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:mod:`itertools` --- Functions creating iterators for efficient looping
=======================================================================
.. module:: itertools
:synopsis: Functions creating iterators for efficient looping.
.. moduleauthor:: Raymond Hettinger <python@rcn.com>
.. sectionauthor:: Raymond Hettinger <python@rcn.com>
.. versionadded:: 2.3
This module implements a number of :term:`iterator` building blocks inspired by
constructs from the Haskell and SML programming languages. Each has been recast
in a form suitable for Python.
The module standardizes a core set of fast, memory efficient tools that are
useful by themselves or in combination. Standardization helps avoid the
readability and reliability problems which arise when many different individuals
create their own slightly varying implementations, each with their own quirks
and naming conventions.
The tools are designed to combine readily with one another. This makes it easy
to construct more specialized tools succinctly and efficiently in pure Python.
For instance, SML provides a tabulation tool: ``tabulate(f)`` which produces a
sequence ``f(0), f(1), ...``. This toolbox provides :func:`imap` and
:func:`count` which can be combined to form ``imap(f, count())`` and produce an
equivalent result.
Likewise, the functional tools are designed to work well with the high-speed
functions provided by the :mod:`operator` module.
The module author welcomes suggestions for other basic building blocks to be
added to future versions of the module.
Whether cast in pure python form or compiled code, tools that use iterators are
more memory efficient (and faster) than their list based counterparts. Adopting
the principles of just-in-time manufacturing, they create data when and where
needed instead of consuming memory with the computer equivalent of "inventory".
The performance advantage of iterators becomes more acute as the number of
elements increases -- at some point, lists grow large enough to severely impact
memory cache performance and start running slowly.
.. seealso::
The Standard ML Basis Library, `The Standard ML Basis Library
<http://www.standardml.org/Basis/>`_.
Haskell, A Purely Functional Language, `Definition of Haskell and the Standard
Libraries <http://www.haskell.org/definition/>`_.
.. _itertools-functions:
Itertool functions
------------------
The following module functions all construct and return iterators. Some provide
streams of infinite length, so they should only be accessed by functions or
loops that truncate the stream.
.. function:: chain(*iterables)
Make an iterator that returns elements from the first iterable until it is
exhausted, then proceeds to the next iterable, until all of the iterables are
exhausted. Used for treating consecutive sequences as a single sequence.
Equivalent to::
def chain(*iterables):
# chain('ABC', 'DEF') --> A B C D E F
for it in iterables:
for element in it:
yield element
.. function:: itertools.chain.from_iterable(iterable)
Alternate constructor for :func:`chain`. Gets chained inputs from a
single iterable argument that is evaluated lazily. Equivalent to::
@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
.. versionadded:: 2.6
.. function:: combinations(iterable, r)
Return successive *r* length combinations of elements in the *iterable*.
Combinations are emitted in lexicographic sort order. So, if the
input *iterable* is sorted, the combination tuples will be produced
in sorted order.
Elements are treated as unique based on their position, not on their
value. So if the input elements are unique, there will be no repeat
values in each combination.
Each result tuple is ordered to match the input order. So, every
combination is a subsequence of the input *iterable*.
Equivalent to::
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)
indices = range(r)
yield tuple(pool[i] for i in indices)
while 1:
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)
The code for :func:`combinations` can be also expressed as a subsequence
of :func:`permutations` after filtering entries where the elements are not
in sorted order (according to their position in the input pool)::
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)
.. versionadded:: 2.6
.. function:: count([n])
Make an iterator that returns consecutive integers starting with *n*. If not
specified *n* defaults to zero. Often used as an argument to :func:`imap` to
generate consecutive data points. Also, used with :func:`izip` to add sequence
numbers. Equivalent to::
def count(n=0):
# count(10) --> 10 11 12 13 14 ...
while True:
yield n
n += 1
.. function:: cycle(iterable)
Make an iterator returning elements from the iterable and saving a copy of each.
When the iterable is exhausted, return elements from the saved copy. Repeats
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
saved.append(element)
while saved:
for element in saved:
yield element
Note, this member of the toolkit may require significant auxiliary storage
(depending on the length of the iterable).
.. function:: dropwhile(predicate, iterable)
Make an iterator that drops elements from the iterable as long as the predicate
is true; afterwards, returns every element. Note, the iterator does not produce
*any* output until the predicate first becomes false, so it may have a lengthy
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):
yield x
break
for x in iterable:
yield x
.. function:: groupby(iterable[, key])
Make an iterator that returns consecutive keys and groups from the *iterable*.
The *key* is a function computing a key value for each element. If not
specified or is ``None``, *key* defaults to an identity function and returns
the element unchanged. Generally, the iterable needs to already be sorted on
the same key function.
The operation of :func:`groupby` is similar to the ``uniq`` filter in Unix. It
generates a break or new group every time the value of the key function changes
(which is why it is usually necessary to have sorted the data using the same key
function). That behavior differs from SQL's GROUP BY which aggregates common
elements regardless of their input order.
