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_NOT_SET = object()
class Slot:
"""A descriptor that provides a slot.
This is useful for types that can't have slots via __slots__,
e.g. tuple subclasses.
"""
__slots__ = ('initial', 'default', 'readonly', 'instances', 'name')
def __init__(self, initial=_NOT_SET, *,
default=_NOT_SET,
readonly=False,
):
self.initial = initial
self.default = default
self.readonly = readonly
# The instance cache is not inherently tied to the normal
# lifetime of the instances. So must do something in order to
# avoid keeping the instances alive by holding a reference here.
# Ideally we would use weakref.WeakValueDictionary to do this.
# However, most builtin types do not support weakrefs. So
# instead we monkey-patch __del__ on the attached class to clear
# the instance.
self.instances = {}
self.name = None
def __set_name__(self, cls, name):
if self.name is not None:
raise TypeError('already used')
self.name = name
try:
slotnames = cls.__slot_names__
except AttributeError:
slotnames = cls.__slot_names__ = []
slotnames.append(name)
self._ensure___del__(cls, slotnames)
def __get__(self, obj, cls):
if obj is None: # called on the class
return self
try:
value = self.instances[id(obj)]
except KeyError:
if self.initial is _NOT_SET:
value = self.default
else:
value = self.initial
self.instances[id(obj)] = value
if value is _NOT_SET:
raise AttributeError(self.name)
# XXX Optionally make a copy?
return value
def __set__(self, obj, value):
if self.readonly:
raise AttributeError(f'{self.name} is readonly')
# XXX Optionally coerce?
self.instances[id(obj)] = value
def __delete__(self, obj):
if self.readonly:
raise AttributeError(f'{self.name} is readonly')
self.instances[id(obj)] = self.default # XXX refleak?
def _ensure___del__(self, cls, slotnames): # See the comment in __init__().
try:
old___del__ = cls.__del__
except AttributeError:
old___del__ = (lambda s: None)
else:
if getattr(old___del__, '_slotted', False):
return
def __del__(_self):
for name in slotnames:
delattr(_self, name)
old___del__(_self)
__del__._slotted = True
cls.__del__ = __del__
def set(self, obj, value):
"""Update the cached value for an object.
This works even if the descriptor is read-only. This is
particularly useful when initializing the object (e.g. in
its __new__ or __init__).
"""
self.instances[id(obj)] = value
class classonly:
"""A non-data descriptor that makes a value only visible on the class.
This is like the "classmethod" builtin, but does not show up on
instances of the class. It may be used as a decorator.
"""
def __init__(self, value):
self.value = value
self.getter = classmethod(value).__get__
self.name = None
def __set_name__(self, cls, name):
if self.name is not None:
raise TypeError('already used')
self.name = name
def __get__(self, obj, cls):
if obj is not None:
raise AttributeError(self.name)
# called on the class
return self.getter(None, cls)
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