python documentation声明在查找特殊方法时可能会绕过__getattribute__。这是通过语言语法或内置函数隐式调用的结果。
例如，
elem = container[0]
不等于:
elem = container.__getattribute__('__getitem__')[0]
下面是另一个例子:
class WrappedList:
def __init__(self):
object.__setattr__(self, 'interal_list', ['apple', 'pear', 'orange'])
def __getattribute__(self, attr_name):
interal_list = object.__getattribute__(self, 'interal_list')
attr = getattr(interal_list, attr_name)
return attr
wl = WrappedList()
print("\nSTART TEST 01 ------------------------")
try:
print(wl[0]) # throws TypeError: 'WrappedList' object does not support indexing
except TypeError as e:
print(e)
print("\nSTART TEST 02 ------------------------")
try:
getitem = getattr(wl, '__getitem__')
print(getitem(0)) # works just fine
except TypeError as e:
print(e)
我想编写一个名为MagicOverrider的类，其中任何继承自MagicOverrider的类总是调用__getattribute__，而不是绕过它。我的问题是我们怎样才能做到这一点？
我尝试了以下方法:
class MagicOverrider:
def __call__(self, *args, **kwargs):
f = getattr(self, '__call__')
return f(*args, **kwargs)
def __iter__(self, *args, **kwargs):
f = getattr(self, '__iter__')
return f(*args, **kwargs)
def __getitem__(self, *args, **kwargs):
f = getattr(self, '__getitem__')
return f(*args, **kwargs)
def __setitem__(self, *args, **kwargs):
f = getattr(self, '__setitem__')
return f(*args, **kwargs)
def __add__(self, *args, **kwargs):
f = getattr(self, '__add__')
return f(*args, **kwargs)
def __sub__(self, *args, **kwargs):
f = getattr(self, '__sub__')
return f(*args, **kwargs)
def __mul__(self, *args, **kwargs):
f = getattr(self, '__mul__')
return f(*args, **kwargs)
def __truediv__(self, *args, **kwargs):
f = getattr(self, '__truediv__')
return f(*args, **kwargs)
def __floordiv__(self, *args, **kwargs):
f = getattr(self, '__floordiv__')
return f(*args, **kwargs)
def __mod__(self, *args, **kwargs):
f = getattr(self, '__mod__')
return f(*args, **kwargs)
def __divmod__(self, *args, **kwargs):
f = getattr(self, '__divmod__')
return f(*args, **kwargs)
def __pow__(self, *args, **kwargs):
f = getattr(self, '__pow__')
return f(*args, **kwargs)
def __lshift__(self, *args, **kwargs):
f = getattr(self, '__lshift__')
return f(*args, **kwargs)
def __rshift__(self, *args, **kwargs):
f = getattr(self, '__rshift__')
return f(*args, **kwargs)
def __and__(self, *args, **kwargs):
f = getattr(self, '__and__')
return f(*args, **kwargs)
def __xor__(self, *args, **kwargs):
f = getattr(self, '__xor__')
return f(*args, **kwargs)
def __or__(self, *args, **kwargs):
f = getattr(self, '__or__')
return f(*args, **kwargs)
def __radd__(self, *args, **kwargs):
f = getattr(self, '__radd__')
return f(*args, **kwargs)
def __rsub__(self, *args, **kwargs):
f = getattr(self, '__rsub__')
return f(*args, **kwargs)
def __rmul__(self, *args, **kwargs):
f = getattr(self, '__rmul__')
return f(*args, **kwargs)
def __rtruediv__(self, *args, **kwargs):
f = getattr(self, '__rtruediv__')
return f(*args, **kwargs)
def __rfloordiv__(self, *args, **kwargs):
f = getattr(self, '__rfloordiv__')
return f(*args, **kwargs)
def __rmod__(self, *args, **kwargs):
f = getattr(self, '__rmod__')
return f(*args, **kwargs)
def __rdivmod__(self, *args, **kwargs):
f = getattr(self, '__rdivmod__')
return f(*args, **kwargs)
def __rpow__(self, *args, **kwargs):
f = getattr(self, '__rpow__')
