Question

我正在阅读Python 3 here的文档：

如果生成器代码直接或间接引发StopIteration，则会将其转换为RuntimeError（保留StopIteration作为新的异常原因）。

我不明白，有人可以解释吗？

这是我在Python 3.6中尝试过的方法，但似乎没有发现任何问题：

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，两个对gen2()的调用都引发了StopIteration，但仍未转换为RuntimeError。

Answer 1

您错过了此更改适用于Python 3.7及更高版本。 除非您先启用from __future__导入功能（自Python 3.5起可用），否则您将不会在Python 3.6或更早版本中看到转换。

在您链接的同一页面上：

在版本3.5中进行了更改：通过RuntimeError引入了from __future__ import generator_stop转换，请参阅PEP 479。

版本3.7中的更改：默认情况下为所有代码启用PEP 479：生成器中引发的StopIteration错误将转换为RuntimeError。

PEP 479 -- Change StopIteration handling inside generators进一步详细说明了进行此更改的原因及其应用方式。对于在Python 3.7上运行的代码，输出将变为：

>>> 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，我在yield 42上添加了gen2()行以使其成为生成器。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一个生成器对象，并且函数主体开始处于暂停状态，而调用普通函数将立即执行该主体：

>>> 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Python 3.6，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

StopIteration何时会转换为RuntimeError？

1 个答案: