New submission from Haoyang <mahaoyang7...@163.com>:
Here is the bug-triggered code snippet in the file uploaded
class A:
def __init__(self, b=[]):
print('b = ', b)
self.a = b
for i in range(3):
a = A()
a.a.append(1)
print(a.a)
It seems that when I pass a list "b" to __init__ function with default value
empty list. Every time I create a new instance of this class and append one new
variable to "self.a", the default value of "b" changed at the next time I
create another instance of class A.
The outcome of this code snippet is
a = []
[1]
a = [1]
[1, 1]
a = [1, 1]
[1, 1, 1]
I am new to python. Is it a legal behavior in python? If yes, what is the
principle beneath it? Thanks in advance!
----------
files: test.py
messages: 384765
nosy: haoyang9804
priority: normal
severity: normal
status: open
title: __init__ function may incur an incorrect behavior if passing a list as
a parameter and set its default value as empty
type: behavior
versions: Python 3.9
Added file: https://bugs.python.org/file49732/test.py
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue42883>
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