Python 3.3+Iterators & generatorsterm

yield from

generators

Delegates yielding to another generator or iterable. It simplifies nested generators.

Syntax

yield from iterable

It is more than shorthand for a for loop with yield: yield from forwards send() and throw() into the subgenerator, and whatever the subgenerator returns becomes the value of the yield from expression itself. For plain iteration the savings are cosmetic; the real payoff is coroutines and recursive traversal (trees, nested lists), which is why Python 3.3 introduced it.

Examples

def chain(*iterables):
    for it in iterables:
        yield from it
print(list(chain([1,2],[3,4],[5])))  # → [1,2,3,4,5]

def flatten(lst):
    for item in lst:
        if isinstance(item, list):
            yield from flatten(item)
        else:
            yield item
print(list(flatten([1,[2,[3,4]],5])))  # → [1,2,3,4,5]

def gen_a():
    yield 1; yield 2
def gen_b():
    yield from gen_a()
    yield 3
print(list(gen_b()))  # → [1,2,3]

# yield from с range
def gen_range(n):
    yield from range(n)
print(list(gen_range(5)))  # → [0,1,2,3,4]

# yield from возвращает значение (return в под-генераторе)
def sub():
    yield 1; yield 2
    return 'done'
def main():
    result = yield from sub()
    print('sub returned:', result)  # → done
list(main())  # → [1,2]

See also

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