return f(*args, **kwargs)
def __rlshift__(self, *args, **kwargs):
f = getattr(self, '__rlshift__')
return f(*args, **kwargs)
def __rrshift__(self, *args, **kwargs):
f = getattr(self, '__rrshift__')
return f(*args, **kwargs)
def __rand__(self, *args, **kwargs):
f = getattr(self, '__rand__')
return f(*args, **kwargs)
def __rxor__(self, *args, **kwargs):
f = getattr(self, '__rxor__')
return f(*args, **kwargs)
def __neg__(self, *args, **kwargs):
f = getattr(self, '__neg__')
return f(*args, **kwargs)
def __pos__(self, *args, **kwargs):
f = getattr(self, '__pos__')
return f(*args, **kwargs)
def __abs__(self, *args, **kwargs):
f = getattr(self, '__abs__')
return f(*args, **kwargs)
def __invert__(self, *args, **kwargs):
f = getattr(self, '__invert__')
return f(*args, **kwargs)
def __complex__(self, *args, **kwargs):
f = getattr(self, '__complex__')
return f(*args, **kwargs)
def __int__(self, *args, **kwargs):
f = getattr(self, '__int__')
return f(*args, **kwargs)
def __float__(self, *args, **kwargs):
f = getattr(self, '__float__')
return f(*args, **kwargs)
def __round__(self, *args, **kwargs):
f = getattr(self, '__round__')
return f(*args, **kwargs)
def __index__(self, *args, **kwargs):
f = getattr(self, '__index__')
return f(*args, **kwargs)
def __eq__(self, *args, **kwargs):
f = getattr(self, '__eq__')
return f(*args, **kwargs)
def __ne__(self, *args, **kwargs):
f = getattr(self, '__ne__')
return f(*args, **kwargs)
def __lt__(self, *args, **kwargs):
f = getattr(self, '__lt__')
return f(*args, **kwargs)
def __le__(self, *args, **kwargs):
f = getattr(self, '__le__')
return f(*args, **kwargs)
def __gt__(self, *args, **kwargs):
f = getattr(self, '__gt__')
return f(*args, **kwargs)
def __ge__(self, *args, **kwargs):
f = getattr(self, '__ge__')
return f(*args, **kwargs)
def __bool__(self, *args, **kwargs):
f = getattr(self, '__bool__')
return f(*args, **kwargs)
def __new__(self, *args, **kwargs):
f = getattr(self, '__new__')
return f(*args, **kwargs)
def __del__(self, *args, **kwargs):
f = getattr(self, '__del__')
return f(*args, **kwargs)
def __slots__(self, *args, **kwargs):
f = getattr(self, '__slots__')
return f(*args, **kwargs)
def __hash__(self, *args, **kwargs):
f = getattr(self, '__hash__')
return f(*args, **kwargs)
def __instancecheck__(self, *args, **kwargs):
f = getattr(self, '__instancecheck__')
return f(*args, **kwargs)
def __subclasscheck__(self, *args, **kwargs):
f = getattr(self, '__subclasscheck__')
return f(*args, **kwargs)
def __subclasshook__(self, *args, **kwargs):
f = getattr(self, '__subclasshook__')
return f(*args, **kwargs)
def __ror__(self, *args, **kwargs):
f = getattr(self, '__ror__')
return f(*args, **kwargs)
def __iadd__(self, *args, **kwargs):
f = getattr(self, '__iadd__')
return f(*args, **kwargs)
def __isub__(self, *args, **kwargs):
f = getattr(self, '__isub__')
return f(*args, **kwargs)
def __imul__(self, *args, **kwargs):
f = getattr(self, '__imul__')
return f(*args, **kwargs)
def __itruediv__(self, *args, **kwargs):
f = getattr(self, '__itruediv__')
return f(*args, **kwargs)
def __ifloordiv__(self, *args, **kwargs):
f = getattr(self, '__ifloordiv__')
return f(*args, **kwargs)
def __imod__(self, *args, **kwargs):
f = getattr(self, '__imod__')
return f(*args, **kwargs)
