text: ##############################################################################
  # 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.
  ##############################################################################
  """Implementation of interface declarations
  
  There are three flavors of declarations:
  
    - Declarations are used to simply name declared interfaces.
  
    - ImplementsDeclarations are used to express the interfaces that a
      class implements (that instances of the class provides).
  
      Implements specifications support inheriting interfaces.
  
    - ProvidesDeclarations are used to express interfaces directly
      provided by objects.
  
  """
  __docformat__ = 'restructuredtext'
  
  import sys
  from types import FunctionType
  from types import MethodType
  from types import ModuleType
  import weakref
  
  from zope.interface.advice import addClassAdvisor
  from zope.interface.interface import InterfaceClass
  from zope.interface.interface import SpecificationBase
  from zope.interface.interface import Specification
  from zope.interface._compat import CLASS_TYPES as DescriptorAwareMetaClasses
  from zope.interface._compat import PYTHON3
  from zope.interface._compat import _use_c_impl
  
  # Registry of class-implementation specifications
  BuiltinImplementationSpecifications = {}
  
  _ADVICE_ERROR = ('Class advice impossible in Python3.  '
                   'Use the @%s class decorator instead.')
  
  _ADVICE_WARNING = ('The %s API is deprecated, and will not work in Python3  '
                     'Use the @%s class decorator instead.')
  
  class named(object):
  
      def __init__(self, name):
          self.name = name
  
      def __call__(self, ob):
          ob.__component_name__ = self.name
          return ob
  
  
  class Declaration(Specification):
      """Interface declarations"""
  
      def __init__(self, *interfaces):
          Specification.__init__(self, _normalizeargs(interfaces))
  
      def changed(self, originally_changed):
          Specification.changed(self, originally_changed)
          try:
              del self._v_attrs
          except AttributeError:
              pass
  
      def __contains__(self, interface):
          """Test whether an interface is in the specification
          """
  
          return self.extends(interface) and interface in self.interfaces()
  
      def __iter__(self):
          """Return an iterator for the interfaces in the specification
          """
          return self.interfaces()
  
      def flattened(self):
          """Return an iterator of all included and extended interfaces
          """
          return iter(self.__iro__)
  
      def __sub__(self, other):
          """Remove interfaces from a specification
          """
          return Declaration(
              *[i for i in self.interfaces()
                if not [j for j in other.interfaces()
                        if i.extends(j, 0)]
              ]
          )
  
      def __add__(self, other):
          """Add two specifications or a specification and an interface
          """
          seen = {}
          result = []
          for i in self.interfaces():
              seen[i] = 1
              result.append(i)
          for i in other.interfaces():
              if i not in seen:
                  seen[i] = 1
                  result.append(i)
  
          return Declaration(*result)
  
      __radd__ = __add__
  
  
  ##############################################################################
  #
  # Implementation specifications
  #
  # These specify interfaces implemented by instances of classes
  
  class Implements(Declaration):
  
      # class whose specification should be used as additional base
      inherit = None
  
      # interfaces actually declared for a class
      declared = ()
  
      __name__ = '?'
  
      @classmethod
      def named(cls, name, *interfaces):
          # Implementation method: Produce an Implements interface with
          # a fully fleshed out __name__ before calling the constructor, which
          # sets bases to the given interfaces and which may pass this object to
          # other objects (e.g., to adjust dependents). If they're sorting or comparing
          # by name, this needs to be set.
          inst = cls.__new__(cls)
          inst.__name__ = name
          inst.__init__(*interfaces)
          return inst
  
      def __repr__(self):
          return '<implementedBy %s>' % (self.__name__)
  
      def __reduce__(self):
          return implementedBy, (self.inherit, )
  
      def __cmp(self, other):
          # Yes, I did mean to name this __cmp, rather than __cmp__.
          # It is a private method used by __lt__ and __gt__.
          # This is based on, and compatible with, InterfaceClass.
          # (The two must be mutually comparable to be able to work in e.g., BTrees.)
          # Instances of this class generally don't have a __module__ other than
          # `zope.interface.declarations`, whereas they *do* have a __name__ that is the
          # fully qualified name of the object they are representing.
  
