Dynamically add member function to an instance of a class in Python

Question

When I do the following:

class C:
 pass

def f( self ):
 print self

a = C()

a.f = f

a.f()

I get the following error at the line a.f(): TypeError: f() takes exactly 1 argument (0 given)

The problem appears to be that when f is added to the instance, a, it is treated like a function that is stored inside of a, rather than an actual member function. If I change a.f() to a.f(a), then I get the intended effect, but is there a way to make python interpret the original example this way? In other words, is there a way to add a member function to an instance of a class at runtime?

Thanks

GWW · Accepted Answer · 2019-08-21 15:00:20Z

27

For Python 2.X you can use:

import types
class C:
    pass

def f(self):
    print self

a = C()
a.f = types.MethodType(f,a)
a.f()

For Python 3.X:

import types
class C(object):
    pass

def f(self):
    print(self)

a = C()
a.f = types.MethodType(f,a)
a.f()

edited Aug 21, 2019 at 15:00

answered May 25, 2011 at 1:42

GWW

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5 Comments

Dave Over a year ago

This is the way to add the function to the specific instance a (other instances of class C won't have f).

Jules Gagnon-Marchand Over a year ago

C.f = types.MethodType(f,None,C) would be the code to add the function to any existing or eventual new instances of the class C

LudvigH Over a year ago

What python version is this for? the print statement suggest python 2.*. Doesnt work in python 3.

GWW Over a year ago

@LudvigH: I have modified my answer to include support for python 3.X.

GWW Over a year ago

No problem. Thanks for pointing this out it's hard to remember if old answers need to be updated.

JBernardo · Accepted Answer · 2011-05-25 02:56:48Z

5

You should put f in the class, not in the instance...

 class C:
     pass

 def f(self):
    print(self)

 a = C()
 C.f = f
 a.f()

For the interpreter myObject.foo() is the same as myClass.foo(myObject) when the object doesn't hold anything named foo, but a function placed inside a object is just a function.

answered May 25, 2011 at 2:56

JBernardo

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1 Comment

Dave Over a year ago

This is the way to add the function to all instances of class C (and it's sub-classes).

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