Length.py

##############################################################################
#
# Copyright (c) 2001, 2002 Zope Corporation and Contributors.
# All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE
#
##############################################################################

import persistent

class Length(persistent.Persistent):
    """BTree lengths are too expensive to compute

    Objects that use BTrees need to keep track of lengths themselves.
    This class provides an object for doing this.

    As a bonus, the object support application-level conflict
    resolution.

    It is tempting to to assign length objects to __len__ attributes
    to provide instance-specific __len__ methods. However, this no
    longer works as expected, because new-style classes cache
    class-defined slot methods (like __len__) in C type slots.  Thus,
    instance-define slot fillers are ignores.

    """

    def __init__(self, v=0):
        self.value = v

    def __getstate__(self):
        return self.value

    def __setstate__(self, v):
        self.value = v

    def set(self, v):
        self.value = v

    def _p_resolveConflict(self, old, s1, s2):
        return s1 + s2 - old

    def _p_independent(self):
        # My state doesn't depend on or materially effect the state of
        # other objects.
        return 1

    def change(self, delta):
        self.value += delta

    def __call__(self, *args):
        return self.value