configparser bug: section is emptied if you assign a section to itself

[simonltwick]

BPO	32108
Nosy	@ambv, @serhiy-storchaka, @corona10
PRs	bpo-32108: Fix configparser when a section is assigned to itself. #4607 bpo-32108: Don't clear configparser values if key is assigned to itself #7588

^{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 2019-02-17.01:34:39.553>
created_at = <Date 2017-11-21.17:46:10.957>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'configparser bug: section is emptied if you assign a section to itself'
updated_at = <Date 2019-02-17.01:34:39.553>
user = 'https://bugs.python.org/simonltwick'

bugs.python.org fields:

activity = <Date 2019-02-17.01:34:39.553>
actor = 'cheryl.sabella'
assignee = 'none'
closed = True
closed_date = <Date 2019-02-17.01:34:39.553>
closer = 'cheryl.sabella'
components = ['Library (Lib)']
creation = <Date 2017-11-21.17:46:10.957>
creator = 'simonltwick'
dependencies = []
files = []
hgrepos = []
issue_num = 32108
keywords = ['patch']
message_count = 5.0
messages = ['306675', '306682', '307131', '315425', '319400']
nosy_count = 4.0
nosy_names = ['lukasz.langa', 'serhiy.storchaka', 'simonltwick', 'corona10']
pr_nums = ['4607', '7588']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32108'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[simonltwick]

If you assign a ConfigParser section back to the parent ConfigParser object (say after updating the section), the section is emptied.

(I realise now that you don't need to assign the section back to the parent as it's a proxy for the section in the parent already - but still it does not behave as you would expect):
code:
from configparser import ConfigParser
config = ConfigParser()
config['test'] = {'key': 'value'}
section = config['test']
section['key'] = 'different'
print("before: config['test'] is %s" % dict(config['test']))
config['test'] = section
print("after: config['test'] is %s" % dict(config['test']))
# the section is now printed as {}

[ambv]

Confirmed.

[serhiy-storchaka]

PR 4607 doesn't look a correct solution to me. I don't know a precedence of calling deepcopy() for a value in __setitem__(). deepcopy() is too heavy, calling it can slow down normal cases. Using deepcopy likely means a bad design.

[ambv]

I agree, the fix needs to be changed. What we probably want is to discover this kind of assignment and special-case *that*.

[ambv]

New changeset 33cd058 by Łukasz Langa (Cheryl Sabella) in branch 'master':
bpo-32108: Don't clear configparser values if key is assigned to itself (GH-7588)
33cd058