Iterators
- Create
- Using iterators
- Map
- Shrink
- Grow
- Aggregate
- Test
- Convert
Create
| What | How | Details |
| Create 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 n times | itertools.repeat(123, 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) | |
| Index-element pairs from list | enumerate(lst) | |
| Key-value pairs from dict | enumerate(dct) | |
| Recycle through list | itertools.cycle(lst) | |
| Recycle through list n times | itertools.product(lst, n) | |
| Concatenate lists | itertools.chain(lst1, lst2) | |
| Create n copies of iterable | itertools.tee(x, 2)
or
x1, x2 = itertools.tee(x) | |
Using iterators
Note that each operation consumes element(s) of the iterator!
| What | How | Details |
| Iterate over iterator elements | for i in iter:
print(i) | |
| Consume fully | collections.deque(x, maxlen=0) | |
| Get 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)
or
next(filter(lambda e: e == 1, x)) | |
| Get last element according to criterion | ? | |
| Get index of first true element | ? | |
| Get index of first false element | ? | |
| Get index of last true element | ? | |
| Get index of last false element | ? | |
| Get remaining length | sum(1 for _ in iter) | |
Map
| What | How |
| Map elements | x + 1 for x in iter |
| Map elements with function | map(fun, iter) |
| Invert boolean iterator | map(lambda x: not x, bool_iter)
or
map(operator.not_, bool_iter) |
| Cumulative sum | itertools.accumulate(iter) |
| Accumulate function (reduce() with keeping all results) | itertools.accumulate(iter, fun) |
| 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) |
Shrink
| What | How | 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) | |
| 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) | |
| Filter iterables based on boolean function | filter(lambda x: x[0] != x[1], zip(iter1, iter2)) | |
| Inverse filter iterable based on a boolean iterable | itertools.filterfalse(mask, x) | |
| Take while predicate is true | itertools.takewhile(lambda x: x < 5, x) | |
| Drop while predicate is true | itertools.dropwhile(lambda x: x < 5, x) | |
Grow
| What | How | Details |
| Append iterable | itertools.chain(iter, append_iter) | |
| Prepend iterable | itertools.chain(prepend_iter, iter) | |
| Create combinations of pairs | itertools.combinations(x, 2) | |
| Create combinations of size n | itertools.combinations(x, n) | |
| Create combinations of pairs with replacement (so including pairs of (A, A)) | itertools.combinations_with_replacement(x, 2) | |
| Create combinations of size n with replacement | itertools.combinations_with_replacement(x, n) | |
| Create permutations | itertools.permutations(x) | |
| 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)) | |
| Chain iterables | itertools.chain.from_iterable(x, y, ...) | |
Aggregate
| What | How | Details |
| Min | min(iter) | |
| Max | max(iter) | |
| Sum | sum(iter) | |
| Hash | hash(iter) | |
| Reduce | reduce(binary_fun, iter) | |
| Group by | for key, group in itertools.groupby(robots, key=lambda x: x['faction']):
print(key)
print(list(group)) | |
Test
| What | How | Details |
| All elements are true | all(bool_iter) | |
| All elements are false | not any(bool_iter) | |
| All elements are equal | g = groupby(x)
return next(g, True) and not next(g, False) | |
| All elements are true conditional on function | all(map(boolFun, iter)) | |
| Any element is true | any(bool_iter) | |
| Any element is false | not all(bool_iter) | |
Convert
| What | How | Details |
| List | list(iter) | |
| Tuple | tuple(iter) | |
| Dict | dict(zip(key_iter, value_iter)) | |