我有一个上下文装饰器,它在完成后会产生副作用。我注意到,如果我使用听写理解,则不会出现副作用。
from contextlib import contextmanager
import traceback
import sys
accumulated = []
@contextmanager
def accumulate(s):
try:
yield
finally:
print("Appending %r to accumulated" % s)
accumulated.append(s)
def iterate_and_accumulate(iterable):
for item in iterable:
with accumulate(item):
yield item
def boom_unless_zero(i):
if i > 0:
raise RuntimeError("Boom!")
try:
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
except:
traceback.print_exc()
print(accumulated)
print('\n=====\n')
try:
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
except:
traceback.print_exc()
print(accumulated)
print('Finished!')
输出:
$ python2 boom3.py
Appending 0 to accumulated
Traceback (most recent call last):
File "boom3.py", line 25, in <module>
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
File "boom3.py", line 25, in <dictcomp>
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
File "boom3.py", line 22, in boom_unless_zero
raise RuntimeError("Boom!")
RuntimeError: Boom!
[0]
=====
Appending 0 to accumulated
Appending 1 to accumulated
Traceback (most recent call last):
File "boom3.py", line 34, in <module>
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
File "boom3.py", line 34, in <dictcomp>
{i: boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])}
File "boom3.py", line 22, in boom_unless_zero
raise RuntimeError("Boom!")
RuntimeError: Boom!
[0, 0, 1]
Finished!
Appending 1 to accumulated
奇怪的是,副作用发生在我的脚本“完成”之后。这意味着如果用户正在使用字典理解,则他们不能使用我的上下文装饰器。
我注意到这种行为在 Python 3 上消失了,如果我写 [boom_unless_zero(i) for i in iterate_and_accumulate([0, 1])]
也不会发生这种行为而不是听写理解。
为什么会这样?
最佳答案
来自 https://docs.python.org/2/reference/simple_stmts.html#the-yield-statement :
As of Python version 2.5, the yield statement is now allowed in the try clause of a try ... finally construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator’s close() method will be called, allowing any pending finally clauses to execute.
换句话说,挂起的 finally 子句在生成器-迭代器关闭之前不会执行,无论是显式关闭还是作为垃圾收集(refcount 或循环)的结果。似乎 Python 2 列表理解和 Python 3 在垃圾收集 iterable 方面更有效。
如果您想明确关闭生成器-迭代器:
from contextlib import closing
try:
with closing(iter(iterate_and_accumulate(a))) as it:
{i: boom_unless_zero(i) for i in it}
except:
traceback.print_exc()
print(accumulated)
我查看了潜在的问题;似乎问题是生成器迭代器由异常回溯状态持有,因此另一种解决方法是调用 sys.exc_clear()
:
import sys
try:
{i: boom_unless_zero(i) for i in iterate_and_accumulate(a)}
except:
traceback.print_exc()
try:
sys.exc_clear()
except AttributeError:
pass
print(accumulated)
在 Python 3 中,词法异常处理程序系统 (http://bugs.python.org/issue3021) 意味着异常状态在从处理程序 block 退出时被清除,因此 sys.exc_clear()
不是必需的(实际上不存在)。
关于python - 为什么这个上下文管理器的行为与字典理解不同?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/24507404/