Python 2.7：“有什么收获了吗？”

Question

Within a generator function, how can I find out if it has yielded anything yet?

def my_generator(stuff):
    # complex logic that interprets stuff and may or may not yield anything

    # I only want to yield this if nothing has been yielded yet.
    yield 'Nothing could be done with the input.'

Answer 1

You'd need to keep a flag yourself, or restructure the code at the top. If things are too complex, it sounds like your function might do too much.

Also, if that's your message, it sounds like you may want an exception instead.

Answer 2

A simple way to keep track yourself, is to wrap the complex logic into an inner generator.

What's great about this, is it does not require any change to that complex logic.

def my_generator(stuff):
    def inner_generator():
        # complex logic that interprets stuff and may or may not yield anything
        if stuff:
            yield 11 * stuff

    # I only want to yield this if nothing has been yielded yet.
    have_yielded = False
    for x in inner_generator():
        have_yielded = True
        yield x

    if not have_yielded:
        yield 'Nothing could be done with the input.'

Test #1:

print(list(my_generator(1)))

=>

[11]

Test #2:

print(list(my_generator(None)))

=>

['Nothing could be done with the input.']

---

--- An Alternative ---

More complicated code, that is probably a premature optimization. Avoids repeatedly setting have_yielded to True. Only works if your generator never produces "None" as its first value:

    ...
    # I only want to yield this if nothing has been yielded yet.
    have_yielded = False
    g = inner_generator()
    x = next(g, None)
    if x is not None:
        yield x
        have_yielded = True
    for x in g:
        yield x

    if not have_yielded:
        yield 'Nothing could be done with the input.'