Python itertools模块详解

迭代器         参数         结果                                                例子
count()     start, [step]   start, start+step, start+2*step, ...                count(10) --> 10 11 12 13 14 ...
cycle()     p               p0, p1, ... plast, p0, p1, ...                      cycle('ABCD') --> A B C D A B C D ...
repeat()    elem [,n]       elem, elem, elem, ... endlessly or up to n times    repeat(10, 3) --> 10 10 10

迭代器          参数            结果                                        例子
chain()     p, q, ...           p0, p1, ... plast, q0, q1, ...              chain('ABC', 'DEF') --> A B C D E F
compress()  data, selectors     (d[0] if s[0]), (d[1] if s[1]), ...         compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
dropwhile() pred, seq           seq[n], seq[n+1], starting when pred fails  dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1
groupby()   iterable[, keyfunc] sub-iterators grouped by value of keyfunc(v)
ifilter()   pred, seq           elements of seq where pred(elem) is True    ifilter(lambda x: x%2, range(10)) --> 1 3 5 7 9
ifilterfalse()  pred, seq       elements of seq where pred(elem) is False   ifilterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8
islice()    seq, [start,] stop [, step] elements from seq[start:stop:step]  islice('ABCDEFG', 2, None) --> C D E F G
imap()      func, p, q, ...     func(p0, q0), func(p1, q1), ...             imap(pow, (2,3,10), (5,2,3)) --> 32 9 1000
starmap()   func, seq           func(*seq[0]), func(*seq[1]), ...           starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
tee()       it, n               it1, it2 , ... itn splits one iterator into n
takewhile() pred, seq           seq[0], seq[1], until pred fails            takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4
izip()      p, q, ...           (p[0], q[0]), (p[1], q[1]), ...             izip('ABCD', 'xy') --> Ax By
izip_longest()  p, q, ...       (p[0], q[0]), (p[1], q[1]), ...             izip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-

迭代器          参数                        结果
product()       p, q, ... [repeat=1]        cartesian product, equivalent to a nested for-loop
permutations()  p[, r]                      r-length tuples, all possible orderings, no repeated elements
combinations()  p, r                        r-length tuples, in sorted order, no repeated elements
combinations_with_replacement() p, r        r-length tuples, in sorted order, with repeated elements
product('ABCD', repeat=2)                   AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD
permutations('ABCD', 2)                     AB AC AD BA BC BD CA CB CD DA DB DC
combinations('ABCD', 2)                     AB AC AD BC BD CD
combinations_with_replacement('ABCD', 2)    AA AB AC AD BB BC BD CC CD DD

def count(start=0, step=1):
    # count(10) --> 10 11 12 13 14 ...
    # count(2.5, 0.5) -> 2.5 3.0 3.5 ...
    n = start
    while True:
        yield n
        n += step

from itertools import *

for i in izip(count(1), ['a', 'b', 'c']):
    print i

(1, 'a')
(2, 'b')
(3, 'c')

def repeat(object, times=None):
    # repeat(10, 3) --> 10 10 10
    if times is None:
        while True:
            yield object
    else:
        for i in xrange(times):
            yield object

from itertools import *

for i in repeat('over-and-over', 5):
    print i

over-and-over
over-and-over
over-and-over
over-and-over
over-and-over

def chain(*iterables):
    # chain('ABC', 'DEF') --> A B C D E F
    for it in iterables:
        for element in it:
            yield element

from itertools import *

for i in chain([1, 2, 3], ['a', 'b', 'c']):
    print i
1
2
3
a
b
c

from itertools import chain, imap
def flatmap(f, items):
    return chain.from_iterable(imap(f, items))
>>> list(flatmap(os.listdir, dirs))
>>> ['settings.py', 'wsgi.py', 'templates', 'app.py',
     'templates', 'index.html, 'config.json']

def compress(data, selectors):
    # compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
    return (d for d, s in izip(data, selectors) if s)
itertools.dropwhile(predicate, iterable)

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):
            yield x
            break
    for x in iterable:
        yield x

from itertools import *

def should_drop(x):
    print 'Testing:', x
    return (x<1)

for i in dropwhile(should_drop, [ -1, 0, 1, 2, 3, 4, 1, -2 ]):
    print 'Yielding:', i

Testing: -1
Testing: 0
Testing: 1
Yielding: 1
Yielding: 2
Yielding: 3
Yielding: 4
Yielding: 1
Yielding: -2

