| Python 3.6.5 Documentation > "itertools" — Functions creating iterators for efficient looping
"itertools" — Functions creating iterators for efficient looping
****************************************************************
======================================================================
This module implements a number of *iterator* building blocks inspired
by constructs from APL, Haskell, and SML. 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. Together, they form
an “iterator algebra” making it possible to construct 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), ...". The same effect can be
achieved in Python by combining "map()" and "count()" to form "map(f,
count())".
These tools and their built-in counterparts also work well with the
high-speed functions in the "operator" module. For example, the
multiplication operator can be mapped across two vectors to form an
efficient dot-product: "sum(map(operator.mul, vector1, vector2))".
**Infinite iterators:**
+--------------------+-------------------+---------------------------------------------------+-------------------------------------------+
| Iterator | Arguments | Results | Example |
+====================+===================+===================================================+===========================================+
| "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" |
+--------------------+-------------------+---------------------------------------------------+-------------------------------------------+
**Iterators terminating on the shortest input sequence:**
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| Iterator | Arguments | Results | Example |
+==============================+==============================+===================================================+===============================================================+
| "accumulate()" | p [,func] | p0, p0+p1, p0+p1+p2, … | "accumulate([1,2,3,4,5]) --> 1 3 6 10 15" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "chain()" | p, q, … | p0, p1, … plast, q0, q1, … | "chain('ABC', 'DEF') --> A B C D E F" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "chain.from_iterable()" | iterable | p0, p1, … plast, q0, q1, … | "chain.from_iterable(['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" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "filterfalse()" | pred, seq | elements of seq where pred(elem) is false | "filterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "groupby()" | iterable[, key] | sub-iterators grouped by value of key(v) | |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "islice()" | seq, [start,] stop [, step] | elements from seq[start:stop:step] | "islice('ABCDEFG', 2, None) --> C D E F G" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "starmap()" | func, seq | func(*seq[0]), func(*seq[1]), … | "starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "takewhile()" | pred, seq | seq[0], seq[1], until pred fails | "takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "tee()" | it, n | it1, it2, … itn splits one iterator into n | |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
| "zip_longest()" | p, q, … | (p[0], q[0]), (p[1], q[1]), … | "zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-" |
+------------------------------+------------------------------+---------------------------------------------------+---------------------------------------------------------------+
**Combinatoric iterators:**
+------------------------------------------------+----------------------+---------------------------------------------------------------+
| Iterator | Arguments | Results |
+================================================+======================+===============================================================+
| "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" |
+------------------------------------------------+----------------------+---------------------------------------------------------------+
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.
itertools.accumulate(iterable[, func])
Make an iterator that returns accumulated sums, or accumulated
results of other binary functions (specified via the optional
*func* argument). If *func* is supplied, it should be a function
of two arguments. Elements of the input *iterable* may be any type
that can be accepted as arguments to *func*. (For example, with the
default operation of addition, elements may be any addable type
including "Decimal" or "Fraction".) If the input iterable is empty,
the output iterable will also be empty.
Roughly equivalent to:
def accumulate(iterable, func=operator.add):
'Return running totals'
# accumulate([1,2,3,4,5]) --> 1 3 6 10 15
# accumulate([1,2,3,4,5], operator.mul) --> 1 2 6 24 120
it = iter(iterable)
try:
total = next(it)
except StopIteration:
return
yield total
for element in it:
total = func(total, element)
yield total
There are a number of uses for the *func* argument. It can be set
to "min()" for a running minimum, "max()" for a running maximum, or
"operator.mul()" for a running product. Amortization tables can be
built by accumulating interest and applying payments. First-order
recurrence relations can be modeled by supplying the initial value
in the iterable and using only the accumulated total in *func*
argument:
>>> data = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
>>> list(accumulate(data, operator.mul)) # running product
[3, 12, 72, 144, 144, 1296, 0, 0, 0, 0]
>>> list(accumulate(data, max)) # running maximum
[3, 4, 6, 6, 6, 9, 9, 9, 9, 9]
# Amortize a 5% loan of 1000 with 4 annual payments of 90
>>> cashflows = [1000, -90, -90, -90, -90]
>>> list(accumulate(cashflows, lambda bal, pmt: bal*1.05 + pmt))
[1000, 960.0, 918.0, 873.9000000000001, 827.5950000000001]
# Chaotic recurrence relation https://en.wikipedia.org/wiki/Logistic_map
>>> logistic_map = lambda x, _: r * x * (1 - x)
>>> r = 3.8
>>> x0 = 0.4
>>> inputs = repeat(x0, 36) # only the initial value is used
>>> [format(x, '.2f') for x in accumulate(inputs, logistic_map)]
['0.40', '0.91', '0.30', '0.81', '0.60', '0.92', '0.29', '0.79', '0.63',
'0.88', '0.39', '0.90', '0.33', '0.84', '0.52', '0.95', '0.18', '0.57',
'0.93', '0.25', '0.71', '0.79', '0.63', '0.88', '0.39', '0.91', '0.32',
'0.83', '0.54', '0.95', '0.20', '0.60', '0.91', '0.30', '0.80', '0.60']
See "functools.reduce()" for a similar function that returns only
the final accumulated value.
