Instance remembers old values from former life

[warnhold]

BPO	2561
Nosy	@birkenfeld
Files	class_with_listparamdefault.py

^{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 2008-04-06.08:30:51.074>
created_at = <Date 2008-04-06.07:31:58.201>
labels = ['type-bug']
title = 'Instance remembers old values from former life'
updated_at = <Date 2008-04-06.08:30:51.037>
user = 'https://bugs.python.org/warnhold'

bugs.python.org fields:

activity = <Date 2008-04-06.08:30:51.037>
actor = 'georg.brandl'
assignee = 'none'
closed = True
closed_date = <Date 2008-04-06.08:30:51.074>
closer = 'georg.brandl'
components = []
creation = <Date 2008-04-06.07:31:58.201>
creator = 'warnhold'
dependencies = []
files = ['9959']
hgrepos = []
issue_num = 2561
keywords = []
message_count = 2.0
messages = ['65024', '65025']
nosy_count = 2.0
nosy_names = ['georg.brandl', 'warnhold']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2561'
versions = ['Python 2.5']

[warnhold]

I don't know if it is a bug or a feature but the result seems to be
wrong for me: A constructor argument remembers its values from the last call

[birkenfeld]

Yes, this is expected. Default values are only evaluated once, at
function creation time. If you need to have a mutable default argument
that is assigned to something and changed, use something like this:

def f(arg=None):
    if arg is None:
        arg = []
    ...