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
        self.keyfunc = key
        self.it = iter(iterable)
        self.tgtkey = self.currkey = self.currvalue = object()
    def __iter__(self):
        return self
    def next(self):
        while self.currkey == self.tgtkey:
            self.currvalue = next(self.it)    # Exit on StopIteration
            self.currkey = self.keyfunc(self.currvalue)
        self.tgtkey = self.currkey
        return (self.currkey, self._grouper(self.tgtkey))
    def _grouper(self, tgtkey):
        while self.currkey == tgtkey:
            yield self.currvalue
            self.currvalue = next(self.it)    # Exit on StopIteration
            self.currkey = self.keyfunc(self.currvalue)

from itertools import groupby
qs = [{'date' : 1},{'date' : 2}]
[(name, list(group)) for name, group in itertools.groupby(qs, lambda p:p['date'])]

Out[77]: [(1, [{'date': 1}]), (2, [{'date': 2}])]

>>> from itertools import *
>>> a = ['aa', 'ab', 'abc', 'bcd', 'abcde']
>>> for i, k in groupby(a, len):
...     print i, list(k)
...
2 ['aa', 'ab']
3 ['abc', 'bcd']
5 ['abcde']

from itertools import *
from operator import itemgetter

d = dict(a=1, b=2, c=1, d=2, e=1, f=2, g=3)
di = sorted(d.iteritems(), key=itemgetter(1))
for k, g in groupby(di, key=itemgetter(1)):
    print k, map(itemgetter(0), g)

1 ['a', 'c', 'e']
2 ['b', 'd', 'f']
3 ['g']

def ifilter(predicate, iterable):
    # ifilter(lambda x: x%2, range(10)) --> 1 3 5 7 9
    if predicate is None:
        predicate = bool
    for x in iterable:
        if predicate(x):
            yield x

from itertools import *

def check_item(x):
    print 'Testing:', x
    return (x<1)

for i in ifilter(check_item, [ -1, 0, 1, 2, 3, 4, 1, -2 ]):
    print 'Yielding:', i

Testing: -1
Yielding: -1
Testing: 0
Yielding: 0
Testing: 1
Testing: 2
Testing: 3
Testing: 4
Testing: 1
Testing: -2
Yielding: -2

def ifilterfalse(predicate, iterable):
    # ifilterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8
    if predicate is None:
        predicate = bool
    for x in iterable:
        if not predicate(x):
            yield x

from itertools import *

def check_item(x):
    print 'Testing:', x
    return (x<1)

for i in ifilterfalse(check_item, [ -1, 0, 1, 2, 3, 4, 1, -2 ]):
    print 'Yielding:', i

Testing: -1
Testing: 0
Testing: 1
Yielding: 1
Testing: 2
Yielding: 2
Testing: 3
Yielding: 3
Testing: 4
Yielding: 4
Testing: 1
Yielding: 1
Testing: -2

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 = iter(xrange(s.start or 0, s.stop or sys.maxint, s.step or 1))
    nexti = next(it)
    for i, element in enumerate(iterable):
        if i == nexti:
            yield element
            nexti = next(it)

from itertools import *

print 'Stop at 5:'
for i in islice(count(), 5):
    print i

print 'Start at 5, Stop at 10:'
for i in islice(count(), 5, 10):
    print i

print 'By tens to 100:'
for i in islice(count(), 0, 100, 10):
    print i

Stop at 5:
0
1
2
3
4
Start at 5, Stop at 10:
5
6
7
8
9
By tens to 100:
0
10
20
30
40
50
60
70
80
90

def imap(function, *iterables):
    # imap(pow, (2,3,10), (5,2,3)) --> 32 9 1000
    iterables = map(iter, iterables)
    while True:
        args = [next(it) for it in iterables]
        if function is None:
            yield tuple(args)
        else:
            yield function(*args)

from itertools import *

print 'Doubles:'
for i in imap(lambda x:2*x, xrange(5)):
    print i

print 'Multiples:'
for i in imap(lambda x,y:(x, y, x*y), xrange(5), xrange(5,10)):
    print '%d * %d = %d' % i

Doubles:
0
2
4
6
8
Multiples:
0 * 5 = 0
1 * 6 = 6
2 * 7 = 14
3 * 8 = 24
4 * 9 = 36

def starmap(function, iterable):
    # starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
    for args in iterable:
        yield function(*args)

from itertools import *

values = [(0, 5), (1, 6), (2, 7), (3, 8), (4, 9)]
for i in starmap(lambda x,y:(x, y, x*y), values):
    print '%d * %d = %d' % i

