An array as attribute of an object is shared between instances

[pluginnieulq]

BPO	38370
Nosy	@pfmoore, @tjguk, @zware, @zooba

^{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 2019-10-04.11:56:58.415>
created_at = <Date 2019-10-04.11:47:21.646>
labels = ['type-bug', '3.7', 'OS-windows']
title = 'An array as attribute of an object is shared between instances'
updated_at = <Date 2019-10-04.12:08:41.227>
user = 'https://bugs.python.org/pluginnieulq'

bugs.python.org fields:

activity = <Date 2019-10-04.12:08:41.227>
actor = 'plugin nieulq'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-04.11:56:58.415>
closer = 'mark.dickinson'
components = ['Windows']
creation = <Date 2019-10-04.11:47:21.646>
creator = 'plugin nieulq'
dependencies = []
files = []
hgrepos = []
issue_num = 38370
keywords = []
message_count = 4.0
messages = ['353928', '353929', '353930', '353935']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'steve.dower', 'plugin nieulq']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38370'
versions = ['Python 3.7']

[pluginnieulq]

I am using this version on Windows :
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul 8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)].
When an attribute of an object is an array, the array is shared between instances as shown in my example.
When executed, the output generated by the script is:
Value1
Value2
Value3
Value4
--------------------------------
Value1
Value2
Value3
Value4

It should be :

Value1
Value2
--------------------------------
Value3
Value4

[pluginnieulq]

I am using this version on Windows :
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul 8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)].
When an attribute of an object is an array, the array is shared between instances as shown in my example.

class AClass(object):
    def __init__(self, val=[]):
        self.contents = val

    def add_content(self, newcontent):
        self.contents.append(newcontent)
        return self

    def print(self):
        for val in self.contents:
            print(str(val))


if __name__ == '__main__':
    sc = AClass()
    sc = sc.add_content("Value1").add_content("Value2")

    sb = AClass()
    sb = sb.add_content("Value3").add_content("Value4")

    sc.print()
    print("--------------------------------")
    sb.print()

When executed, the output generated by the script is:
Value1
Value2
Value3
Value4
--------------------------------
Value1
Value2
Value3
Value4

It should be :

Value1
Value2
--------------------------------
Value3
Value4

[mdickinson]

This is a common gotcha. Take a look at: https://docs.python.org/3/faq/programming.html#why-are-default-values-shared-between-objects

[pluginnieulq]

Ah ... Ok. My bad.
Thank you for pointing this out to me, Mark.
This is definitely a strange behavior.