[python] decorator implemented with class to wrap class method

# python2
Python 2.7.5 (default, Dec  8 2017, 16:39:59) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-25)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class tracer:
...     def __init__(self, func):
...             self.func = func
...             self.count = 0
...     def __call__(self, *args, **kargs):
...             self.count += 1
...             return self.func(*args, **kargs)
... 
>>> class C:
...     @tracer
...     def f(self):
...             pass
... 
>>> c = C()
>>> c.f()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 7, in __call__
TypeError: f() takes exactly 1 argument (0 given)
>>> C.f()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 7, in __call__
TypeError: f() takes exactly 1 argument (0 given)
>>> C.f.func
0x7f0efb4ad0c8>
>>> c.f.func
0x7f0efb4ad0c8>
>>> 

this is because decoration occurs at function defining phase:

@tracer
def f(self):
...

is the same as

f = tracer(f)

when c.f() is called, it is same as tracer(f)(), this won’t trigger function f __get__ method to pass self of c implicitly to f, thus will result in TypeError because no arguments passed.

how to solve this?
in __call__, the instance must be passed as first argument, how to retrieve the instance from call like c.f()? the answer is to use descriptor protocol

class tracer:
    def __init__(self, func):
        self.calls = 0
        self.func = func
    def __call__(self, *args, **kargs):
        self.calls += 1
        print('call %s to %s' % (self.calls, self.func.__name__))
        return self.func(*args, **kargs)
    def __get__(self, instance, owner):
        def wrapper(*args, **kargs):
            return self(instance, *args, **kargs)
        return wrapper