New class instance not initializing variables of type list

[ArturoInzunza]

BPO	35439
Nosy	@ammaraskar

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2018-12-08.00:43:21.083>
created_at = <Date 2018-12-08.00:26:46.907>
labels = ['type-bug', 'invalid']
title = 'New class instance not initializing variables of type list'
updated_at = <Date 2018-12-08.00:43:21.068>
user = 'https://bugs.python.org/ArturoInzunza'

bugs.python.org fields:

activity = <Date 2018-12-08.00:43:21.068>
actor = 'ammar2'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-08.00:43:21.083>
closer = 'ammar2'
components = []
creation = <Date 2018-12-08.00:26:46.907>
creator = 'Arturo Inzunza'
dependencies = []
files = []
hgrepos = []
issue_num = 35439
keywords = []
message_count = 2.0
messages = ['331370', '331371']
nosy_count = 2.0
nosy_names = ['ammar2', 'Arturo Inzunza']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35439'
versions = ['Python 3.6']

[ArturoInzunza]

List type variables in a class are not reset on new instances of the class.

Example:

class Klazz:
    lst = []
    def __init__(self, va):
        print(self.lst)
        self.lst.append(va)


k = Klazz(1)
[]      -> This is correct as the lst value is empty on class instantiation

k2 = Klazz(2)
[1]   -> This is wrong, a totally new instance of the class retains the value of a previous class instance lst variable

k3 = Klazz(3)
[1, 2]  -> And so on... new instances all share the same list

[ammaraskar]

This is expected behavior, take a look at this section in the tutorial on classes: https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables

Unlike say Java, member variables need to be initialized in the constructor.