Iterators

Create

Action	Code	Details
Indefinite counter	itertools.count()
Create counter starting from n	itertools.count(n)
Create counter starting from n with step size s	itertools.count(n, s)
Create limited counter from n to m	?
Repeat constant value v by n times	itertools.repeat(v, n)
2D coordinates grid from (0, 0) to (n - 1, m - 1 )	itertools.product(range(n), range(m))
From string (over chars)	iter(str)
From list	iter(lst)
Enumeration pairs (index, element) from list	enumerate(lst)
From list, indefinitely	itertools.cycle(lst)
From list, n times	itertools.product(lst, n)
Concatenate lists	itertools.chain(lst1, lst2)
Key-value pairs from dict	enumerate(dct)
Create n copies of iterable	itertools.tee(x, 2)
Create n copies of iterable	x1, x2 = itertools.tee(x)

Usage

Action	Code	Details
Loop over iterator	for i in iter: print(i)

Test

Note that all tests consume.

Action	Code	Details
All elements are true	all(bool_iter)
Any element is true	any(bool_iter)
All elements are false	not any(bool_iter)
Any element is false	not all(bool_iter)
All elements are equal	g = groupby(x) next(g, True) and not next(g, False)
All elements are true conditional on function	all(map(boolFun, iter))

Extract

Operations (partially) consume the iterable unless mentioned otherwise.

Action	Code	Details
Get remaining length	sum(1 for _ in iter)	Fully consumes
Next element	next(iter)
Get next element with default if depleted	next(iter, 0)
Get n th element	next(itertools.islice(x, n, None), default)
Get first element according to criterion	next(e for e in x if e == 1)
Get first element according to criterion	next(filter(lambda e: e == 1, x))
Get index of first true element	?
Get index of first false element	?

Update

Action	Code	Details
Clear (consume fully)	collections.deque(x, maxlen=0)

Derive

Action	Code	Details
First n items of iterable	itertools.islice(x, n)
Last n items of iterable	iter(collections.deque(x, maxlen = n))
Drop first n items of iterable	itertools.islice(x, start=n)
Drop while predicate is true	itertools.dropwhile(lambda x: x < 5, x)
Slice based on (index) integer iterable	?
Filter based on boolean function	filter(bool_fun, iterable)
Filter based on boolean iterable	itertools.compress(x, mask_iter)
Inverse filter iterable based on a boolean iterable	itertools.filterfalse(mask, x)
Take while predicate is true	itertools.takewhile(lambda x: x < 5, x)
Filter iterables based on boolean function	filter(lambda x: x[0] != x[1], zip(iter1, iter2))

Transform iterable

Using the derived iterable consumes the original one as well.

Action	Code	Details
Transform elements by expression	x + 1 for x in iter
Map elements with function	map(fun, iter)
Invert boolean iterator	map(lambda x: not x, bool_iter)
Invert boolean iterator	map(operator.not_, bool_iter)
Cumulative sum	itertools.accumulate(iter)
Accumulate function (reduce() with keeping all results)	itertools.accumulate(iter, fun)

Grow

Action	Code	Details
Append iterable	itertools.chain(iter, append_iter)
Prepend iterable	itertools.chain(prepend_iter, iter)
Cartesian product of two or more iterables	itertools.product(x, y, ...)
Repeat iterable indefinitely	itertools.cycle(x)
Repeat iterable n times	itertools.repeat(x, n)
Repeat each element of iterable n times	itertools.chain.from_iterable(itertools.repeat(tuple(x), n))

Shrink

Combine iterables

Action	Code	Details
Chain iterables (concatenate)	itertools.chain.from_iterable(x, y, ...)
Combine iterable elements as tuple, stop on shortest iterable	zip(x, y, ...)
Combine iterable elements as tuple, expect equal length	zip(x, y, ..., strict=True)	Throws error if an iterable is depleted prematurely
Combine iterable elements as tuple until all iterables are exhausted, with default value for depleted iterables	itertools.zip_longest(x, y, ..., fillvalue = None)
Create combinations of pairs	itertools.combinations(x, 2)
Create combinations of size n	itertools.combinations(x, n)
Create combinations of size n with replacement	itertools.combinations_with_replacement(x, n)
Create combinations of pairs with replacement (so including pairs of (A, A))	itertools.combinations_with_replacement(x, 2)
Create permutations	itertools.permutations(x)

Convert

Action	Code	Details
List	list(iter)
Tuple	tuple(iter)
Dict	dict(zip(key_iter, value_iter))