Tell me about yield from — when is it used and how does it work?

yield from is used in Python to delegate part of a generator to another generator or iterable object. It simplifies nested generators by allowing all values from the nested generator to be 'pushed out' without an explicit loop.

When you call yield from iterable, Python automatically iterates over iterable, yielding its elements one by one, and correctly handles sending values and exceptions into the nested generator.

Example:

# Generator that delegates to another generator
def subgen():
    yield 1
    yield 2

def main_gen():
    yield from subgen()
    yield 3

for value in main_gen():
    print(value)

Output:

1
2
3

Thus, yield from simplifies code and improves readability when working with nested generators.