何时将StopIteration转换为RuntimeError？

Question

我正在阅读Python3 这里的文档

如果生成器代码直接或间接引发StopIteration，则将其转换为RuntimeError (保留StopIteration作为新异常的原因)。

我不明白，有人能解释一下吗？

这是我在Python3.6中尝试过的，但似乎没有发现：

def gen1():
    yield from [1, 2, 3]
    raise StopIteration

def gen2():
    raise StopIteration

try:
    a = list(gen1())
    # a == [1, 2, 3]
except RuntimeError:
    print("Caught")

try:
    a = gen1()
    next(a), next(a), next(a), next(a), next(a)
except RuntimeError:
    print("Caught")

try:
    gen2()
except RuntimeError:
    print("Caught")

try:
    a = list(gen2())
except RuntimeError:
    print("Caught")

特别是，对gen2()的两个调用都会引发StopIteration，但仍未转换为RuntimeError。

Answer 1

您忽略了这个更改适用于Python3.7和更高版本。您将不会看到Python3.6或更高版本中的转换，除非您首先使用from __future__导入启用该特性(在Python3.5中可用)。

从您链接的同一页：

在3.5版中更改:通过RuntimeError引入了from __future__ import generator_stop转换，参见PEP 479。 在版本3.7中更改:默认情况下为所有代码启用PEP 479 :在生成器中引发的StopIteration错误将转换为RuntimeError。

佩普479 --进一步详细介绍了做出此更改的原因及其应用方式。对于运行在Python3.7上的代码，输出如下：

>>> import sys
>>> sys.version_info
sys.version_info(major=3, minor=7, micro=0, releaselevel='final', serial=0)
>>> def gen1():
...     yield from [1, 2, 3]
...     raise StopIteration
...
>>> def gen2():
...     yield 42  # make this an actual generator
...     raise StopIteration
...
>>> try:
...     a = list(gen1())
... except RuntimeError:
...     print("Caught")
...
Caught
>>> try:
...     a = gen1()
...     next(a), next(a), next(a), next(a), next(a)
... except RuntimeError:
...     print("Caught")
...
Caught
>>> try:
...     a = list(gen2())
... except RuntimeError:
...     print("Caught")
...
Caught

请注意，我向yield 42添加了一个gen2()行，使其成为一个生成器。如果体内没有yield或yield from，则会得到一个常规函数。调用生成器函数将生成生成器对象，函数体开始暂停，而调用普通函数则立即执行主体：

>>> def normal():
...     raise StopIteration
...
>>> def generator():
...     raise StopIteration
...     yield  # never reached, but this is now a generator
...
>>> normal()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in normal
StopIteration
>>> generator()
<generator object generator at 0x105831ed0>
>>> next(generator())
Traceback (most recent call last):
  File "<stdin>", line 2, in generator
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration

对于Python3.6，您可以使用from __future__ import generator_stop编译器开关(在编写脚本或模块时在代码顶部使用它)：

>>> import sys
>>> sys.version_info
sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)
>>> def generator():
...     raise StopIteration
...     yield
...
>>> next(generator())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in generator
StopIteration
>>> from __future__ import generator_stop
>>> def generator():  # re-define it so it is compiled anew
...     raise StopIteration
...     yield
...
>>> next(generator())
Traceback (most recent call last):
  File "<stdin>", line 2, in generator
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: generator raised StopIteration