为什么 next 会引发“StopIteration",但“for"会正常返回?
问题描述
在这段代码中,为什么使用 for
导致没有 StopIteration
还是 for
循环捕获所有异常然后静默退出?在这种情况下,为什么我们会有多余的 return
??或者是raise StopIteration
导致:return None
?
In this piece of code, why does using for
result in no StopIteration
or is the for
loop trapping all exceptions and then silently exiting?
In which case, why do we have the extraneous return
?? Or is the
raise StopIteration
caused by: return None
?
#!/usr/bin/python3.1
def countdown(n):
print("counting down")
while n >= 9:
yield n
n -= 1
return
for x in countdown(10):
print(x)
c = countdown(10)
next(c)
next(c)
next(c)
假设 StopIteration
被触发:return None
.GeneratorExit
是什么时候生成的?
Assuming StopIteration
is being triggered by: return None
.
When is GeneratorExit
generated?
def countdown(n):
print("Counting down from %d" % n)
try:
while n > 0:
yield n
n = n - 1
except GeneratorExit:
print("Only made it to %d" % n)
如果我手动执行:
c = countdown(10)
c.close() #generates GeneratorExit??
在这种情况下,为什么我看不到回溯?
In which case why don't I see a traceback?
解决方案
for
循环显式监听 StopIteration
.
for
语句的目的是循环遍历迭代器提供的序列,异常用于表示迭代器现在完成;for
不会捕获被迭代的对象引发的其他异常,只是那个.
The purpose of the for
statement is to loop over the sequence provided by an iterator and the exception is used to signal that the iterator is now done; for
doesn't catch other exceptions raised by the object being iterated over, just that one.
这是因为 StopIteration
是正常的、预期的信号,它告诉正在迭代的任何人没有更多的东西要产生.
That's because StopIteration
is the normal, expected signal to tell whomever is iterating that there is nothing more to be produced.
生成器函数是一种特殊的迭代器;它确实会在函数完成时引发 StopIteration
(即,当它返回时,是的,return None
引发 StopIteration
).这是迭代器的要求;他们必须在完成后提出StopIteration
;事实上,一旦一个 StopIteration
被引发,试图从他们那里获取另一个元素(通过 next()
,或者调用 .next()
(py 2) 或 .__next__()
(py 3) 迭代器上的方法)必须总是再次引发 StopIteration
.
A generator function is a special kind of iterator; it indeed raises StopIteration
when the function is done (i.e. when it returns, so yes, return None
raises StopIteration
). It is a requirement of iterators; they must raise StopIteration
when they are done; in fact, once a StopIteration
has been raised, attempting to get another element from them (through next()
, or calling the .next()
(py 2) or .__next__()
(py 3) method on the iterator) must always raise StopIteration
again.
GeneratorExit
是向other 方向通信的异常.您正在显式关闭一个带有 yield
表达式的生成器,Python 将该关闭传递给生成器的方式是在该函数内部引发 GeneratorExit
.您在 countdown
中显式捕获该异常,其目的是让生成器在关闭时根据需要清理资源.
GeneratorExit
is an exception to communicate in the other direction. You are explicitly closing a generator with a yield
expression, and the way Python communicates that closure to the generator is by raising GeneratorExit
inside of that function. You explicitly catch that exception inside of countdown
, its purpose is to let a generator clean up resources as needed when closing.
GeneratorExit
不会传播给调用者;请参阅 generator.close()
文档一个>.
A GeneratorExit
is not propagated to the caller; see the generator.close()
documentation.
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