python - Python 2.7：“有什么产出了吗？”

Question

在生成器函数中，我怎样才能知道它是否已经产生了任何东西？

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

您需要自己保留一个标志，或在顶部重构代码。如果事情太复杂，听起来你的函数可能做的太多了。

此外，如果这是您的信息，听起来您可能想要一个例外。

Answer 2

跟踪自己的一种简单方法是将复杂的逻辑包装到内部生成器中。

这样做的好处是，它不需要对复杂的逻辑进行任何更改。

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.'

测试#1：

print(list(my_generator(1)))

=>

[11]

测试#2：

print(list(my_generator(None)))

=>

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

---

- - 替代 - -

更复杂的代码，那可能是过早的优化。避免重复将 have_yielded 设置为 True。仅当您的生成器从不产生“无”作为其第一个值时才有效：

    ...
    # 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.'