How Do Generator Yield and Return Interact in Python?

Generator Yield and Return Interplay in Python

Prior to Python 3.3, combining return and yield within a function definition would result in an error. However, in Python 3.3 and later, this behavior has been updated.

Python 3.3 Behavior

In Python 3.3, the return statement within a generator function is now treated similarly to a raise StopIteration() statement. As a result, using both return and yield in a generator is now equivalent to raising a StopIteration exception with the specified value.

Example:

Consider the following code:

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

When the f() function is called, it will no longer raise a runtime error. Instead, the __next__() function of the generator object (x) will raise a StopIteration exception with the value 3:

<code class="python">x = f()
try:
    print(x.__next__())
except StopIteration as e:
    print(e.value)  # Prints "3"</code>

Yielding the Returned Value

If a generator function is delegated using the yield from syntax, the returned value of the return statement will be the result of the delegated generator.

Example:

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

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

# Iterate through the generator:
for _ in g():
    pass  # Prints "1"</code>

In this example, the f() generator is delegated to the g() generator using yield from. As a result, the value returned by return 1 in the f() generator becomes the result of the g() generator.

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