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Created on 2021-09-11 02:55 by ohwphil, last changed 2022-04-11 14:59 by admin. This issue is now closed.
| Messages (3) | |||
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| msg401625 - (view) | Author: 2106 Hyunwoo Oh (ohwphil) | Date: 2021-09-11 02:55 | |
If you do the following:
lists=[[]]*100
lists[1].append('text')
print(lists[2])
you can see lists[2] contains 'text' even though it was appended to lists[1] in the text. A little more investigation with the id() function can show that the lists are shallowly copied when list multiplication occurs. I think this can confuse users when they try to use list multiplication to create nested lists, as they expected a deep copy to occur.
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| msg401626 - (view) | Author: Steven D'Aprano (steven.daprano) *  |
Date: 2021-09-11 03:07 | |
This is not a bug. No copy is made at all, neither shallow nor deep. This is described in the documentation for built-in types: https://docs.python.org/3/library/stdtypes.html#common-sequence-operations and is similar to the issue in the FAQs: https://docs.python.org/3/faq/programming.html#id16 I acknowledge that this is sometimes confusing for beginners, but it is just one of those things that programmers have to learn. Sequence multiplication does not copy the items, it replicates references to the same item. |
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| msg401627 - (view) | Author: Steven D'Aprano (steven.daprano) * |
Date: 2021-09-11 03:09 | |
Oh, I forgot: this exact issue is also in the FAQs. https://docs.python.org/3/faq/programming.html#id46 |
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| History | |||
|---|---|---|---|
| Date | User | Action | Args |
| 2022-04-11 14:59:49 | admin | set | github: 89332 |
| 2021-09-11 03:09:16 | steven.daprano | set | messages: + msg401627 |
| 2021-09-11 03:07:46 | steven.daprano | set | status: open -> closed nosy: + steven.daprano messages: + msg401626 resolution: not a bug stage: resolved |
| 2021-09-11 03:00:08 | ohwphil | set | components: + Interpreter Core, - Demos and Tools |
| 2021-09-11 02:55:07 | ohwphil | create | |