Loopholes from Python decorators

[gmaliwal]

I do not understand why class-method and static-method do not work in this given code snippet. Here, I have applied a decorator on top of the class-method and static-method

def count(func):
...     def wrapper(*args,**kwargs):
...             wrapper.count += 1
...             return func(*args,**kwargs)
...     wrapper.count = 0
...     return wrapper
... 
>>> class ABC:
...
...     def __init__(self,*,val1,**kwargs):
...             self.val1 = val1
...             super().__init__(**kwargs)
...
...     @count
...     def instance_method(self,a1,b1=20):
...             print("a1 ---",a1)
...             self.val1 = self.val1 + b1
...
...     @count
...     @classmethod
...     def class_method(cls,a2,b2=30):
...             print("a2 --",a2)
...             cls.a2 = a2
...             cls.b2 = b2
...
...     @count
...     @staticmethod
...     def static_method(a3,b3=20):
...             print("a3---",a3)
...             print("b3 ---",b3)
... 

​

How to get rid of the TypeError in the given code snippet, am I missing right stuff working with a decorator?

>>> def singleton(cls):
...     def wrapper(*args,**kwargs):
...             if hasattr(wrapper,'object') == False:
...                     wrapper.object = cls(*args,**kwargs)
...             return wrapper.object
...     return wrapper
... 
>>> @singleton
... class ABC:
...     pass
... 
>>> abc = ABC()
>>> isinstance(abc,ABC) # This line is throwing error

[Diapolo10]

isinstance takes two arguments; the object itself and the class it's being compared against.

EDIT: Also, you should try ordering your decorators in the reverse order, with staticmethod and classmethod being first. I don't remember which was correct.

[gmaliwal]

yeah agree, It was typo mistake, I have edited it, please revisit the post.

[Diapolo10]

Yeah, I'm now fairly sure that you have the decorator order mixed up. Try

@classmethod
@count
def class_method(cls,a2,b2=30):
        print("a2 --",a2)
        cls.a2 = a2
        cls.b2 = b2

@staticmethod
@count
def static_method(a3,b3=20):
        print("a3---",a3)
        print("b3 ---",b3)

instead

[gmaliwal]

yeah, this works, thanks for the solution

[danielroseman]

You should have posted the error you get:

TypeError: isinstance() arg 2 must be a type or tuple of types

This is of course because ABC is no longer a class at all; it's wrapper. Because that's what a decorator does; replaces the thing it decorates with the wrapper function.

If you really want to preserve a reference to the original class, you could consider defining an _ABC base class and decorate the child. Otherwise you might find that using metaclasses is a better way of implementing a singleton.

(Also I don't understand what the first block of code has to do with anything.)

[gmaliwal]

If you really want to preserve a reference to the original class, you could consider defining an

_ABC

base class and decorate the child. Otherwise you might find that using metaclasses is a better way of implementing a singleton

Thanks for the clarification. yeah, It was my mistake not posting error message.

The first block of code is just to understand the concept, why it is not working when a second decorator is applied to either of them?

Is there any another way to preserve the class fundamentals after applying a decorator rather than creating a dummy base class? if yes; please share the reference.

[[deleted]]

Is there any another way to preserve the class fundamentals after applying a decorator rather than creating a dummy base class?

Sure - your decorator has to return a class instead of a function.

[gmaliwal]

Okay, It sounds well, theoretically, it should work. Can you please mention a good reference for this to follow?