def __ipow__(self, *args, **kwargs):
f = getattr(self, '__ipow__')
return f(*args, **kwargs)
def __ilshift__(self, *args, **kwargs):
f = getattr(self, '__ilshift__')
return f(*args, **kwargs)
def __irshift__(self, *args, **kwargs):
f = getattr(self, '__irshift__')
return f(*args, **kwargs)
def __iand__(self, *args, **kwargs):
f = getattr(self, '__iand__')
return f(*args, **kwargs)
def __ixor__(self, *args, **kwargs):
f = getattr(self, '__ixor__')
return f(*args, **kwargs)
def __repr__(self, *args, **kwargs):
f = getattr(self, '__repr__')
return f(*args, **kwargs)
def __str__(self, *args, **kwargs):
f = getattr(self, '__str__')
return f(*args, **kwargs)
def __cmp__(self, *args, **kwargs):
f = getattr(self, '__cmp__')
return f(*args, **kwargs)
def __rcmp__(self, *args, **kwargs):
f = getattr(self, '__rcmp__')
return f(*args, **kwargs)
def __nonzero__(self, *args, **kwargs):
f = getattr(self, '__nonzero__')
return f(*args, **kwargs)
def __unicode__(self, *args, **kwargs):
f = getattr(self, '__unicode__')
return f(*args, **kwargs)
但是，我的解决方案至少有两个问题:
- 如果在Python的 future 版本中引入新的魔术方法，它将不再起作用
- 第一行，
class MagicOverrider:，抛出TypeError: 'function' object is not iterable
最佳答案
这很棘手。
因为当通过语言结构触发魔法方法时，Python 不会经历正常情况下使用的正常属性检索路径(即使用 __getattribute__ 等):相反，每当特殊情况时，方法被分配给一个类，它在类本身的二进制数据结构中被标记(这是由Python解释器中的C代码完成的)。这样做是为了使这种用法是快捷方式 - 否则，仅仅为了获得执行的正确方法(例如添加或项目检索)就会需要太多代码。而且，很容易出现一些无限递归循环。
所以 - 魔术方法总是直接通过 Python 检索 - 没有 __getattribute__。
可以做的就是让 magicmethods 本身在运行时触发 __getattribute__。如果他们得到任何与自己不同的结果，他们就会调用该结果。只需要小心避免无限递归。
至于潜在的魔术方法:因为无论如何这都需要一个元类，所以只需在创建将强制 __getattribute__ 的类时让元类包装所需类的所有魔术方法即可.
下面的代码执行此操作，并包含一个示例类，该示例类在 __getitem__ 上放置一个临时包装器:
from functools import wraps
from threading import local as thread_local
from types import MethodType
def wrap(name, method):
local_flag = thread_local()
@wraps(method)
def wrapper(*args, **kw):
local_method = method
if not getattr(local_flag, "running", False) and args and not isinstance(args[0], type):
local_flag.running = True
# trigger __getattribute__:
self = args[0]
cls = self.__class__
retrieved = cls.__getattribute__(self, name)
if not retrieved is wrapper:
local_method = retrieved
if isinstance(local_method, MethodType):
args = args[1:]
result = local_method(*args, **kw)
local_flag.running = False
return result
wrapper._wrapped = True
return wrapper
class MetaOverrider(type):
def __init__(cls, name, bases, namespace, **kwd):
super().__init__(name, bases, namespace, **kwd)
for name in dir(cls):
if not (name.startswith("__") and name.endswith("__")):
continue
if name in ("__getattribute__", "__class__", "__init__"):
continue
magic_method = getattr(cls, name)
if not callable(magic_method) or getattr(magic_method, "_wrapped", False):
continue
setattr(cls, name, wrap(name, magic_method))
class TestOverriding(list, metaclass=MetaOverrider):
def __getattribute__(self, attrname):
attr = super().__getattribute__(attrname)
if attrname == "__getitem__":
original = attr
def printergetitem(self, index):
print("Getting ", index)
return original(index)
attr = printergetitem
return attr
它适用于任何魔术方法 - 但当然，如果您在创建类后将魔术方法分配给类本身，它将隐藏所使用的包装方法。但对于 __getattribute__ 本身添加的任何魔法包装来说，它应该可以工作。
关于python - 我们如何强制为魔术方法(特殊方法)调用 getattribute() ？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48966182/