Dynamically adding methods to a class...

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Wed Jul 30 07:09:01 CEST 2008

En Tue, 29 Jul 2008 01:17:13 -0300, Piyush Anonymous  
<piyush.subscription at gmail.com> escribi�:

> class MyObject:
>    def __init__(self, name):
>      self.name = name
>    def do_this_default(self):
>       print "default do_this implementation for %s" % self.name
> def custom_do_this(): #method to be added
>    print "custom do_this implementation for %s" % self.name

You forget the self argument (this explains the error you got).

> def funcToMethod(func,clas,method_name=None):
>     """Adds func to class so it is an accessible method; use method_name  
> to
> specify the name to be used for calling the method.
>     The new method is accessible to any instance immediately."""
>     import new
>     method = new.instancemethod(func,None,clas)
>     print method
>     if not method_name: method_name=func.__name__
>     clas.__dict__[method_name]=func

It's a lot easier than that; just add the *function* object to the class  
setattr(clas, method_name, func)
If you know the name when you write the code, just set the attribute:

py> o = MyObject("hello")
py> MyObject.custom_do_this = custom_do_this
py> o.custom_do_this()
custom do_this implementation for hello

new.instancemethod is useful to add specific methods to individual  
instances, but it's not used to add methods globally to the class.

> Error I am getting;
> TypeError: custom_do_this() takes no arguments (1 given)

Add the self argument and you're done.

> Why am I getting it?
> Also how can I do this in new style class (inherited from 'object')?

It's the same thing, no difference.

Gabriel Genellina

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