How Do Return and Yield Work in Python Generators?

Return and Yield in Python Generators

In Python prior to version 3.3, using both return and yield statements simultaneously within a generator function definition would result in an error. However, this behavior has changed in Python 3.3.

Consider the following code:

<code class="python">def f():
  return 3
  yield 2</code>

Calling x = f() will create a generator, and x.__next__() will raise a StopIteration exception. This behavior differs from simply return 3, which would have returned the value 3.

This is because in Python 3.3, return within a generator is now equivalent to raise StopIteration(). Consequently, the StopIteration exception contains the value 3 in the code snippet above. Accessing the value attribute of the exception will retrieve this value.

Additionally, yield from allows делегировать generators to other generators. Consider the following example:

<code class="python">def f():
    return 1
    yield 2

def g():
    x = yield from f()
    print(x)

for _ in g():
    pass</code>

This code prints 1. g delegates execution to f, and the value returned by f (i.e., 1) is assigned to x. However, the yield 2 statement in f is not executed since the execution has been delegated to g.

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