ConfigParser.items returns items present in `DEFAULTSECT` when section argument given.

[chrBrd]

BPO	33333
Nosy	@ambv, @chrBrd
PRs	bpo-33251: Prevent ConfigParser.items returning items present in vars. #6446 bpo-33333 #6567

^{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 = None
created_at = <Date 2018-04-22.11:20:36.565>
labels = ['type-bug', '3.8', 'docs']
title = 'ConfigParser.items returns items present in `DEFAULTSECT` when section argument given.'
updated_at = <Date 2018-04-22.22:05:28.728>
user = 'https://github.com/chrBrd'

bugs.python.org fields:

activity = <Date 2018-04-22.22:05:28.728>
actor = 'chrBrd'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-04-22.11:20:36.565>
creator = 'chrBrd'
dependencies = []
files = []
hgrepos = []
issue_num = 33333
keywords = ['patch']
message_count = 4.0
messages = ['315608', '315625', '315629', '315630']
nosy_count = 3.0
nosy_names = ['docs@python', 'lukasz.langa', 'chrBrd']
pr_nums = ['6446', '6567']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33333'
versions = ['Python 3.8']

[chrBrd]

According to ConfigParser.items docs:

When section is not given, return a list of section_name, section_proxy pairs, including DEFAULTSECT.

Otherwise, return a list of name, value pairs for the options in the given section. Optional arguments have the same meaning as for the get() method.

https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.items

However due to d = self._defaults.copy() on line 843 of configparser.py the default section is always included, even when a section argument is specified.

[chrBrd]

The fix for the actual issue is quite trivial (and should fix bpo-33251 at the same time). However ExtendedInterpolation::_interpolate_some relies on the options from the default section always being returned.

Not only does this make the fix more complicated, it also makes me worry that aligning behaviour to match the documentation (which I personally feel makes more sense) will breake backwards compatibility.

[ambv]

See line 379 in configparser.py. BasicInterpolation is also assumed to work for values from the default section. This is why the default section exists in the first place.

More importantly, as you're pointing out, even though this is an omission of the documentation, existing code in the wild probably assumes that items() include pairs from the default section. This is how ConfigParser worked in Python 2. If you don't want this behavior, use cfgparser[section].items() instead which does what you want.

If you think this is helpful, go ahead and extend the documentation of ConfigParser.items() to point out this weirdness. But we won't be going forward with PR 6567.

[chrBrd]

I'll make the documentation changes instead, not overly surprised to hear this is intentional.