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Created on 2020-11-01 05:28 by kaiyutony, last changed 2022-04-11 14:59 by admin. This issue is now closed.
| Messages (3) | |||
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| msg380115 - (view) | Author: Kaiyu Zheng (kaiyutony) | Date: 2020-11-01 05:28 | |
In the following toy example, with Python 3.7.4
class Foo:
def __init__(self, a=set()):
self.a = a
foo1 = Foo()
foo2 = Foo()
foo1.a.add(1)
print(foo2.a)
This shows {1}. This means that modifying the .a field of foo1 changed that of foo2. I was not expecting this behavior, as I thought that when the constructor is called, an empty set is created for the parameter `a`. But instead, what seems to happen is that a set() is created, and then shared between instances of Foo. What is the reason for this? What is the benefit? It adds a lot of confusion
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| msg380117 - (view) | Author: Steven D'Aprano (steven.daprano) *  |
Date: 2020-11-01 06:24 | |
Sorry, this is not a bug, but working as designed. Default values for functions and methods are only created once, when the function is created, not on every call. This is a FAQ: https://docs.python.org/3/faq/programming.html#why-are-default-values-shared-between-objects |
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| msg380118 - (view) | Author: Steven D'Aprano (steven.daprano) * |
Date: 2020-11-01 06:34 | |
See also my comment here: https://bugs.python.org/msg361451 |
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| History | |||
|---|---|---|---|
| Date | User | Action | Args |
| 2022-04-11 14:59:37 | admin | set | github: 86393 |
| 2020-11-01 06:34:38 | steven.daprano | set | messages: + msg380118 |
| 2020-11-01 06:24:35 | steven.daprano | set | status: open -> closed nosy: + steven.daprano messages: + msg380117 resolution: not a bug stage: resolved |
| 2020-11-01 05:28:43 | kaiyutony | create | |