装饰和包装的函数没有将 self 传递给实例方法

Question

I am trying to run some instance methods as background threads using a decorator. Several nested functions are chained (as found there ) to make it work:

import traceback
from functools import partial
from threading import Thread


def backgroundThread(name=''):
    def fnWrapper(decorated_func):
        def argsWrapper(name, *inner_args, **inner_kwargs):
            def exceptionWrapper(fn, *args, **kwargs):
                try:
                    fn(*args, **kwargs)
                except:
                    traceback.print_exc()

            if not name:
                name = decorated_func.__name__

            th = Thread(
                name=name,
                target=exceptionWrapper,
                args=(decorated_func, ) + inner_args,
                kwargs=inner_kwargs
            )
            th.start()

        return partial(argsWrapper, name)

    return fnWrapper


class X:
    @backgroundThread()
    def myfun(self, *args, **kwargs):
        print(args, kwargs)
        print("myfun was called")
        #1 / 0


x = X()
x.myfun(1, 2, foo="bar")
x.myfun()

Output/Error (on Windows, Python 3.6.6):

(2,) {'foo': 'bar'}
myfun was called
Traceback (most recent call last):
  File "t3.py", line 11, in exceptionWrapper
    fn(*args, **kwargs)
TypeError: myfun() missing 1 required positional argument: 'self'

The code works partly, how to be able to 'bind' self to the call: x.myfun() which takes no arguments ?

Answer 1

Fundamentally, the problem is that @backgroundThread() doesn't wrap an instance method x.myfun ; it wraps the function X.myfun that is namespaced to the class.

We can inspect the wrapped result:

>>> X.myfun
functools.partial(<function backgroundThread.<locals>.fnWrapper.<locals>.argsWrapper at 0x7f0a1e2e7a60>, '')

This is not usable as a method, because functools.partial is not a descriptor:

>>> X.myfun.__get__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'functools.partial' object has no attribute '__get__'
>>> class Y:
...     # no wrapper
...     def myfun(self, *args, **kwargs):
...         print(args, kwargs)
...         print("myfun was called")
...         #1 / 0
... 
>>> Y.myfun.__get__
<method-wrapper '__get__' of function object at 0x7f0a1e2e7940>

Because X.myfun is not usable as a descriptor, when it is looked up via x.myfun , it is called like an ordinary function. self does not receive the value of x , but instead of the first argument that was passed, resulting in the wrong output for the (1, 2, foo='bar') case and the exception for the () case.

---

Instead of having argsWrapper accept a name and then binding it with partial , we can just use the name from the closure - since we are already doing that with decorated_func anyway. Thus:

def backgroundThread(name=''):
    def fnWrapper(decorated_func):
        def argsWrapper(*inner_args, **inner_kwargs):
            nonlocal name
            def exceptionWrapper(fn, *args, **kwargs):
                try:
                    fn(*args, **kwargs)
                except:
                    traceback.print_exc()
            if not name:
                name = decorated_func.__name__
            th = Thread(
                name=name,
                target=exceptionWrapper,
                args=(decorated_func, ) + inner_args,
                kwargs=inner_kwargs
            )
            th.start()
        return argsWrapper
    return fnWrapper

Here, nonlocal name is needed so that argsWrapper has access to a name from a scope that is not the immediate closure, but also isn't global.