          # Note, though, that equality and hashing are still identity based. This
          # accounts for things like nested objects that have the same name (typically
          # only in tests) and is consistent with pickling. As far as comparisons to InterfaceClass
          # goes, we'll never have equal name and module to those, so we're still consistent there.
          # Instances of this class are essentially intended to be unique and are
          # heavily cached (note how our __reduce__ handles this) so having identity
          # based hash and eq should also work.
          if other is None:
              return -1
  
          n1 = (self.__name__, self.__module__)
          n2 = (getattr(other, '__name__', ''), getattr(other, '__module__', ''))
  
          # This spelling works under Python3, which doesn't have cmp().
          return (n1 > n2) - (n1 < n2)
  
      def __hash__(self):
          return Declaration.__hash__(self)
  
      # We want equality to be based on identity. However, we can't actually
      # implement __eq__/__ne__ to do this because sometimes we get wrapped in a proxy.
      # We need to let the proxy types implement these methods so they can handle unwrapping
      # and then rely on: (1) the interpreter automatically changing `implements == proxy` into
      # `proxy == implements` (which will call proxy.__eq__ to do the unwrapping) and then
      # (2) the default equality semantics being identity based.
  
      def __lt__(self, other):
          c = self.__cmp(other)
          return c < 0
  
      def __le__(self, other):
          c = self.__cmp(other)
          return c <= 0
  
      def __gt__(self, other):
          c = self.__cmp(other)
          return c > 0
  
      def __ge__(self, other):
          c = self.__cmp(other)
          return c >= 0
  
  def _implements_name(ob):
      # Return the __name__ attribute to be used by its __implemented__
      # property.
      # This must be stable for the "same" object across processes
      # because it is used for sorting. It needn't be unique, though, in cases
      # like nested classes named Foo created by different functions, because
      # equality and hashing is still based on identity.
      # It might be nice to use __qualname__ on Python 3, but that would produce
      # different values between Py2 and Py3.
      return (getattr(ob, '__module__', '?') or '?') + \
          '.' + (getattr(ob, '__name__', '?') or '?')
  
  
  @_use_c_impl
  def implementedBy(cls):
      """Return the interfaces implemented for a class' instances
  
        The value returned is an `~zope.interface.interfaces.IDeclaration`.
      """
      try:
          spec = cls.__dict__.get('__implemented__')
      except AttributeError:
  
          # we can't get the class dict. This is probably due to a
          # security proxy.  If this is the case, then probably no
          # descriptor was installed for the class.
  
          # We don't want to depend directly on zope.security in
          # zope.interface, but we'll try to make reasonable
          # accommodations in an indirect way.
  
          # We'll check to see if there's an implements:
  
          spec = getattr(cls, '__implemented__', None)
          if spec is None:
              # There's no spec stred in the class. Maybe its a builtin:
              spec = BuiltinImplementationSpecifications.get(cls)
              if spec is not None:
                  return spec
              return _empty
  
          if spec.__class__ == Implements:
              # we defaulted to _empty or there was a spec. Good enough.
              # Return it.
              return spec
  
          # TODO: need old style __implements__ compatibility?
          # Hm, there's an __implemented__, but it's not a spec. Must be
          # an old-style declaration. Just compute a spec for it
          return Declaration(*_normalizeargs((spec, )))
  
      if isinstance(spec, Implements):
          return spec
  
      if spec is None:
          spec = BuiltinImplementationSpecifications.get(cls)
          if spec is not None:
              return spec
  
      # TODO: need old style __implements__ compatibility?
      spec_name = _implements_name(cls)
      if spec is not None:
          # old-style __implemented__ = foo declaration
          spec = (spec, ) # tuplefy, as it might be just an int
          spec = Implements.named(spec_name, *_normalizeargs(spec))
          spec.inherit = None    # old-style implies no inherit
          del cls.__implemented__ # get rid of the old-style declaration
      else:
          try:
              bases = cls.__bases__
          except AttributeError:
              if not callable(cls):
                  raise TypeError("ImplementedBy called for non-factory", cls)
              bases = ()
  
          spec = Implements.named(spec_name, *[implementedBy(c) for c in bases])
          spec.inherit = cls
  
      try:
          cls.__implemented__ = spec
          if not hasattr(cls, '__providedBy__'):
              cls.__providedBy__ = objectSpecificationDescriptor
  
          if (isinstance(cls, DescriptorAwareMetaClasses)
              and
              '__provides__' not in cls.__dict__):
              # Make sure we get a __provides__ descriptor
              cls.__provides__ = ClassProvides(
                  cls,
                  getattr(cls, '__class__', type(cls)),
                  )
  
      except TypeError:
          if not isinstance(cls, type):
              raise TypeError("ImplementedBy called for non-type", cls)
          BuiltinImplementationSpecifications[cls] = spec
  
      return spec
  
  
  def classImplementsOnly(cls, *interfaces):
      """Declare the only interfaces implemented by instances of a class
  
