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Clean Code in Python

Further Uses of the yield from Construction

Explore how to leverage Python's yield from construction to capture return values from nested generators and facilitate data sending and exception handling in coroutines. Understand how yield from enhances generator chains and opens possibilities for asynchronous programming by managing execution flow and communication between coroutines efficiently.

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Let's explore the real use of the yield from construction.

Capturing the value returned by a subgenerator

In the following example, we have a generator that calls another two nested generators, producing values in a sequence. Each one of these nested generators returns a value, and we will see how the top-level generator is able to effectively capture the return value since it's calling the internal generators through yield from:

Python 3.8
from log import logger
def sequence(name, start, end):
    logger.info("%s started at %i", name, start)
    yield from range(start, end)
    logger.info("%s finished at %i", name, end)
    return end
def main():
    step1 = yield from sequence("first", 0, 5)
    step2 = yield from sequence("second", step1, 10)
    return step1 + step2

This is a possible execution of the code in main while it's being iterated:

Python 3.8
g = main()
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))

The first line of main delegates into the internal generator and produces the values, extracting them directly from it. This is nothing new, as we have already seen. Notice, though, how the sequence() generator function returns the end value, which is assigned in the first line to the variable named step1, and how this value is correctly used at the start of the following instance of that generator. ...