0 * 5 = 0
1 * 6 = 6
2 * 7 = 14
3 * 8 = 24
4 * 9 = 36

def tee(iterable, n=2):
    it = iter(iterable)
    deques = [collections.deque() for i in range(n)]
    def gen(mydeque):
        while True:
            if not mydeque:             # when the local deque is empty
                newval = next(it)       # fetch a new value and
                for d in deques:        # load it to all the deques
                    d.append(newval)
            yield mydeque.popleft()
    return tuple(gen(d) for d in deques)

from itertools import *

r = islice(count(), 5)
i1, i2 = tee(r)

for i in i1:
    print 'i1:', i
for i in i2:
    print 'i2:', i

i1: 0
i1: 1
i1: 2
i1: 3
i1: 4
i2: 0
i2: 1
i2: 2
i2: 3
i2: 4

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
        else:
            break

from itertools import *

def should_take(x):
    print 'Testing:', x
    return (x<2)

for i in takewhile(should_take, [ -1, 0, 1, 2, 3, 4, 1, -2 ]):
    print 'Yielding:', i

Testing: -1
Yielding: -1
Testing: 0
Yielding: 0
Testing: 1
Yielding: 1
Testing: 2

izip(iter1, iter2, ... iterN):
返回:(it1[0],it2 [0], it3[0], ..), (it1[1], it2[1], it3[1], ..)...

def izip(*iterables):
    # izip('ABCD', 'xy') --> Ax By
    iterators = map(iter, iterables)
    while iterators:
        yield tuple(map(next, iterators))

from itertools import *

for i in izip([1, 2, 3], ['a', 'b', 'c']):
    print i
(1, 'a')
(2, 'b')
(3, 'c')

class ZipExhausted(Exception):
    pass

def izip_longest(*args, **kwds):
    # izip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-
    fillvalue = kwds.get('fillvalue')
    counter = [len(args) - 1]
    def sentinel():
        if not counter[0]:
            raise ZipExhausted
        counter[0] -= 1
        yield fillvalue
    fillers = repeat(fillvalue)
    iterators = [chain(it, sentinel(), fillers) for it in args]
    try:
        while iterators:
            yield tuple(map(next, iterators))
    except ZipExhausted:
        pass

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:
        result = [x+[y] for x in result for y in pool]
    for prod in result:
        yield tuple(prod)

import itertools
a = (1, 2, 3)
b = ('A', 'B', 'C')
c = itertools.product(a,b)
for elem in c:
    print elem

(1, 'A')
(1, 'B')
(1, 'C')
(2, 'A')
(2, 'B')
(2, 'C')
(3, 'A')
(3, 'B')
(3, 'C')

def permutations(iterable, r=None):
    # 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
    if r > n:
        return
    indices = range(n)
    cycles = range(n, n-r, -1)
    yield tuple(pool[i] for i in indices[:r])
    while n:
        for i in reversed(range(r)):
            cycles[i] -= 1
            if cycles[i] == 0:
                indices[i:] = indices[i+1:] + indices[i:i+1]
                cycles[i] = n - i
            else:
                j = cycles[i]
                indices[i], indices[-j] = indices[-j], indices[i]
                yield tuple(pool[i] for i in indices[:r])
                break
        else:
            return
也可以用product实现

def permutations(iterable, r=None):
    pool = tuple(iterable)
    n = len(pool)
    r = n if r is None else r
    for indices in product(range(n), repeat=r):
        if len(set(indices)) == r:
            yield tuple(pool[i] for i in indices)

def combinations(iterable, r):
    # combinations('ABCD', 2) --> AB AC AD BC BD CD
    # combinations(range(4), 3) --> 012 013 023 123
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
        return
    indices = range(r)
    yield tuple(pool[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else:
            return
        indices[i] += 1
        for j in range(i+1, r):
            indices[j] = indices[j-1] + 1
        yield tuple(pool[i] for i in indices)

#或者
def combinations(iterable, r):
    pool = tuple(iterable)
    n = len(pool)
    for indices in permutations(range(n), r):
        if sorted(indices) == list(indices):
            yield tuple(pool[i] for i in indices)

def combinations_with_replacement(iterable, r):
    # combinations_with_replacement('ABC', 2) --> AA AB AC BB BC CC
    pool = tuple(iterable)
    n = len(pool)
    if not n and r:
        return
    indices = [0] * r
    yield tuple(pool[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != n - 1:
                break
        else:
            return
        indices[i:] = [indices[i] + 1] * (r - i)
        yield tuple(pool[i] for i in indices)
或者
def combinations_with_replacement(iterable, r):
    pool = tuple(iterable)
    n = len(pool)
    for indices in product(range(n), repeat=r):
        if sorted(indices) == list(indices):
            yield tuple(pool[i] for i in indices)

def take(n, iterable):
    "Return first n items of the iterable as a list"
    return list(islice(iterable, n))

def tabulate(function, start=0):
    "Return function(0), function(1), ..."
    return imap(function, count(start))

def consume(iterator, n):
    "Advance the iterator n-steps ahead. If n is none, consume entirely."
    # 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 empty 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"
    return next(islice(iterable, n, None), default)

def quantify(iterable, pred=bool):
    "Count how many times the predicate is true"
    return sum(imap(pred, iterable))

def padnone(iterable):
    """Returns the sequence elements and then returns None indefinitely.

