Iterator is onetime. So this is not a bug. Iterable may return an iterator that iterate from the start. Containers including list, tuple, and dict are re-iterable. On the other hand, file object is one shot iterable. 2024年10月7日(月) 21:22 RAUL GONZALO DIAZ <raul.gonzalodiaz@telefonica.com>:

Good morning,

In the tutorial Docs, chapter 9 (Clases), section 9.8 (Iterators)…

(this is the link 9. Classes — Python 3.12.7 documentation <https://docs.python.org/3/tutorial/classes.html#iterators>)

…there is an example of __iter__() and __next__() methods explaining the mechanics of the iterator protocol.

The example is a bit confusing because in the example:

rev = Reverse('spam')

*>>> *iter(rev)

<__main__.Reverse object at 0x00A1DB50>

*>>> **for* char *in* rev:

*... * print(char)

*...*

m

a

p

s

it behaves as expected, but if you repeat the for statement just afterwards, it does nothing:

*for* char *in* rev:

*... * print(char)

*...*

*>>>*

The problem is in the definition of the __iter__() method, because it does not reset the index.

This is the code that appears in the tutorial:

*class* *Reverse*:

*"""Iterator for looping over a sequence backwards."""*

*def* __init__(self, data):

self.data = data

self.index = len(data)

*def* __iter__(self):

*return* self

*def* __next__(self):

*if* self.index == 0:

*raise* *StopIteration*

self.index = self.index - 1

*return* self.data[self.index]

and this is my proposal:

*class* *Reverse*:

*"""Iterator for looping over a sequence backwards."""*

*def* __init__(self, data):

self.data = data

*def* __iter__(self):

self.index = len(self.data)

*return* self

*def* __next__(self):

*if* self.index == 0:

*raise* *StopIteration*

self.index = self.index - 1

*return* self.data[self.index]

Note: this is not perfect, because it does not admit nested iterations over the same object (that’s the reason why generally iterator objects (different from the original data) are necessary).

Regards.

*Raúl Gonzalo Díaz*

*Automatización Red y Servicios | Soporte de Red*

Edif. Torras i Bages, c/Cinca, 39-41 <https://www.google.com/maps/search/c%2FCinca,+39-41?entry=gmail&source=g>, Planta 1 | 08030 Barcelona, España (GMT +1)

Telf. +34 93 403 06 61

Email raul.gonzalodiaz@telefonica.com

­

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