Mutable object change flag

[rhettinger]

BPO	555251
Nosy	@loewis, @rhettinger
Files	cache.dif: Context diff for listobject.c and listobject.h

^{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 2002-10-07.21:32:31.000>
created_at = <Date 2002-05-13.00:22:27.000>
labels = ['interpreter-core']
title = 'Mutable object change flag'
updated_at = <Date 2002-10-07.21:32:31.000>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2002-10-07.21:32:31.000>
actor = 'loewis'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2002-05-13.00:22:27.000>
creator = 'rhettinger'
dependencies = []
files = ['4269']
hgrepos = []
issue_num = 555251
keywords = ['patch']
message_count = 2.0
messages = ['40017', '40018']
nosy_count = 2.0
nosy_names = ['loewis', 'rhettinger']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue555251'
versions = ['Python 2.3']

[rhettinger]

This patch is a proof-of-concept for marking state
changes in mutable objects. It is meant as a simpler,
faster alternative to a full built-in observer pattern.

Here's the whole protocol:
Mutable objects choosing to implement the protocol
provide a writable attribute, cachevalid, which is
initialized to zero. Upon any update to the object,
the attribute is reset to zero. Callers who set the
attribute to one can later check to see if an update
has occurred (as indicated by a zero).

>>> a = list('abcd')
>>> a.cachevalid
0
>>> a.cachevalid=1
>>> a.cachevalid
1
>>> a[2] = 'k'
>>> a.cachevalid
0

The proof-of-concept patch is implemented only for
lists. A production version would need to include
dictionaries.

[loewis]

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Raymond, are you still interested in this approach?