    Useful for emulating the behavior of the built-in map() function.
    """
    return chain(iterable, repeat(None))

def ncycles(iterable, n):
    "Returns the sequence elements n times"
    return chain.from_iterable(repeat(tuple(iterable), n))

def dotproduct(vec1, vec2):
    return sum(imap(operator.mul, vec1, vec2))

def flatten(listOfLists):
    "Flatten one level of nesting"
    return chain.from_iterable(listOfLists)

def repeatfunc(func, times=None, *args):
    """Repeat calls to func with specified arguments.

    Example:  repeatfunc(random.random)
    """
    if times is None:
        return starmap(func, repeat(args))
    return starmap(func, repeat(args, times))

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

def grouper(iterable, n, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

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)
    while pending:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            pending -= 1
            nexts = cycle(islice(nexts, pending))

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

def unique_everseen(iterable, key=None):
    "List unique elements, preserving order. Remember all elements ever seen."
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in ifilterfalse(seen.__contains__, iterable):
            seen_add(element)
            yield element
    else:
        for element in iterable:
            k = key(element)
            if k not in seen:
                seen_add(k)
                yield element

def unique_justseen(iterable, key=None):
    "List unique elements, preserving order. Remember only the element just seen."
    # unique_justseen('AAAABBBCCDAABBB') --> A B C D A B
    # unique_justseen('ABBCcAD', str.lower) --> A B C A D
    return imap(next, imap(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:
        bsddbiter = iter_except(db.next, bsddb.error, db.first)
        heapiter = iter_except(functools.partial(heappop, h), IndexError)
        dictiter = iter_except(d.popitem, KeyError)
        dequeiter = iter_except(d.popleft, IndexError)
        queueiter = iter_except(q.get_nowait, Queue.Empty)
        setiter = iter_except(s.pop, KeyError)

    """
    try:
        if first is not None:
            yield first()
        while 1:
            yield func()
    except exception:
        pass

def random_product(*args, **kwds):
    "Random selection from itertools.product(*args, **kwds)"
    pools = map(tuple, args) * kwds.get('repeat', 1)
    return tuple(random.choice(pool) for pool in pools)

def random_permutation(iterable, r=None):
    "Random selection from itertools.permutations(iterable, r)"
    pool = tuple(iterable)
    r = len(pool) if r is None else r
    return tuple(random.sample(pool, r))

def random_combination(iterable, r):
    "Random selection from itertools.combinations(iterable, r)"
    pool = tuple(iterable)
    n = len(pool)
    indices = sorted(random.sample(xrange(n), r))
    return tuple(pool[i] for i in indices)

def random_combination_with_replacement(iterable, r):
    "Random selection from itertools.combinations_with_replacement(iterable, r)"
    pool = tuple(iterable)
    n = len(pool)
    indices = sorted(random.randrange(n) for i in xrange(r))
    return tuple(pool[i] for i in indices)

def tee_lookahead(t, i):
    """Inspect the i-th upcomping value from a tee object
    while leaving the tee object at its current position.

    Raise an IndexError if the underlying iterator doesn't
    have enough values.

    """
    for value in islice(t.__copy__(), i, None):
        return value
    raise IndexError(i)

from itertools import chain, islice
def chunks(iterable, size, format=iter):
    it = iter(iterable)
    while True:
        yield format(chain((it.next(),), islice(it, size - 1)))

>>> l = ["a", "b", "c", "d", "e", "f", "g"]
>>> for chunk in chunks(l, 3, tuple):...
        print chunk...
("a", "b", "c")
("d", "e", "f")
("g",)

>>> horses = [1, 2, 3, 4]
>>> races = itertools.permutations(horses)
>>> print(races)
<itertools.permutations object at 0xb754f1dc]]>
>>> print(list(itertools.permutations(horses)))
[(1, 2, 3, 4),
 (1, 2, 4, 3),
 (1, 3, 2, 4),
 (1, 3, 4, 2),
 (1, 4, 2, 3),
 (1, 4, 3, 2),
 (2, 1, 3, 4),
 (2, 1, 4, 3),
 (2, 3, 1, 4),
 (2, 3, 4, 1),
 (2, 4, 1, 3),
 (2, 4, 3, 1),
 (3, 1, 2, 4),
 (3, 1, 4, 2),
 (3, 2, 1, 4),
 (3, 2, 4, 1),
 (3, 4, 1, 2),
 (3, 4, 2, 1),
 (4, 1, 2, 3),
 (4, 1, 3, 2),
 (4, 2, 1, 3),
 (4, 2, 3, 1),
 (4, 3, 1, 2),
 (4, 3, 2, 1)]