python - Why can't I use yield with return?

Question

I would like you to consider the following code:

def func(alist):
    if len(alist) == 1:
        return arg * 2
    for item in alist:
        yield item * 2

When I run it, I get this error:

SyntaxError: 'return' with argument inside generator

Now, I realize that I cannot do this. However, I would like to know why. What exactly is going on behind the scenes that is causing Python to throw the SyntaxError?

Answer 1

Python has to decide whether a function is a generator at bytecode compilation time. This is because the semantics of generators say that none of the code in a generator function runs before the first next call; the generator function returns a generator iterator that, when next is called, runs the generator code. Thus, Python can't decide whether a function should be a generator or not by running it until it hits a yield or a return; instead, the presence of a yield in a function signals that the function is a generator.