The returned group is itself an iterator that shares the underlying iterable
with :func:`groupby`. Because the source is shared, when the :func:`groupby`
object is advanced, the previous group is no longer visible. So, if that data
is needed later, it should be stored as a list::
groups = []
uniquekeys = []
data = sorted(data, key=keyfunc)
for k, g in groupby(data, keyfunc):
groups.append(list(g)) # Store group iterator as a list
uniquekeys.append(k)
: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
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 = self.it.next() # 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 = self.it.next() # Exit on StopIteration
self.currkey = self.keyfunc(self.currvalue)
.. versionadded:: 2.4
.. function:: ifilter(predicate, iterable)
Make an iterator that filters elements from iterable returning only those for
which the predicate is ``True``. If *predicate* is ``None``, return the items
that are true. Equivalent to::
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
.. function:: ifilterfalse(predicate, iterable)
Make an iterator that filters elements from iterable returning only those for
which the predicate is ``False``. If *predicate* is ``None``, return the items
that are false. Equivalent to::
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
.. function:: imap(function, *iterables)
Make an iterator that computes the function using arguments from each of the
iterables. If *function* is set to ``None``, then :func:`imap` returns the
arguments as a tuple. Like :func:`map` but stops when the shortest iterable is
exhausted instead of filling in ``None`` for shorter iterables. The reason for
the difference is that infinite iterator arguments are typically an error for
:func:`map` (because the output is fully evaluated) but represent a common and
useful way of supplying arguments to :func:`imap`. Equivalent to::
def imap(function, *iterables):
# imap(pow, (2,3,10), (5,2,3)) --> 32 9 1000
iterables = map(iter, iterables)
while True:
args = [it.next() for it in iterables]
if function is None:
yield tuple(args)
else:
yield function(*args)
.. function:: islice(iterable, [start,] stop [, step])
Make an iterator that returns selected elements from the iterable. If *start* is
non-zero, then elements from the iterable are skipped until start is reached.
Afterward, elements are returned consecutively unless *step* is set higher than
one which results in items being skipped. If *stop* is ``None``, then iteration
continues until the iterator is exhausted, if at all; otherwise, it stops at the
specified position. Unlike regular slicing, :func:`islice` does not support
negative values for *start*, *stop*, or *step*. Can be used to extract related
fields from data where the internal structure has been flattened (for example, a
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 = iter(xrange(s.start or 0, s.stop or sys.maxint, s.step or 1))
nexti = it.next()
for i, element in enumerate(iterable):
if i == nexti:
yield element
nexti = it.next()
If *start* is ``None``, then iteration starts at zero. If *step* is ``None``,
then the step defaults to one.
.. versionchanged:: 2.5
accept ``None`` values for default *start* and *step*.
.. 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):
# izip('ABCD', 'xy') --> Ax By
iterables = map(iter, iterables)
while iterables:
result = [it.next() for it in iterables]
yield tuple(result)
.. versionchanged:: 2.4
When no iterables are specified, returns a zero length iterator instead of
raising a :exc:`TypeError` exception.
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:`izip_longest` instead.
.. function:: izip_longest(*iterables[, fillvalue])
Make an iterator that aggregates elements from each of the iterables. If the
iterables are of uneven length, missing values are filled-in with *fillvalue*.
Iteration continues until the longest iterable is exhausted. Equivalent to::
def izip_longest(*args, **kwds):
# izip_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):
yield tup
except IndexError:
pass
If one of the iterables is potentially infinite, then the :func:`izip_longest`
function should be wrapped with something that limits the number of calls (for
example :func:`islice` or :func:`takewhile`).
.. versionadded:: 2.6
.. function:: permutations(iterable[, r])
Return successive *r* length permutations of elements in the *iterable*.
If *r* is not specified or is ``None``, then *r* defaults to the length
of the *iterable* and all possible full-length permutations
are generated.
Permutations are emitted in lexicographic sort order. So, if the
input *iterable* is sorted, the permutation tuples will be produced
in sorted order.
Elements are treated as unique based on their position, not on their
value. So if the input elements are unique, there will be no repeat
values in each permutation.
Equivalent to::
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
indices = range(n)
cycles = range(n-r+1, n+1)[::-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
The code for :func:`permutations` can be also expressed as a subsequence of
:func:`product`, filtered to exclude entries with repeated elements (those
from the same position in the input pool)::
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)
.. versionadded:: 2.6
.. function:: product(*iterables[, repeat])
Cartesian product of input iterables.
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 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.
To compute the product of an iterable with itself, specify the number of
repetitions with the optional *repeat* keyword argument. For example,
``product(A, repeat=4)`` means the same as ``product(A, A, A, A)``.
