带有返回语句的生成器 [英] Generator with return statement

问题描述

在报道期间，我在以下案例(python 3.4)上挠头了

During my coverage, I scratched my head on the following case (python 3.4)

def simple_gen_function(str_in, sep=""):
    if sep == "":
        yield str_in[0]
        for c in str_in[1:]:
            yield c
    else:
        return str_in
        # yield from str_in

str_in = "je teste "
t = "".join(simple_gen_function(str_in))
p = "".join(simple_gen_function(str_in, "\n"))

print("%r %r" % (t, p))
# 'je teste' ''

使用生成器中的return，在使用yield from str_in时未达到"返回值，我得到了预期的结果.

Using return in the generator, the return was not "reached" while using yield from str_in I have the expected result.

这个问题似乎很简单，但是我相信在生成器中使用return可以实现.

The question seems simple, but I believed that using return in a generator, it was in reached.

推荐答案

在函数主体中yield的存在将其转变为生成器函数，而不是普通函数.在生成器函数中，使用return表示生成器已结束，没有更多元素了".通过将生成器方法的第一条语句设为return str_in，可以确保您拥有不返回任何元素的生成器.

The presence of yield in a function body turns it into a generator function instead of a normal function. And in a generator function, using return is a way of saying "The generator has ended, there are no more elements." By having the first statement of a generator method be return str_in, you are guaranteed to have a generator that returns no elements.

正如评论所提到的，返回值 用作StopIteration异常的参数，该异常在生成器结束时引发.参见:

As a comment mentions, the return value is used as an argument to the StopIteration exception that gets raised when the generator has ended. See:

>>> gen = simple_gen_function("hello", "foo")
>>> next(gen)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration: hello

如果def中任何位置都有yield，则它是生成器！

在注释中，提问者提到他们认为执行yield语句时，该函数动态地变成了生成器.但这不是它的工作方式！该决定是在代码被执行之前做出的.如果Python在def下的任何位置都找到了yield，它将把def变成生成器函数.

If there's a yield anywhere in your def, it's a generator!

In the comments, the asker mentions they thought the function turned into a generator dynamically, when the yield statement is executed. But this is not how it works! The decision is made before the code is ever excuted. If Python finds a yield anywhere at all under your def, it turns that def into a generator function.

请参见以下简短示例:

>>> def foo():
...     if False:
...         yield "bar"
...     return "baz"
>>> foo()
<generator object foo at ...>
>>> # The return value "baz" is only exposed via StopIteration
>>> # You probably shouldn't use this behavior.
>>> next(foo())
Traceback (most recent call last):
  ...
StopIteration: baz
>>> # Nothing is ever yielded from the generator, so it generates no values.
>>> list(foo())
[]

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