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Python: yield from

yield from gen abbreviates 
for val in gen:
    yield val
However, yield from does much more behind the scenes. For example, values passed to a generator with .send() are transparently handled.

Simple example

This Stackoverflow answer was helpful to understand yield from 
def yieldXYZ():

    yield 'X'
    yield 'Y'
    yield 'Z'


def yieldFromYieldXYZ():

    for i in range(3):
        yield from yieldXYZ()


for x in yieldFromYieldXYZ():            
    print(x)
When executed, it prints 
X
Y
Z
X
Y
Z
X
Y
Z

Modifying yielded elements

In the following example, the function squaredNubmers uses yield from to create lists with two elements: the first is the number yielded from numbers while the second one is the number squared: 
def numbers():
    yield  7
    yield 28
    yield 12
    yield  3


def squaredNubmers():
    yield from ( [x, x**2] for x in numbers() )


for i in squaredNubmers():
    print(f'{i[0]} -> {i[1]}')
I am not sure if this is a form of list comprehension.

Flattening nested lists

def flatten(listOrElem):

    if isinstance(listOrElem, list):
       for x in listOrElem:
           yield from flatten(x)

    else:
       yield listOrElem


for i in flatten( [ 1, ['a', 'b', ['I', 'II', 'III', 'IV' ], 'c' ], 2, 3] ):
    print(i)

TODO

This stackoverflow comment suggests that yield as would have been a better wording than yield from.

See also

yield
PEP-380

Index

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