Zope Subversion Repository

Zope: zope.security/trunk/src/zope/security/decorator.py

Fixed circular import beween ``zope.security.decorator`` and ``zope.security.proxy`` by moving the code leading to the ``ImportError`` from proxy to decorator

##############################################################################
#
# Copyright (c) 2003 Zope Foundation 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.
#
##############################################################################
"""Decorator support

Decorators are proxies that are mostly transparent but that may provide
additional features.
"""
__docformat__ = "reStructuredText"

from zope.interface.declarations import ObjectSpecification
from zope.proxy import getProxiedObject, ProxyBase
from zope.proxy.decorator import SpecificationDecoratorBase
from zope.security.checker import selectChecker, CombinedChecker
from zope.security.proxy import Proxy
from zope.security.proxy import getChecker


class DecoratedSecurityCheckerDescriptor(object):
    """Descriptor for a Decorator that provides a decorated security checker.

    To illustrate, we'll create a class that will be proxied:

      >>> class Foo(object):
      ...     a = 'a'

    and a class to proxy it that uses a decorated security checker:

      >>> class Wrapper(ProxyBase):
      ...     b = 'b'
      ...     __Security_checker__ = DecoratedSecurityCheckerDescriptor()

    Next we'll create and register a checker for `Foo`:

      >>> from zope.security.checker import NamesChecker, defineChecker
      >>> fooChecker = NamesChecker(['a'])
      >>> defineChecker(Foo, fooChecker)

    along with a checker for `Wrapper`:

      >>> wrapperChecker = NamesChecker(['b'])
      >>> defineChecker(Wrapper, wrapperChecker)

    Using `selectChecker()`, we can confirm that a `Foo` object uses
    `fooChecker`:

      >>> foo = Foo()
      >>> selectChecker(foo) is fooChecker
      True
      >>> fooChecker.check(foo, 'a')
      >>> fooChecker.check(foo, 'b')  # doctest: +ELLIPSIS
      Traceback (most recent call last):
      ForbiddenAttribute: ('b', <zope.security.decorator.Foo object ...>)

    and that a `Wrapper` object uses `wrappeChecker`:

      >>> wrapper = Wrapper(foo)
      >>> selectChecker(wrapper) is wrapperChecker
      True
      >>> wrapperChecker.check(wrapper, 'b')
      >>> wrapperChecker.check(wrapper, 'a')  # doctest: +ELLIPSIS
      Traceback (most recent call last):
      ForbiddenAttribute: ('a', <zope.security.decorator.Foo object ...>)

    (Note that the object description says `Foo` because the object is a
    proxy and generally looks and acts like the object it's proxying.)

    When we access wrapper's ``__Security_checker__`` attribute, we invoke
    the decorated security checker descriptor. The decorator's job is to make
    sure checkers from both objects are used when available. In this case,
    because both objects have checkers, we get a combined checker:

      >>> checker = wrapper.__Security_checker__
      >>> type(checker)
      <class 'zope.security.checker.CombinedChecker'>
      >>> checker.check(wrapper, 'a')
      >>> checker.check(wrapper, 'b')

    The decorator checker will work even with security proxied objects. To
    illustrate, we'll proxify `foo`:

      >>> from zope.security.proxy import ProxyFactory
      >>> secure_foo = ProxyFactory(foo)
      >>> secure_foo.a
      'a'
      >>> secure_foo.b  # doctest: +ELLIPSIS
      Traceback (most recent call last):
      ForbiddenAttribute: ('b', <zope.security.decorator.Foo object ...>)

    when we wrap the secured `foo`:

      >>> wrapper = Wrapper(secure_foo)

    we still get a combined checker:

      >>> checker = wrapper.__Security_checker__
      >>> type(checker)
      <class 'zope.security.checker.CombinedChecker'>
      >>> checker.check(wrapper, 'a')
      >>> checker.check(wrapper, 'b')

    The decorator checker has three other scenarios:

      - the wrapper has a checker but the proxied object doesn't
      - the proxied object has a checker but the wrapper doesn't
      - neither the wrapper nor the proxied object have checkers

    When the wrapper has a checker but the proxied object doesn't:

      >>> from zope.security.checker import NoProxy, _checkers
      >>> del _checkers[Foo]
      >>> defineChecker(Foo, NoProxy)
      >>> selectChecker(foo) is None
      True
      >>> selectChecker(wrapper) is wrapperChecker
      True

    the decorator uses only the wrapper checker:

      >>> wrapper = Wrapper(foo)
      >>> wrapper.__Security_checker__ is wrapperChecker
      True

    When the proxied object has a checker but the wrapper doesn't:

      >>> del _checkers[Wrapper]
      >>> defineChecker(Wrapper, NoProxy)
      >>> selectChecker(wrapper) is None
      True
      >>> del _checkers[Foo]
      >>> defineChecker(Foo, fooChecker)
      >>> selectChecker(foo) is fooChecker
      True

    the decorator uses only the proxied object checker:

      >>> wrapper.__Security_checker__ is fooChecker
      True

    Finally, if neither the wrapper not the proxied have checkers:

      >>> del _checkers[Foo]
      >>> defineChecker(Foo, NoProxy)
      >>> selectChecker(foo) is None
      True
      >>> selectChecker(wrapper) is None
      True

    the decorator doesn't have a checker:

      >>> wrapper.__Security_checker__
      Traceback (most recent call last):
        ...
      AttributeError: 'Foo' has no attribute '__Security_checker__'

    __Security_checker__ cannot be None, otherwise Checker.proxy blows
    up:

      >>> checker.proxy(wrapper) is wrapper
      True

    """
    def __get__(self, inst, cls=None):
        if inst is None:
            return self
        else:
            proxied_object = getProxiedObject(inst)
            if type(proxied_object) is Proxy:
                checker = getChecker(proxied_object)
            else:
                checker = getattr(proxied_object, '__Security_checker__', None)
                if checker is None:
                    checker = selectChecker(proxied_object)
            wrapper_checker = selectChecker(inst)
            if wrapper_checker is None and checker is None:
                raise AttributeError("%r has no attribute %r" %
                                     (proxied_object.__class__.__name__,
                                      '__Security_checker__'))
            elif wrapper_checker is None:
                return checker
            elif checker is None:
                return wrapper_checker
            else:
                return CombinedChecker(wrapper_checker, checker)

    def __set__(self, inst, value):
        raise TypeError("Can't set __Security_checker__ on a decorated object")


class SecurityCheckerDecoratorBase(ProxyBase):
    """Base class for a proxy that provides additional security declarations."""

    __Security_checker__ = DecoratedSecurityCheckerDescriptor()


class DecoratorBase(SpecificationDecoratorBase, SecurityCheckerDecoratorBase):
    """Base class for a proxy that provides both additional interfaces and
    security declarations."""


# zope.location was made independent of security. To work together with
# security, we re-inject the DecoratedSecurityCheckerDescriptor onto the
# location proxy from here.
# This is the only sane place we found for doing it: it kicks in as soon
# as someone starts using security proxies.
import zope.location.location
zope.location.location.LocationProxy.__Security_checker__ = (
    DecoratedSecurityCheckerDescriptor())

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