator instead of a list. Used for - lock-step iteration over several iterables at a time. Equivalent to:: - - def izip(*iterables): - iterables = map(iter, iterables) - while iterables: - result = [next(it) for it in iterables] - yield tuple(result) - - When no iterables are specified, return a zero length iterator. - - The left-to-right evaluation order of the iterables is guaranteed. This - makes possible an idiom for clustering a data series into n-length groups - using ``izip(*[iter(s)]*n)``. - - :func:`izip` should only be used with unequal length inputs when you don't - care about trailing, unmatched values from the longer iterables. If those - values are important, use :func:`zip_longest` instead. - - .. function:: zip_longest(*iterables[, fillvalue]) Make an iterator that aggregates elements from each of the iterables. If the @@ -302,13 +292,14 @@ loops that truncate the stream. Iteration continues until the longest iterable is exhausted. Equivalent to:: def zip_longest(*args, **kwds): + # zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D- fillvalue = kwds.get('fillvalue') def sentinel(counter = ([fillvalue]*(len(args)-1)).pop): yield counter() # yields the fillvalue, or raises IndexError fillers = repeat(fillvalue) iters = [chain(it, sentinel(), fillers) for it in args] try: - for tup in izip(*iters): + for tup in zip(*iters): yield tup except IndexError: pass @@ -337,7 +328,8 @@ loops that truncate the stream. Equivalent to:: def permutations(iterable, r=None): - 'permutations(range(3), 2) --> (0,1) (0,2) (1,0) (1,2) (2,0) (2,1)' + # permutations('ABCD', 2) --> AB AC AD BA BC BD CA CB CD DA DB DC + # permutations(range(3)) --> 012 021 102 120 201 210 pool = tuple(iterable) n = len(pool) r = n if r is None else r @@ -379,8 +371,8 @@ loops that truncate the stream. Equivalent to nested for-loops in a generator expression. For example, ``product(A, B)`` returns the same as ``((x,y) for x in A for y in B)``. - The leftmost iterators are in the outermost for-loop, so the output tuples - cycle like an odometer (with the rightmost element changing on every + The leftmost iterators correspond to the outermost for-loop, so the output + tuples cycle like an odometer (with the rightmost element changing on every iteration). This results in a lexicographic ordering so that if the inputs iterables are sorted, the product tuples are emitted in sorted order. @@ -393,6 +385,8 @@ loops that truncate the stream. actual implementation does not build up intermediate results in memory:: def product(*args, **kwds): + # product('ABCD', 'xy') --> Ax Ay Bx By Cx Cy Dx Dy + # product(range(2), repeat=3) --> 000 001 010 011 100 101 110 111 pools = map(tuple, args) * kwds.get('repeat', 1) result = [[]] for pool in pools: @@ -405,10 +399,11 @@ loops that truncate the stream. Make an iterator that returns *object* over and over again. Runs indefinitely unless the *times* argument is specified. Used as argument to :func:`map` for - invariant parameters to the called function. Also used with :func:`izip` to + invariant parameters to the called function. Also used with :func:`zip` to create an invariant part of a tuple record. Equivalent to:: def repeat(object, times=None): + # repeat(10, 3) --> 10 10 10 if times is None: while True: yield object @@ -426,6 +421,7 @@ loops that truncate the stream. between ``function(a,b)`` and ``function(*c)``. Equivalent to:: def starmap(function, iterable): + # starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000 for args in iterable: yield function(*args) @@ -440,6 +436,7 @@ loops that truncate the stream. predicate is true. Equivalent to:: def takewhile(predicate, iterable): + # takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4 for x in iterable: if predicate(x): yield x @@ -481,34 +478,6 @@ Examples The following examples show common uses for each tool and demonstrate ways they can be combined. :: - >>> amounts = [120.15, 764.05, 823.14] - >>> for checknum, amount in izip(count(1200), amounts): - ... print('Check %d is for $%.2f' % (checknum, amount)) - ... - Check 1200 is for $120.15 - Check 1201 is for $764.05 - Check 1202 is for $823.14 - - >>> import operator - >>> for cube in map(operator.pow, range(1,5), repeat(3)): - ... print(cube) - ... - 1 - 8 - 27 - 64 - - >>> reportlines = ['EuroPython', 'Roster', '', 'alex', '', 'laura', - ... '', 'martin', '', 'walter', '', 'mark'] - >>> for name in islice(reportlines, 3, None, 2): - ... print(name.title()) - ... - Alex - Laura - Martin - Walter - Mark - # Show a dictionary sorted and grouped by value >>> from operator import itemgetter >>> d = dict(a=1, b=2, c=1, d=2, e=1, f=2, g=3) @@ -556,15 +525,18 @@ which incur interpreter overhead. :: return list(islice(seq, n)) def enumerate(iterable): - return izip(count(), iterable) + return zip(count(), iterable) def tabulate(function): "Return function(0), function(1), ..." return map(function, count()) + def items(mapping): + return zip(mapping.keys(), mapping.values()) + def nth(iterable, n): "Returns the nth item or raise StopIteration" - return islice(iterable, n, None).next() + return next(islice(iterable, n, None)) def all(seq, pred=None): "Returns True if pred(x) is true for every element in the iterable" @@ -617,18 +589,21 @@ which incur interpreter overhead. :: def pairwise(iterable): "s -> (s0,s1), (s1,s2), (s2, s3), ..." a, b = tee(iterable) - next(b, None) - return izip(a, b) + for elem in b: + break + return zip(a, b) - def grouper(n, iterable, padvalue=None): + def grouper(n, iterable, fillvalue=None): "grouper(3, 'abcdefg', 'x') --> ('a','b','c'), ('d','e','f'), ('g','x','x')" - return izip(*[chain(iterable, repeat(padvalue, n-1))]*n) + args = [iter(iterable)] * n + kwds = dict(fillvalue=fillvalue) + return zip_longest(*args, **kwds) def roundrobin(*iterables): "roundrobin('abc', 'd', 'ef') --> 'a', 'd', 'e', 'b', 'f', 'c'" # Recipe credited to George Sakkis pending = len(iterables) - nexts = cycle(iter(it).next for it in iterables) + nexts = cycle(iter(it).__next__ for it in iterables) while pending: try: for next in nexts: @@ -644,3 +619,9 @@ which incur interpreter overhead. :: for n in xrange(2**len(pairs)): yield set(x for m, x in pairs if m&n) + def compress(data, selectors): + "compress('abcdef', [1,0,1,0,1,1]) --> a c e f" + for d, s in zip(data, selectors): + if s: + yield d + |