readProxy.py

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#!/usr/bin/env python

# 
# LSST Data Management System
# Copyright 2008, 2009, 2010 LSST Corporation.
# 
# This product includes software developed by the
# LSST Project (http://www.lsst.org/).
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
# 
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# 
# You should have received a copy of the LSST License Statement and 
# the GNU General Public License along with this program.  If not, 
# see <http://www.lsstcorp.org/LegalNotices/>.
#

# -*- python -*-

"""This module defines the ReadProxy class."""

class ReadProxy(object):
    """ReadProxy provides a lazy-loading object that is initialized by a
    callback function set in ReadProxy's constructor.  Adapted from
    peak.util.proxies.LazyProxy, which was written by Phillip J. Eby
    (peak@eby-sarna.com)."""

    __slots__ = ('__cache__', '__callback__', '__subject__')

    def __init__(self, func):
        set_callback(self, func)

    def __getattribute__(self, attr, oga=object.__getattribute__):
        subject = oga(self, '__subject__')
        if attr == '__subject__':
            return subject
        return getattr(subject, attr)

    def __setattr__(self, attr, val):
        setattr(self.__subject__, attr, val)

    def __delattr__(self, attr):
        delattr(self.__subject__, attr)

    def __nonzero__(self):
        return bool(self.__subject__)

    def __getitem__(self, arg):
        return self.__subject__[arg]

    def __setitem__(self, arg, val):
        self.__subject__[arg] = val

    def __delitem__(self, arg):
        del self.__subject__[arg]

    def __getslice__(self, i, j):
        return self.__subject__[i:j]

    def __setslice__(self, i, j, val):
        self.__subject__[i:j] = val

    def __delslice__(self, i, j):
        del self.__subject__[i:j]

    def __contains__(self, ob):
        return ob in self.__subject__

    for name in 'repr str hash len abs complex int long float iter oct hex'.split():
        exec "def __%s__(self): return %s(self.__subject__)" % (name, name)

    for name in 'cmp', 'coerce', 'divmod':
        exec "def __%s__(self, ob): return %s(self.__subject__, ob)" % (name,
                name)

    for name, op in [
        ('lt', '<'), ('gt', '>'), ('le', '<='), ('ge', '>='),
        ('eq', '=='), ('ne', '!=')
        ]:
        exec "def __%s__(self, ob): return self.__subject__ %s ob" % (name, op)

    for name, op in [('neg', '-'), ('pos', '+'), ('invert', '~')]:
        exec "def __%s__(self): return %s self.__subject__" % (name, op)

    for name, op in [
        ('or','|'),  ('and','&'), ('xor','^'),
        ('lshift','<<'), ('rshift','>>'),
        ('add','+'), ('sub','-'), ('mul','*'), ('div','/'),
        ('mod','%'), ('truediv','/'), ('floordiv','//')
        ]:
        exec (
        "def __%(name)s__(self,ob):\n"
        "    return self.__subject__ %(op)s ob\n"
        "\n"
        "def __r%(name)s__(self,ob):\n"
        "    return ob %(op)s self.__subject__\n"
        "\n"
        "def __i%(name)s__(self,ob):\n"
        "    self.__subject__ %(op)s=ob\n"
        "    return self\n"
        ) % locals()

    del name, op

    # Oddball signatures

    def __rdivmod__(self, ob):
        return divmod(ob, self.__subject__)

    def __pow__(self, *args):
        return pow(self.__subject__, *args)

    def __ipow__(self,ob):
        self.__subject__ **= ob
        return self

    def __rpow__(self,ob):
        return pow(ob, self.__subject__)

get_callback = ReadProxy.__callback__.__get__
set_callback = ReadProxy.__callback__.__set__
get_cache = ReadProxy.__cache__.__get__
set_cache = ReadProxy.__cache__.__set__

def __subject__(self, get_cache=get_cache, set_cache=set_cache):
    try:
        return get_cache(self)
    except AttributeError:
        set_cache(self, get_callback(self)())
        return get_cache(self)

ReadProxy.__subject__ = property(__subject__, set_cache)
del __subject__