何时将StopIteration转换为RuntimeError？

Question

我读的Python 3文档在这里:

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,有谁能解释一下？

这是我在Python 3.6中尝试过的,但似乎没有抓到:

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,两次调用都gen2()提升了StopIteration,但仍未转换成RuntimeError.

Answer 1

您错过了此更改适用于Python 3.7及更高版本.您不会在Python 3.6或更早版本中看到转换,除非您from __future__首先使用导入启用该功能(从Python 3.5开始提供).

从您链接的同一页面:

版本3.5中已更改:引入了RuntimeError转换from __future__ import generator_stop,请参阅PEP 479.

在3.7版中更改:默认情况下为所有代码启用PEP 479:StopIteration生成器中引发的错误转换为a RuntimeError.

PEP 479 - 更改生成器内部的StopIteration处理进一步详细说明了为何进行此更改以及如何应用此更改.对于在Python 3.7上运行的代码,输出变为:

>>> 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,我添加了yield 42一行gen2()以使其成为生成器.没有yield或yield from在身体中,你会得到一个常规的功能.调用生成器函数会生成一个生成器对象,并且函数体开始暂停,而调用普通函数会立即执行正文:

>>> 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Python 3.6,您将使用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