TypeError: __class__ assignment only supported for

问题:

I'm trying to copy functions from an arbitrary 'base' class into my new object. However, I'm getting the following error with this sample code.

class my_base:
    def print_hey():
        print("HEY")

    def get_one():
        print(1)

class my_ext:
    def __init__(self, base):
        methods = [method for method in dir(base) if callable(getattr(base, method))]
        for method in methods:
            setattr(self, method, getattr(base, method))


me = my_ext(my_base)
me.get_one()

The above gets this error on the call to setattr.

 TypeError: __class__ assignment only supported for heap types or ModuleType subclasses

The statement works if I type it into the prompt after defining the above.

回答1:

The problem here is that all objects in python have a __class__ attribute that stores the type of the object:

>>> my_base.__class__
<class 'type'>
>>> type(my_base)
<class 'type'>

Since calling a class is how you create an instance of that class, they're considered callables and pass the callable test:

>>> callable(my_base)
True
>>> my_base()
<__main__.my_base object at 0x7f2ea5304208>

And when your code tries to assign something to the __class__ attribute the TypeError you've observed is thrown:

>>> object().__class__ = int
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment only supported for heap types or ModuleType subclasses

---

So you need to be more specific about which attributes should be copied.

You could filter out attributes with double underscores:

methods = [method for method in dir(base) if not method.startswith('__')
                                             and callable(getattr(base, method))]

Or you could filter out classes:

methods = [method for method in dir(base) if callable(getattr(base, method)) and
                                     not isinstance(getattr(base, method), type)]

Or you could only allow functions by comparing to types.FunctionType:

methods = [method for method in dir(base) if callable(getattr(base, method)) and
                           isinstance(getattr(base, method), types.FunctionType)]

回答2:

Your instance object has many attributes that shouldn't be reassigned, and most of them pass the callable test:

>>> [item for item in dir(my_base) if callable(getattr(my_base, item))]
['__class__', '__delattr__', '__dir__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'get_one', 'print_hey']

You should restrict most of them. You could simply check for item.startswith('__') but I'm not sure what you want to do with __repr__ and friends. Perhaps check for underbars after a whitelist?

回答3:

Turns out that __class__ is callable and that there are many other callables as well.

I just want the functions so the following does the job:

 import types
 . . . 
  methods = [method for method in dir(base) if isinstance(getattr(base, method), types.FunctionType)]
 . . .