Why is using the return operator inside a generator in Python invalid or causes errors?

In Python, a generator is a function that uses the yield keyword to produce values step by step. Using the return statement inside a generator is not forbidden, but if return is used with a value, it will raise a SyntaxError in Python 3.7 and above.

The reason is that return in a generator is used to end the iteration, and if it contains a value, it contradicts the concept of a generator, which should produce values via yield. In Python 3.3+, you can use return without a value to stop the generator, or return value inside a generator raises a StopIteration exception with that value, which is rarely used directly.

Example of a correct generator:

def gen():
    yield 1
    yield 2
    return  # just ending the generator

for val in gen():
    print(val)

If you write return 5 inside a generator, then when trying to get the next value, a StopIteration exception will be raised with the attribute value=5, which is usually not handled explicitly and can lead to errors.

Thus, the return operator with a value inside a generator is undesirable because it does not return a value to the calling code, but only signals the end of iteration, which can lead to unexpected errors.