[Python-checkins] Descriptor HowTo: Improve the fidelity of the member object simulation (GH-23475) (GH-23479)

Mon Nov 23 14:32:05 EST 2020

https://github.com/python/cpython/commit/2019e4ff983dae8c17b616941b7a6b3a077278a6
commit: 2019e4ff983dae8c17b616941b7a6b3a077278a6
branch: 3.9
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: rhettinger <rhettinger at users.noreply.github.com>
date: 2020年11月23日T11:31:45-08:00
summary:
Descriptor HowTo: Improve the fidelity of the member object simulation (GH-23475) (GH-23479)
files:
M Doc/howto/descriptor.rst

diff --git a/Doc/howto/descriptor.rst b/Doc/howto/descriptor.rst
index 76987fda22b59..8c6e90319a7f3 100644
--- a/Doc/howto/descriptor.rst
+++ b/Doc/howto/descriptor.rst
@@ -1079,6 +1079,8 @@ simulation where the actual C structure for slots is emulated by a private
 ``_slotvalues`` list. Reads and writes to that private structure are managed
 by member descriptors::
 
+ null = object()
+
 class Member:
 
 def __init__(self, name, clsname, offset):
@@ -1091,20 +1093,28 @@ by member descriptors::
 def __get__(self, obj, objtype=None):
 'Emulate member_get() in Objects/descrobject.c'
 # Also see PyMember_GetOne() in Python/structmember.c
- return obj._slotvalues[self.offset]
+ value = obj._slotvalues[self.offset]
+ if value is null:
+ raise AttributeError(self.name)
+ return value
 
 def __set__(self, obj, value):
 'Emulate member_set() in Objects/descrobject.c'
 obj._slotvalues[self.offset] = value
 
+ def __delete__(self, obj):
+ 'Emulate member_delete() in Objects/descrobject.c'
+ value = obj._slotvalues[self.offset]
+ if value is null:
+ raise AttributeError(self.name)
+ obj._slotvalues[self.offset] = null
+
 def __repr__(self):
 'Emulate member_repr() in Objects/descrobject.c'
 return f'<Member {self.name!r} of {self.clsname!r}>'
 
 The :meth:`type.__new__` method takes care of adding member objects to class
-variables. The :meth:`object.__new__` method takes care of creating instances
-that have slots instead of an instance dictionary. Here is a rough equivalent
-in pure Python::
+variables::
 
 class Type(type):
 'Simulate how the type metaclass adds member objects for slots'
@@ -1117,6 +1127,10 @@ in pure Python::
 mapping[name] = Member(name, clsname, offset)
 return type.__new__(mcls, clsname, bases, mapping)
 
+The :meth:`object.__new__` method takes care of creating instances that have
+slots instead of an instance dictionary. Here is a rough simulation in pure
+Python::
+
 class Object:
 'Simulate how object.__new__() allocates memory for __slots__'
 
@@ -1124,13 +1138,33 @@ in pure Python::
 'Emulate object_new() in Objects/typeobject.c'
 inst = super().__new__(cls)
 if hasattr(cls, 'slot_names'):
- inst._slotvalues = [None] * len(cls.slot_names)
+ empty_slots = [null] * len(cls.slot_names)
+ object.__setattr__(inst, '_slotvalues', empty_slots)
 return inst
 
+ def __setattr__(self, name, value):
+ 'Emulate _PyObject_GenericSetAttrWithDict() Objects/object.c'
+ cls = type(self)
+ if hasattr(cls, 'slot_names') and name not in cls.slot_names:
+ raise AttributeError(
+ f'{type(self).__name__!r} object has no attribute {name!r}'
+ )
+ super().__setattr__(name, value)
+
+ def __delattr__(self, name):
+ 'Emulate _PyObject_GenericSetAttrWithDict() Objects/object.c'
+ cls = type(self)
+ if hasattr(cls, 'slot_names') and name not in cls.slot_names:
+ raise AttributeError(
+ f'{type(self).__name__!r} object has no attribute {name!r}'
+ )
+ super().__delattr__(name)
+
 To use the simulation in a real class, just inherit from :class:`Object` and
 set the :term:`metaclass` to :class:`Type`::
 
 class H(Object, metaclass=Type):
+ 'Instance variables stored in slots'
 
 slot_names = ['x', 'y']
 
@@ -1143,11 +1177,11 @@ At this point, the metaclass has loaded member objects for *x* and *y*::
 >>> import pprint
 >>> pprint.pp(dict(vars(H)))
 {'__module__': '__main__',
+ '__doc__': 'Instance variables stored in slots',
 'slot_names': ['x', 'y'],
 '__init__': <function H.__init__ at 0x7fb5d302f9d0>,
 'x': <Member 'x' of 'H'>,
- 'y': <Member 'y' of 'H'>,
- '__doc__': None}
+ 'y': <Member 'y' of 'H'>}
 
 When instances are created, they have a ``slot_values`` list where the
 attributes are stored::
@@ -1159,8 +1193,9 @@ attributes are stored::
 >>> vars(h)
 {'_slotvalues': [55, 20]}
 
-Unlike the real ``__slots__``, this simulation does have an instance
-dictionary just to hold the ``_slotvalues`` array. So, unlike the real code,
-this simulation doesn't block assignments to misspelled attributes::
+Misspelled or unassigned attributes will raise an exception::
 
- >>> h.xz = 30 # For actual __slots__ this would raise an AttributeError
+ >>> h.xz
+ Traceback (most recent call last):
+ ...
+ AttributeError: 'H' object has no attribute 'xz'
