python - 在函数上定义 __getattr__ 和 __getitem__ 无效

Question

免责声明 这只是一个元编程练习，没有任何实际意义目的。

我已经在函数对象上分配了 __getitem__ 和 __getattr__ 方法，但是没有效果...

def foo():
  print "foo!"

foo.__getitem__ = lambda name: name
foo.__getattr__ = lambda name: name
foo.baz = 'baz'

健全性检查我们可以将属性分配给函数:

>>> foo.baz
'baz'

整洁。 “魔术 setter/getter ”怎么样？

>>> foo.bar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute 'bar'

>>> foo['foo']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'function' object is not subscriptable

>>> getattr(foo, 'bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute 'bar'

是否可以在函数对象上使用“魔法 setter/getter ”？

Answer 1

不!将 __getitem__ 分配给实例不适用于任何类型的对象:

>>> class A(object):
...   pass
...
>>> a = A()
>>> a.__getattr__ = lambda name: name
>>> a.foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'foo'

并且您不能在内置函数类型上定义__getattr__:

>>> import types
>>> types.FunctionType.__getitem__ = lambda name: name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'function'

并且您不能继承types.FunctionType:

>>> import types
>>> class F(types.FunctionType):
...   pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
type 'function' is not an acceptable base type