While you're at it, it would be nice to fix this ugly asymmetry I found in
descriptors. It seems that descriptor's __get__ is called even when
accessed from a class rather than instance, but __set__ is only invoked from
instances, never from classes:
class Descr(object):
def __get__(self, obj, objtype):
print "__get__ from instance %s, type %s" % (obj, type)
return "foo"
def __set__(self, obj, value):
print "__set__ on instance %s, value %s" % (obj, value)
class Foo(object):
foo = Descr()
print Foo.foo # works
## doesn't work, goes directly to the class dict, not calling __set__
Foo.foo = 123
Because of this problem, I may have to install properties into a class's
metaclass achieve the same effect that I expected to achieve with a simple
descriptor :-(
On 10/06/07, Aahz <aahz at pythoncraft.com> wrote:
>
> On Sun, Jun 10, 2007, Eyal Lotem wrote:
> >
> > Python, probably through the valid assumption that most attribute
> > lookups go to the class, tries to look for the attribute in the class
> > first, and in the instance, second.
> >
> > What Python currently does is quite peculiar!
> > Here's a short description o PyObject_GenericGetAttr:
> >
> > A. Python looks for a descriptor in the _entire_ mro hierarchy
> > (len(mro) class/type check and dict lookups).
> > B. If Python found a descriptor and it has both get and set functions
> > - it uses it to get the value and returns, skipping the next stage.
> > C. If Python either did not find a descriptor, or found one that has
> > no setter, it will try a lookup in the instance dict.
> > D. If Python failed to find it in the instance, it will use the
> > descriptor's getter, and if it has no getter it will use the
> > descriptor itself.
>
> Guido, Ping, and I tried working on this at the sprint for PyCon 2003.
> We were unable to find any solution that did not affect critical-path
> timing. As other people have noted, the current semantics cannot be
> changed. I'll also echo other people and suggest that this discusion be
> moved to python-ideas if you want to continue pushing for a change in
> semantics.
>
> I just did a Google for my notes from PyCon 2003 and it appears that I
> never sent them out (probably because they aren't particularly
> comprehensible). Here they are for the record (from 3/25/2003):
>
> '''
> CACHE_ATTR is the name used to describe a speedup (for new-style classes
> only) in attribute lookup by caching the location of attributes in the
> MRO. Some of the non-obvious bits of code:
>
> * If a new-style class has any classic classes in its bases, we
> can't do attribute caching (we need to weakrefs to the derived
> classes).
>
> * If searching the MRO for an attribute discovers a data descriptor (has
> tp_descr_set), that overrides any attribute that might be in the instance;
> however, the existence of tp_descr_get still permits the instance to
> override its bases (but tp_descr_get is called if there is no instance
> attribute).
>
> * We need to invalidate the cache for the updated attribute in all derived
> classes in the following cases:
>
> * an attribute is added or deleted to the class or its base classes
>
> * an attribute has its status changed to or from being a data
> descriptor
>
> This file uses Python pseudocode to describe changes necessary to
> implement CACHE_ATTR at the C level. Except for class Meta, these are
> all exact descriptions of the work being done. Except for class Meta the
> changes go into object.c (Meta goes into typeobject.c). The pseudocode
> looks somewhat C-like to ease the transformation.
> '''
>
> NULL = object()
>
> def getattr(inst, name):
> isdata, where = lookup(inst.__class__, name)
> if isdata:
> descr = where[name]
> if hasattr(descr, "__get__"):
> return descr.__get__(inst)
> else:
> return descr
> value = inst.__dict__.get(name, NULL)
> if value != NULL:
> return value
> if where == NULL:
> raise AttributError
> descr = where[name]
> if hasattr(descr, "__get__"):
> value = descr.__get__(inst)
> else:
> value = descr
> return value
>
> def setattr(inst, name, value):
> isdata, where = lookup(inst.__class__, name)
> if isdata:
> descr = where[name]
> descr.__set__(inst, value)
> return
> inst.__dict__[name] = value
>
> def lookup(cls, name):
> if cls.__cache__ != NULL:
> pair = cls.__cache__.get(name)
> else:
> pair = NULL
> if pair:
> return pair
> else:
> for c in cls.__mro__:
> where = c.__dict__
> if name in where:
> descr = where[name]
> isdata = hasattr(descr, "__set__")
> pair = isdata, where
> break
> else:
> pair = False, NULL
> if cls.__cache__ != NULL:
> cls.__cache__[name] = pair
> return pair
>
>
> '''
> These changes go into typeobject.c; they are not a complete
> description of what happens during creation/updates, only the
> changes necessary to implement CACHE_ATTRO.
> '''
>
> from types import ClassType
>
> class Meta(type):
> def _invalidate(cls, name):
> if name in cls.__cache__:
> del cls.__cache__[name]
> for c in cls.__subclasses__():
> if name not in c.__dict__:
> self._invalidate(c, name)
> def _build_cache(cls, bases):
> for base in bases:
> if type(base.__class__) is ClassType:
> cls.__cache__ = NULL
> break
> else:
> cls.__cache__ = {}
> def __new__ (cls, bases):
> self._build_cache(cls, bases)
> def __setbases__(cls, bases):
> self._build_cache(cls, bases)
> def __setattr__(cls, name, value):
> if cls.__cache__ != NULL:
> old = cls.__dict__.get(name, NULL)
> wasdata = old != NULL and hasattr(old, "__set__")
> isdata = value != NULL and hasattr(value, "__set__")
> if wasdata != isdata or (old == NULL) != (value === NULL):
> self._invalidate(cls, name)
> type.__setattr__(cls, name, value)
> def __delattr__(cls, name):
> self.__setattr__(cls, name, NULL)
> --
> Aahz (aahz at pythoncraft.com) <*>
> http://www.pythoncraft.com/
>
> "as long as we like the same operating system, things are cool." --piranha
> _______________________________________________
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> Unsubscribe:
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>
--
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert
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