        The arguments after the class are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        The interfaces given (including the interfaces in the specifications)
        replace any previous declarations.
      """
      spec = implementedBy(cls)
      spec.declared = ()
      spec.inherit = None
      classImplements(cls, *interfaces)
  
  
  def classImplements(cls, *interfaces):
      """Declare additional interfaces implemented for instances of a class
  
        The arguments after the class are one or more interfaces or
        interface specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        The interfaces given (including the interfaces in the specifications)
        are added to any interfaces previously declared.
      """
      spec = implementedBy(cls)
      spec.declared += tuple(_normalizeargs(interfaces))
  
      # compute the bases
      bases = []
      seen = {}
      for b in spec.declared:
          if b not in seen:
              seen[b] = 1
              bases.append(b)
  
      if spec.inherit is not None:
  
          for c in spec.inherit.__bases__:
              b = implementedBy(c)
              if b not in seen:
                  seen[b] = 1
                  bases.append(b)
  
      spec.__bases__ = tuple(bases)
  
  def _implements_advice(cls):
      interfaces, classImplements = cls.__dict__['__implements_advice_data__']
      del cls.__implements_advice_data__
      classImplements(cls, *interfaces)
      return cls
  
  
  class implementer(object):
      """Declare the interfaces implemented by instances of a class.
  
        This function is called as a class decorator.
  
        The arguments are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        The interfaces given (including the interfaces in the
        specifications) are added to any interfaces previously
        declared.
  
        Previous declarations include declarations for base classes
        unless implementsOnly was used.
  
        This function is provided for convenience. It provides a more
        convenient way to call `classImplements`. For example::
  
          @implementer(I1)
          class C(object):
              pass
  
        is equivalent to calling::
  
          classImplements(C, I1)
  
        after the class has been created.
        """
  
      def __init__(self, *interfaces):
          self.interfaces = interfaces
  
      def __call__(self, ob):
          if isinstance(ob, DescriptorAwareMetaClasses):
              classImplements(ob, *self.interfaces)
              return ob
  
          spec_name = _implements_name(ob)
          spec = Implements.named(spec_name, *self.interfaces)
          try:
              ob.__implemented__ = spec
          except AttributeError:
              raise TypeError("Can't declare implements", ob)
          return ob
  
  class implementer_only(object):
      """Declare the only interfaces implemented by instances of a class
  
        This function is called as a class decorator.
  
        The arguments are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        Previous declarations including declarations for base classes
        are overridden.
  
        This function is provided for convenience. It provides a more
        convenient way to call `classImplementsOnly`. For example::
  
          @implementer_only(I1)
          class C(object): pass
  
        is equivalent to calling::
  
          classImplementsOnly(I1)
  
        after the class has been created.
        """
  
      def __init__(self, *interfaces):
          self.interfaces = interfaces
  
      def __call__(self, ob):
          if isinstance(ob, (FunctionType, MethodType)):
              # XXX Does this decorator make sense for anything but classes?
              # I don't think so. There can be no inheritance of interfaces
              # on a method pr function....
              raise ValueError('The implementer_only decorator is not '
                               'supported for methods or functions.')
          else:
              # Assume it's a class:
              classImplementsOnly(ob, *self.interfaces)
              return ob
  
  def _implements(name, interfaces, classImplements):
      # This entire approach is invalid under Py3K.  Don't even try to fix
      # the coverage for this block there. :(
      frame = sys._getframe(2)
      locals = frame.f_locals
  
      # Try to make sure we were called from a class def. In 2.2.0 we can't
      # check for __module__ since it doesn't seem to be added to the locals
      # until later on.
      if locals is frame.f_globals or '__module__' not in locals:
          raise TypeError(name+" can be used only from a class definition.")
  
      if '__implements_advice_data__' in locals:
          raise TypeError(name+" can be used only once in a class definition.")
  
      locals['__implements_advice_data__'] = interfaces, classImplements
      addClassAdvisor(_implements_advice, depth=3)
  
  def implements(*interfaces):
      """Declare interfaces implemented by instances of a class
  
        This function is called in a class definition.
  
