__init__ function may incur an incorrect behavior if passing a list as a parameter and set its default value as empty

[haoyang9804]

BPO	42883
Nosy	@serhiy-storchaka, @haoyang9804
Files	test.py: This code snippet trigger this incorrect behavior.

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-01-10.14:58:42.480>
created_at = <Date 2021-01-10.14:43:44.558>
labels = ['type-bug', 'invalid', '3.9']
title = '__init__  function may incur an incorrect behavior if passing a list as a parameter and set its default value as empty'
updated_at = <Date 2021-01-10.15:00:28.855>
user = 'https://github.com/haoyang9804'

bugs.python.org fields:

activity = <Date 2021-01-10.15:00:28.855>
actor = 'haoyang9804'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-10.14:58:42.480>
closer = 'serhiy.storchaka'
components = []
creation = <Date 2021-01-10.14:43:44.558>
creator = 'haoyang9804'
dependencies = []
files = ['49732']
hgrepos = []
issue_num = 42883
keywords = []
message_count = 3.0
messages = ['384765', '384766', '384767']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'haoyang9804']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42883'
versions = ['Python 3.9']

[haoyang9804]

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!

[serhiy-storchaka]

https://docs.python.org/3/faq/programming.html#why-are-default-values-shared-between-objects

[haoyang9804]

Thanks!