New in version 3.2.
Changed in version 3.3: Added the optional *func* parameter.
itertools.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. Roughly equivalent to:
def chain(*iterables):
# chain('ABC', 'DEF') --> A B C D E F
for it in iterables:
for element in it:
yield element
classmethod chain.from_iterable(iterable)
Alternate constructor for "chain()". Gets chained inputs from a
single iterable argument that is evaluated lazily. Roughly
equivalent to:
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
itertools.combinations(iterable, r)
Return *r* length subsequences of elements from the input
*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.
Roughly 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)
if r > n:
return
indices = list(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)
The code for "combinations()" can be also expressed as a
subsequence of "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)
The number of items returned is "n! / r! / (n-r)!" when "0 <= r <=
n" or zero when "r > n".
itertools.combinations_with_replacement(iterable, r)
Return *r* length subsequences of elements from the input
*iterable* allowing individual elements to be repeated more than
once.
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, the generated
combinations will also be unique.
Roughly equivalent to:
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)
The code for "combinations_with_replacement()" can be also
expressed as a subsequence of "product()" after filtering entries
where the elements are not in sorted order (according to their
position in the input pool):
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)
The number of items returned is "(n+r-1)! / r! / (n-1)!" when "n >
0".
New in version 3.1.
itertools.compress(data, selectors)
Make an iterator that filters elements from *data* returning only
those that have a corresponding element in *selectors* that
evaluates to "True". Stops when either the *data* or *selectors*
iterables has been exhausted. Roughly equivalent to:
def compress(data, selectors):
# compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
return (d for d, s in zip(data, selectors) if s)
New in version 3.1.
itertools.count(start=0, step=1)
Make an iterator that returns evenly spaced values starting with
number *start*. Often used as an argument to "map()" to generate
consecutive data points. Also, used with "zip()" to add sequence
numbers. Roughly equivalent to:
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
When counting with floating point numbers, better accuracy can
sometimes be achieved by substituting multiplicative code such as:
"(start + step * i for i in count())".
Changed in version 3.1: Added *step* argument and allowed non-
integer arguments.
itertools.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. Roughly 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).
itertools.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.
Roughly 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
itertools.filterfalse(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. Roughly 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:
if not predicate(x):
yield x
itertools.groupby(iterable, key=None)
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 "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 "groupby()". Because the source is shared, when the
"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)
"groupby()" is roughly equivalent to:
class groupby:
# [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
try:
self.currvalue = next(self.it)
except StopIteration:
return
self.currkey = self.keyfunc(self.currvalue)
itertools.islice(iterable, stop)
itertools.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, "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). Roughly 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)
start, stop, step = s.start or 0, s.stop or sys.maxsize, s.step or 1
it = iter(range(start, stop, step))
try:
nexti = next(it)
except StopIteration:
# Consume *iterable* up to the *start* position.
for i, element in zip(range(start), iterable):
pass
return
try:
for i, element in enumerate(iterable):
if i == nexti:
yield element
nexti = next(it)
except StopIteration:
# Consume to *stop*.
for i, element in zip(range(i + 1, stop), iterable):
pass
If *start* is "None", then iteration starts at zero. If *step* is
"None", then the step defaults to one.
itertools.permutations(iterable, r=None)
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.
Roughly 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
if r > n:
return
indices = list(range(n))
cycles = list(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
The code for "permutations()" can be also expressed as a
subsequence of "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)
The number of items returned is "n! / (n-r)!" when "0 <= r <= n" or
zero when "r > n".
itertools.product(*iterables, repeat=1)
Cartesian product of input iterables.
Roughly 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 nested loops cycle like an odometer with the rightmost element
advancing on every iteration. This pattern creates a lexicographic
ordering so that if the input’s 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 roughly equivalent to the following code, except
that the actual implementation does not build up intermediate
results in memory:
def product(*args, repeat=1):
# 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 = [tuple(pool) for pool in args] * repeat
result = [[]]
for pool in pools:
result = [x+[y] for x in result for y in pool]
for prod in result:
yield tuple(prod)
itertools.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 "map()" for invariant parameters to the called
function. Also used with "zip()" to create an invariant part of a
tuple record.
