fix UserDict.get, setdefault, update

[ping]

BPO	413171
Nosy	@gvanrossum, @loewis
Files	UserDict.patch

^{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 2001-06-18.01:10:09.000>
created_at = <Date 2001-04-02.17:18:02.000>
labels = ['library']
title = 'fix UserDict.get, setdefault, update'
updated_at = <Date 2001-06-18.01:10:09.000>
user = 'https://bugs.python.org/ping'

bugs.python.org fields:

activity = <Date 2001-06-18.01:10:09.000>
actor = 'loewis'
assignee = 'ping'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2001-04-02.17:18:02.000>
creator = 'ping'
dependencies = []
files = ['3242']
hgrepos = []
issue_num = 413171
keywords = ['patch']
message_count = 5.0
messages = ['36252', '36253', '36254', '36255', '36256']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'loewis', 'ping']
pr_nums = []
priority = 'low'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue413171'
versions = []

[ping]

The methods 'get', 'setdefault', and 'update'
on a dictionary are usually implemented (and
thought of) in terms of the lower-level methods
has_key, __getitem__, and __setitem__. The
current implementation of UserDict relays a
call to e.g. x.get() to x.data.get(), which
behaves inconsistently if __getitem__ has been
implemented on x.

One particular big place where this turns up is cgi.
If you get a dict = cgi.SvFormContentDict(), then
dict.get('key') will return a *list* even though
dict['key'] returns a single item!

To make UserDict behave consistently, this patch
fixes get(), update(), and setdefault() to re-use
the other methods. Then the only occurrence of
self.data[k] = v is in __setitem__, the only
occurrence of self.data[k] without assignment is
in __getitem__, etc.

[gvanrossum]

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Let's not fix this in 2.1.

[loewis]

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user_id=21627

I recommend to approve this patch.

[gvanrossum]

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Approved. Check it in already!

[loewis]

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user_id=21627

Committed as UserDict 1.14.