英文:
What is the difference between exhausting a generator using "for i in generator" and next(generator)
问题
我想学习如何使用生成器的返回值（但这不是我现在关心的问题）。
经过搜索，他们说当生成器耗尽时，可以从StopIteration中获取返回值，所以我用以下代码进行了测试：
def my_generator():
yield 1
yield 2
yield 3
return "done"
def exhaust_generator(_gen):
print("===============================================")
print("exhaust_generator")
try:
while True:
print(next(_gen))
except StopIteration as e:
print(f"Return value: '{e.value}'")
def exhaust_generator_iter(_gen):
print("===============================================")
print("exhaust_generator_iter")
try:
for i in _gen:
print(i)
print(next(_gen))
except StopIteration as e:
print(f"Return value: {e.value}")
gen = my_generator()
gen2 = my_generator()
exhaust_generator(gen)
exhaust_generator_iter(gen2)
如您所见，两种耗尽生成器的版本之间的返回值不同，我想知道为什么。
搜索过Google，但没有找到有用的信息。
英文:
I want to learn how to use the return value of a generator (but this not what I'm concerned with now).
After searching, they said that I can get the return value from StopIteration when the generator is exhausted, so I tested it with the following code:
def my_generator():
yield 1
yield 2
yield 3
return "done"
def exhaust_generator(_gen):
print("===============================================\n")
print("exhaust_generator")
try:
while True:
print(next(_gen))
except StopIteration as e:
print(f"Return value: '{e.value}'")
def exhaust_generator_iter(_gen):
print("===============================================\n")
print("exhaust_generator_iter")
try:
for i in _gen:
print(i)
print(next(_gen))
except StopIteration as e:
print(f"Return value: {e.value}")
gen = my_generator()
gen2 = my_generator()
exhaust_generator(gen)
exhaust_generator_iter(gen2)
===============================================
exhaust_generator
1
2
3
Return value: 'done'
===============================================
exhaust_generator_iter
1
2
3
Return value: None
As you can see, the return value is different between the two versions of exhausting the generator and I wonder why.
Searched google but it has not been helpful.
答案1
得分: 5
在你的第一个示例中，使用while时，你捕获了生成器在最初耗尽时引发的第一个StopIteration异常。
在你的第二个示例中，使用for时，你捕获了一个后续的 StopIteration异常，这是因为在生成器已经被for循环用尽并捕获了初始异常后，你调用next()第二次时引发的。
英文:
In your first example with the while, you're catching the first StopIteration exception raised by the generator when it's initially exhausted.
In your second example with the for, you're catching a subsequent StopIteration exception raised by calling next() a second time after the generator has already been exhausted (by the for, which caught that initial exception).
答案2
得分: 2
根据我理解，for 循环内部也会调用 next() 并最终捕获 StopIteration，以便知道何时停止调用 next()。
在 exhaust_generator_iter 中，您的 next() 调用然后引发 第二个 StopIteration 但不带传递值。
此Stack Overflow问题 及其答案探讨了从生成器接收 return 值的方法，包括使用实用程序类或依赖注入。
英文:
As far as I understand, the for loop internally also calls next() and eventually catches the StopIteration, so it knows when to stop calling next().
Your next() call in exhaust_generator_iter then raises a second StopIteration but without the passed value.
This SO question and its answers explore ways to receive the return value from a generator, i.a. using a utility class or dependency injection.
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