New submission from Kaiyu Zheng <kaiyut...@gmail.com>:
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
----------
messages: 380115
nosy: kaiyutony
priority: normal
severity: normal
status: open
title: Unexpected sharing of list/set/dict between instances of the same class,
when the list/set/dict is a default parameter value of the constructor
type: behavior
versions: Python 3.7
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue42227>
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