        The arguments are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        The interfaces given (including the interfaces in the
        specifications) are added to any interfaces previously
        declared.
  
        Previous declarations include declarations for base classes
        unless `implementsOnly` was used.
  
        This function is provided for convenience. It provides a more
        convenient way to call `classImplements`. For example::
  
          implements(I1)
  
        is equivalent to calling::
  
          classImplements(C, I1)
  
        after the class has been created.
      """
      # This entire approach is invalid under Py3K.  Don't even try to fix
      # the coverage for this block there. :(
      if PYTHON3:
          raise TypeError(_ADVICE_ERROR % 'implementer')
      _implements("implements", interfaces, classImplements)
  
  def implementsOnly(*interfaces):
      """Declare the only interfaces implemented by instances of a class
  
        This function is called in a class definition.
  
        The arguments are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        Previous declarations including declarations for base classes
        are overridden.
  
        This function is provided for convenience. It provides a more
        convenient way to call `classImplementsOnly`. For example::
  
          implementsOnly(I1)
  
        is equivalent to calling::
  
          classImplementsOnly(I1)
  
        after the class has been created.
      """
      # This entire approach is invalid under Py3K.  Don't even try to fix
      # the coverage for this block there. :(
      if PYTHON3:
          raise TypeError(_ADVICE_ERROR % 'implementer_only')
      _implements("implementsOnly", interfaces, classImplementsOnly)
  
  ##############################################################################
  #
  # Instance declarations
  
  class Provides(Declaration):  # Really named ProvidesClass
      """Implement ``__provides__``, the instance-specific specification
  
      When an object is pickled, we pickle the interfaces that it implements.
      """
  
      def __init__(self, cls, *interfaces):
          self.__args = (cls, ) + interfaces
          self._cls = cls
          Declaration.__init__(self, *(interfaces + (implementedBy(cls), )))
  
      def __reduce__(self):
          return Provides, self.__args
  
      __module__ = 'zope.interface'
  
      def __get__(self, inst, cls):
          """Make sure that a class __provides__ doesn't leak to an instance
          """
          if inst is None and cls is self._cls:
              # We were accessed through a class, so we are the class'
              # provides spec. Just return this object, but only if we are
              # being called on the same class that we were defined for:
              return self
  
          raise AttributeError('__provides__')
  
  ProvidesClass = Provides
  
  # Registry of instance declarations
  # This is a memory optimization to allow objects to share specifications.
  InstanceDeclarations = weakref.WeakValueDictionary()
  
  def Provides(*interfaces):
      """Cache instance declarations
  
        Instance declarations are shared among instances that have the same
        declaration. The declarations are cached in a weak value dictionary.
      """
      spec = InstanceDeclarations.get(interfaces)
      if spec is None:
          spec = ProvidesClass(*interfaces)
          InstanceDeclarations[interfaces] = spec
  
      return spec
  
  Provides.__safe_for_unpickling__ = True
  
  
  def directlyProvides(object, *interfaces):
      """Declare interfaces declared directly for an object
  
        The arguments after the object are one or more interfaces or interface
        specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
        The interfaces given (including the interfaces in the specifications)
        replace interfaces previously declared for the object.
      """
      cls = getattr(object, '__class__', None)
      if cls is not None and getattr(cls, '__class__', None) is cls:
          # It's a meta class (well, at least it it could be an extension class)
          # Note that we can't get here from Py3k tests:  there is no normal
          # class which isn't descriptor aware.
          if not isinstance(object,
                            DescriptorAwareMetaClasses):
              raise TypeError("Attempt to make an interface declaration on a "
                              "non-descriptor-aware class")
  
      interfaces = _normalizeargs(interfaces)
      if cls is None:
          cls = type(object)
  
      issub = False
      for damc in DescriptorAwareMetaClasses:
          if issubclass(cls, damc):
              issub = True
              break
      if issub:
          # we have a class or type.  We'll use a special descriptor
          # that provides some extra caching
          object.__provides__ = ClassProvides(object, cls, *interfaces)
      else:
          object.__provides__ = Provides(cls, *interfaces)
  
  
  def alsoProvides(object, *interfaces):
      """Declare interfaces declared directly for an object
  
      The arguments after the object are one or more interfaces or interface
      specifications (`~zope.interface.interfaces.IDeclaration` objects).
  
