Iterables: lists, sets, dicts, tuples, strs, iterators
these are functions:
   list, tuple, dict, set, sum, sorted(should be sort), any, all, max, min
treat a string as an iterable and you get the chars
for x in iterable:
filter(boolfunction, iterable)   opaque return, note not method
functools.reduce(f, iterable) = ((((1, 2), 3), 4), 5)
map(f, iterable) returns an opaque iterable

yield makes a function behave like an iterable: primes.py, treeen.py

Iterators: - like lazy evaluation
x = iter(ob) - create an iterator from an iterable 
for pos, val in enumerate([9,8,7,6]):    (it produces tuples)
itertools.count()   intsfrom default 0, can have ,stepsize
itertools.cycle(iterable)  infinite repetition of all
itertools.repeat(item, times), default times is unending
itertools.chain(it1, it2, it3, ...) is append
itertools.combinations(it, r)  all r-tuples, but after (a,b), (b,a) does not appear, r required
itertools.permutations(it, r)  same but both (a,b) and (b,a) appear, r not required
L = [("a", 1), ("a", 2), ("b", 3), ("b", 4), ("b", 9)]
key_func = lambda x: x[0]
for key, group in itertools.groupby(L, key_func):    # pretty much worthless
    print(key + " :", group)
itertools.accumulate(it {, func=operator.add}) 1 2 3 4 -> 1 3 6 10, optional initial = value
zip(iterable, iterable, ...) = iterator
next(it) returns next item, exception StopIteration if at end
class numbers: / def __iter__(self): / self.val = 1 / return self
                 def __next__(self): x = self.val / self.val += 2 / return x
      ob = numbers() / i = iter(ob)
can raise StopIteration