access object members from dynamically added methods in python

Question

I have problem sharing members of a class with its dynamically generated methods.

For example below x accessed from __init__ and from normal_test is different fro x accessed from dynamically bounded methods test and setx:

class Foo:
    def __init__(self):
        self.x = 10
    def normal_test(self):
        print self.x

def bar(self):
    print self.x
def setx_method(self,x):
    self.x = x

setattr(Foo, "test", classmethod(bar))
setattr(Foo, "setx", classmethod(setx_method))

f = Foo();
f.setx(5)
f.test()
f.normal_test()

How should I write the code such that self.x would refer to same x?

Remi Guan · Accepted Answer · 2015-11-09 08:33:41Z

1

Analyzing

setattr(Foo, "setx", classmethod(setx_method))

It's equivalent to

@classmethod
def setx_method(self,x):
    self.x=x

Now when you invoke f.setx(5), it actually binds f's class(ie, Foo) with self and 5 with x and thus it executed Foo.x = 5

In short you have two x in Foo, one is instance member x and one is class member x.

f.setx(5)        # sets class x
f.test()         # print class variable x
f.normal_test()  # print instance variable x

edited Nov 9, 2015 at 8:33

Remi Guan

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answered Nov 9, 2015 at 8:29

Mangu Singh Rajpurohit

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1 Comment

Artyom Over a year ago

Thanks I failed to understand that setting an attribute method on class actually defined a method.. no need of class method.

Community · Accepted Answer · 2017-05-23 12:03:56Z

0

You shouldn't use @classmethod with self, because it points to your class instance, while in @classmethod you should use cls as it points to whole your class.

See here: Meaning of @classmethod and @staticmethod for beginner?

So the simplest solution is just remove classmethod:

setattr(Foo, "test", bar)
setattr(Foo, "setx", setx_method)

edited May 23, 2017 at 12:03

CommunityBot

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answered Nov 9, 2015 at 8:25

Nhor

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