      The interfaces given (including the interfaces in the specifications) are
      added to the interfaces previously declared for the object.
      """
      directlyProvides(object, directlyProvidedBy(object), *interfaces)
  
  
  def noLongerProvides(object, interface):
      """ Removes a directly provided interface from an object.
      """
      directlyProvides(object, directlyProvidedBy(object) - interface)
      if interface.providedBy(object):
          raise ValueError("Can only remove directly provided interfaces.")
  
  
  @_use_c_impl
  class ClassProvidesBase(object):
      # In C, this extends SpecificationBase, so its kind of weird here that it
      # doesn't.
  
      def __get__(self, inst, cls):
          if cls is self._cls:
              # We only work if called on the class we were defined for
  
              if inst is None:
                  # We were accessed through a class, so we are the class'
                  # provides spec. Just return this object as is:
                  return self
  
              return self._implements
  
          raise AttributeError('__provides__')
  
  
  class ClassProvides(Declaration, ClassProvidesBase):
      """Special descriptor for class ``__provides__``
  
      The descriptor caches the implementedBy info, so that
      we can get declarations for objects without instance-specific
      interfaces a bit quicker.
      """
  
      def __init__(self, cls, metacls, *interfaces):
          self._cls = cls
          self._implements = implementedBy(cls)
          self.__args = (cls, metacls, ) + interfaces
          Declaration.__init__(self, *(interfaces + (implementedBy(metacls), )))
  
      def __reduce__(self):
          return self.__class__, self.__args
  
      # Copy base-class method for speed
      __get__ = ClassProvidesBase.__get__
  
  
  def directlyProvidedBy(object):
      """Return the interfaces directly provided by the given object
  
      The value returned is an `~zope.interface.interfaces.IDeclaration`.
      """
      provides = getattr(object, "__provides__", None)
      if (provides is None # no spec
          or
          # We might have gotten the implements spec, as an
          # optimization. If so, it's like having only one base, that we
          # lop off to exclude class-supplied declarations:
          isinstance(provides, Implements)
      ):
          return _empty
  
      # Strip off the class part of the spec:
      return Declaration(provides.__bases__[:-1])
  
  
  def classProvides(*interfaces):
      """Declare interfaces provided directly by a class
  
        This function is called in a class definition.
  
        The arguments are one or more interfaces or interface specifications
        (`~zope.interface.interfaces.IDeclaration` objects).
  
        The given interfaces (including the interfaces in the specifications)
        are used to create the class's direct-object interface specification.
        An error will be raised if the module class has an direct interface
        specification. In other words, it is an error to call this function more
        than once in a class definition.
  
        Note that the given interfaces have nothing to do with the interfaces
        implemented by instances of the class.
  
        This function is provided for convenience. It provides a more convenient
        way to call `directlyProvides` for a class. For example::
  
          classProvides(I1)
  
        is equivalent to calling::
  
          directlyProvides(theclass, I1)
  
        after the class has been created.
      """
      # This entire approach is invalid under Py3K.  Don't even try to fix
      # the coverage for this block there. :(
  
      if PYTHON3:
          raise TypeError(_ADVICE_ERROR % 'provider')
  
      frame = sys._getframe(1)
      locals = frame.f_locals
  
      # Try to make sure we were called from a class def
      if (locals is frame.f_globals) or ('__module__' not in locals):
          raise TypeError("classProvides can be used only from a "
                          "class definition.")
  
      if '__provides__' in locals:
          raise TypeError(
              "classProvides can only be used once in a class definition.")
  
      locals["__provides__"] = _normalizeargs(interfaces)
  
      addClassAdvisor(_classProvides_advice, depth=2)
  
  def _classProvides_advice(cls):
      # This entire approach is invalid under Py3K.  Don't even try to fix
      # the coverage for this block there. :(
      interfaces = cls.__dict__['__provides__']
      del cls.__provides__
      directlyProvides(cls, *interfaces)
      return cls
  
  
  class provider(object):
      """Class decorator version of classProvides"""
  
      def __init__(self, *interfaces):
          self.interfaces = interfaces
  
      def __call__(self, ob):
          directlyProvides(ob, *self.interfaces)
          return ob
  
  
  def moduleProvides(*interfaces):
      """Declare interfaces provided by a module
  
      This function is used in a module definition.
  