Roughly 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 range(times):
yield object
A common use for *repeat* is to supply a stream of constant values
to *map* or *zip*:
>>> list(map(pow, range(10), repeat(2)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
itertools.starmap(function, iterable)
Make an iterator that computes the function using arguments
obtained from the iterable. Used instead of "map()" when argument
parameters are already grouped in tuples from a single iterable
(the data has been “pre-zipped”). The difference between "map()"
and "starmap()" parallels the distinction between "function(a,b)"
and "function(*c)". Roughly equivalent to:
def starmap(function, iterable):
# starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
for args in iterable:
yield function(*args)
itertools.takewhile(predicate, iterable)
Make an iterator that returns elements from the iterable as long as
the predicate is true. Roughly 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
itertools.tee(iterable, n=2)
Return *n* independent iterators from a single iterable.
The following Python code helps explain what *tee* does (although
the actual implementation is more complex and uses only a single
underlying FIFO (first-in, first-out) queue).
Roughly equivalent to:
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
try:
newval = next(it) # fetch a new value and
except StopIteration:
return
for d in deques: # load it to all the deques
d.append(newval)
yield mydeque.popleft()
return tuple(gen(d) for d in deques)
Once "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.
This itertool may require significant auxiliary storage (depending
on how much temporary data needs to be stored). In general, if one
iterator uses most or all of the data before another iterator
starts, it is faster to use "list()" instead of "tee()".
itertools.zip_longest(*iterables, fillvalue=None)
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. Roughly equivalent to:
class ZipExhausted(Exception):
pass
def zip_longest(*args, **kwds):
# zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-
fillvalue = kwds.get('fillvalue')
counter = len(args) - 1
def sentinel():
nonlocal counter
if not counter:
raise ZipExhausted
counter -= 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
If one of the iterables is potentially infinite, then the
"zip_longest()" function should be wrapped with something that
limits the number of calls (for example "islice()" or
"takewhile()"). If not specified, *fillvalue* defaults to "None".
Itertools 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 *generator*s which incur
interpreter overhead.
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 map(function, count(start))
def tail(n, iterable):
"Return an iterator over the last n items"
# tail(3, 'ABCDEFG') --> E F G
return iter(collections.deque(iterable, maxlen=n))
def consume(iterator, n=None):
"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 all_equal(iterable):
"Returns True if all the elements are equal to each other"
g = groupby(iterable)
return next(g, True) and not next(g, False)
def quantify(iterable, pred=bool):
"Count how many times the predicate is true"
return sum(map(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(map(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 zip(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 zip_longest(*args, fillvalue=fillvalue)
def roundrobin(*iterables):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
# Recipe credited to George Sakkis
num_active = len(iterables)
nexts = cycle(iter(it).__next__ for it in iterables)
while num_active:
try:
for next in nexts:
yield next()
except StopIteration:
# Remove the iterator we just exhausted from the cycle.
num_active -= 1
nexts = cycle(islice(nexts, num_active))
def partition(pred, iterable):
'Use a predicate to partition entries into false entries and true entries'
# partition(is_odd, range(10)) --> 0 2 4 6 8 and 1 3 5 7 9
t1, t2 = tee(iterable)
return filterfalse(pred, t1), filter(pred, t2)
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 filterfalse(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 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 builtins.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 True:
yield func()
except exception:
pass
def first_true(iterable, default=False, pred=None):
"""Returns the first true value in the iterable.
If no true value is found, returns *default*
If *pred* is not None, returns the first item
for which pred(item) is true.
"""
# first_true([a,b,c], x) --> a or b or c or x
# first_true([a,b], x, f) --> a if f(a) else b if f(b) else x
return next(filter(pred, iterable), default)
def random_product(*args, repeat=1):
"Random selection from itertools.product(*args, **kwds)"
pools = [tuple(pool) for pool in args] * repeat
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(range(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 range(r))
return tuple(pool[i] for i in indices)
def nth_combination(iterable, r, index):
'Equivalent to list(combinations(iterable, r))[index]'
pool = tuple(iterable)
n = len(pool)
if r < 0 or r > n:
raise ValueError
c = 1
k = min(r, n-r)
for i in range(1, k+1):
c = c * (n - k + i) // i
if index < 0:
index += c
if index < 0 or index >= c:
raise IndexError
result = []
while r:
c, n, r = c*r//n, n-1, r-1
while index >= c:
index -= c
c, n = c*(n-r)//n, n-1
result.append(pool[-1-n])
return tuple(result)
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))
| 