This function is equivalent to the following code, except that the
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:
result = [x+[y] for x in result for y in pool]
for prod in result:
yield tuple(prod)
.. versionadded:: 2.6
.. function:: repeat(object[, times])
Make an iterator that returns *object* over and over again. Runs indefinitely
unless the *times* argument is specified. Used as argument to :func:`imap` for
invariant function parameters. Also used with :func:`izip` to create constant
fields in a tuple record. Equivalent to::
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
.. function:: starmap(function, iterable)
Make an iterator that computes the function using arguments obtained from
the iterable. Used instead of :func:`imap` when argument parameters are already
grouped in tuples from a single iterable (the data has been "pre-zipped"). The
difference between :func:`imap` and :func:`starmap` parallels the distinction
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)
.. versionchanged:: 2.6
Previously, :func:`starmap` required the function arguments to be tuples.
Now, any iterable is allowed.
.. function:: takewhile(predicate, iterable)
Make an iterator that returns elements from the iterable as long as the
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
else:
break
.. function:: tee(iterable[, n=2])
Return *n* independent iterators from a single iterable. The case where ``n==2``
is equivalent to::
def tee(iterable):
def gen(next, data={}):
for i in count():
if i in data:
yield data.pop(i)
else:
data[i] = next()
yield data[i]
it = iter(iterable)
return gen(it.next), gen(it.next)
Note, once :func:`tee` has made a split, the original *iterable* should not be
used anywhere else; otherwise, the *iterable* could get advanced without the tee
objects being informed.
Note, this member of the toolkit may require significant auxiliary storage
(depending on how much temporary data needs to be stored). In general, if one
iterator is going to use most or all of the data before the other iterator, it
is faster to use :func:`list` instead of :func:`tee`.
.. versionadded:: 2.4
.. _itertools-example:
Examples
--------
The following examples show common uses for each tool and demonstrate ways they
can be combined. ::
# 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)
>>> 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']
# Find runs of consecutive numbers using groupby. The key to the solution
# is differencing with a range so that consecutive numbers all appear in
# same group.
>>> data = [ 1, 4,5,6, 10, 15,16,17,18, 22, 25,26,27,28]
>>> for k, g in groupby(enumerate(data), lambda (i,x):i-x):
... print map(operator.itemgetter(1), g)
...
[1]
[4, 5, 6]
[10]
[15, 16, 17, 18]
[22]
[25, 26, 27, 28]
.. _itertools-recipes:
Recipes
-------
This section shows recipes for creating an extended toolset using the existing
itertools as building blocks.
The extended tools offer the same high performance as the underlying toolset.
The superior memory performance is kept by processing elements one at a time
rather than bringing the whole iterable into memory all at once. Code volume is
kept small by linking the tools together in a functional style which helps
eliminate temporary variables. High speed is retained by preferring
"vectorized" building blocks over the use of for-loops and :term:`generator`\s
which incur interpreter overhead. ::
def take(n, seq):
return list(islice(seq, n))
def enumerate(iterable):
return izip(count(), iterable)
def tabulate(function):
"Return function(0), function(1), ..."
return imap(function, count())
def iteritems(mapping):
return izip(mapping.iterkeys(), mapping.itervalues())
def nth(iterable, n):
"Returns the nth item or raise StopIteration"
return islice(iterable, n, None).next()
def all(seq, pred=None):
"Returns True if pred(x) is true for every element in the iterable"
for elem in ifilterfalse(pred, seq):
return False
return True
def any(seq, pred=None):
"Returns True if pred(x) is true for at least one element in the iterable"
for elem in ifilter(pred, seq):
return True
return False
def no(seq, pred=None):
"Returns True if pred(x) is false for every element in the iterable"
for elem in ifilter(pred, seq):
return False
return True
def quantify(seq, pred=None):
"Count how many times the predicate is true in the sequence"
return sum(imap(pred, seq))
def padnone(seq):
"""Returns the sequence elements and then returns None indefinitely.
Useful for emulating the behavior of the built-in map() function.
"""
return chain(seq, repeat(None))
def ncycles(seq, n):
"Returns the sequence elements n times"
return chain.from_iterable(repeat(seq, n))
def dotproduct(vec1, vec2):
return sum(imap(operator.mul, vec1, vec2))
def flatten(listOfLists):
return list(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)
for elem in b:
break
return izip(a, b)
def grouper(n, iterable, fillvalue=None):
"grouper(3, 'abcdefg', 'x') --> ('a','b','c'), ('d','e','f'), ('g','x','x')"
args = [iter(iterable)] * n
kwds = dict(fillvalue=fillvalue)
return izip_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)
while pending:
try:
for next in nexts:
yield next()
except StopIteration:
pending -= 1
nexts = cycle(islice(nexts, pending))
def powerset(iterable):
"powerset('ab') --> set([]), set(['a']), set(['b']), set(['a', 'b'])"
# Recipe credited to Eric Raymond
pairs = [(2**i, x) for i, x in enumerate(iterable)]
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 izip(data, selectors):
if s:
yield d
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