      The arguments are one or more interfaces or interface specifications
      (`~zope.interface.interfaces.IDeclaration` objects).
  
      The given interfaces (including the interfaces in the specifications) are
      used to create the module's direct-object interface specification.  An
      error will be raised if the module already has an interface specification.
      In other words, it is an error to call this function more than once in a
      module definition.
  
      This function is provided for convenience. It provides a more convenient
      way to call directlyProvides. For example::
  
        moduleImplements(I1)
  
      is equivalent to::
  
        directlyProvides(sys.modules[__name__], I1)
      """
      frame = sys._getframe(1)
      locals = frame.f_locals
  
      # Try to make sure we were called from a class def
      if (locals is not frame.f_globals) or ('__name__' not in locals):
          raise TypeError(
              "moduleProvides can only be used from a module definition.")
  
      if '__provides__' in locals:
          raise TypeError(
              "moduleProvides can only be used once in a module definition.")
  
      locals["__provides__"] = Provides(ModuleType,
                                        *_normalizeargs(interfaces))
  
  
  ##############################################################################
  #
  # Declaration querying support
  
  # XXX:  is this a fossil?  Nobody calls it, no unit tests exercise it, no
  #       doctests import it, and the package __init__ doesn't import it.
  def ObjectSpecification(direct, cls):
      """Provide object specifications
  
      These combine information for the object and for it's classes.
      """
      return Provides(cls, direct) # pragma: no cover fossil
  
  @_use_c_impl
  def getObjectSpecification(ob):
  
      provides = getattr(ob, '__provides__', None)
      if provides is not None:
          if isinstance(provides, SpecificationBase):
              return provides
  
      try:
          cls = ob.__class__
      except AttributeError:
          # We can't get the class, so just consider provides
          return _empty
  
      return implementedBy(cls)
  
  
  @_use_c_impl
  def providedBy(ob):
  
      # Here we have either a special object, an old-style declaration
      # or a descriptor
  
      # Try to get __providedBy__
      try:
          r = ob.__providedBy__
      except AttributeError:
          # Not set yet. Fall back to lower-level thing that computes it
          return getObjectSpecification(ob)
  
      try:
          # We might have gotten a descriptor from an instance of a
          # class (like an ExtensionClass) that doesn't support
          # descriptors.  We'll make sure we got one by trying to get
          # the only attribute, which all specs have.
          r.extends
      except AttributeError:
  
          # The object's class doesn't understand descriptors.
          # Sigh. We need to get an object descriptor, but we have to be
          # careful.  We want to use the instance's __provides__, if
          # there is one, but only if it didn't come from the class.
  
          try:
              r = ob.__provides__
          except AttributeError:
              # No __provides__, so just fall back to implementedBy
              return implementedBy(ob.__class__)
  
          # We need to make sure we got the __provides__ from the
          # instance. We'll do this by making sure we don't get the same
          # thing from the class:
  
          try:
              cp = ob.__class__.__provides__
          except AttributeError:
              # The ob doesn't have a class or the class has no
              # provides, assume we're done:
              return r
  
          if r is cp:
              # Oops, we got the provides from the class. This means
              # the object doesn't have it's own. We should use implementedBy
              return implementedBy(ob.__class__)
  
      return r
  
  
  @_use_c_impl
  class ObjectSpecificationDescriptor(object):
      """Implement the `__providedBy__` attribute
  
      The `__providedBy__` attribute computes the interfaces peovided by
      an object.
      """
  
      def __get__(self, inst, cls):
          """Get an object specification for an object
          """
          if inst is None:
              return getObjectSpecification(cls)
  
          provides = getattr(inst, '__provides__', None)
          if provides is not None:
              return provides
  
          return implementedBy(cls)
  
  
  ##############################################################################
  
  def _normalizeargs(sequence, output=None):
      """Normalize declaration arguments
  
      Normalization arguments might contain Declarions, tuples, or single
      interfaces.
  
      Anything but individial interfaces or implements specs will be expanded.
      """
      if output is None:
          output = []
  
      cls = sequence.__class__
      if InterfaceClass in cls.__mro__ or Implements in cls.__mro__:
          output.append(sequence)
      else:
          for v in sequence:
              _normalizeargs(v, output)
  
      return output
  
  _empty = Declaration()
  
  objectSpecificationDescriptor = ObjectSpecificationDescriptor()
  