How can I dynamically add a function to an instance in python upon instantiation

Question

If I have a python class which allows for an options parameter upon instantiation, how can I dynamically set a function to it, based upon the value of that options parameter. For example, if I have the code

def hello1():
 print(self.name,"says hi")
def hello2():
 print(self.name,"says hello")
class A:
 def __init__(self, name, opt=0):
 if opt == 1:
 setattr(self,'hello',hello1)
 else:
 setattr(self,'hello',hello2)
if __name__ == "__main__":
 a1 = A("my")
 a2 = A("name",1)
 a1.hello()
 a2.hello()

I get the traceback error

Traceback (most recent call last):
 File "dynamic_classes.py", line 17, in <module>
 a1.hello()
 File "dynamic_classes.py", line 5, in hello2
 print(self.name,"says hello")
NameError: global name 'self' is not defined

• user395760: Do you realize that getattr(a, 'b', c) is an obscure way to write a.b = c?
• user1876508: I understand that. I am writing a wrapper around zmq which instantiates processes with a special connection method, either a bind, or connect method, which is defined at instantiation
• user395760: So, why do you use setattr (just noticed the typo in my first comment), rather than plain old attribute assignment?
• user1876508: Here is a copy of my current code pastebin.com/9QeY0p0y, maybe this will illuminate this. I do not want to have to write a connect method for each class, also, I will add extra protection to the bind method
• user395760: I am only talking about the lines of the form setattr(self, 'connect', <something>). These are exactly equivalent to self.connect = <something>, so why don't you do that?

Answer 1

Your functions do not define a self parameter, nor will they ever get one.

You need to use methods; you can create these from the functions by treating them as descriptors and explicitly calling .__get__() on them:

def hello1(self):
 print(self.name,"says hi")
def hello2(self):
 print(self.name,"says hello")
class A:
 def __init__(self, name, opt=0):
 if opt == 1:
 setattr(self, 'hello', hello1.__get__(self, type(self))
 else:
 setattr(self, 'hello', hello2.__get__(self, type(self)))

Normally, the .__get__() method is called on functions when accessing them on a class (either directly or via an instance). This doesn't happen for functions added directly on an instance however, so you need to do it manually.

Answer 2

You can also use MethodType from types module.

import types
def hello1(self):
 print(self.name,"says hi")
def hello2(self):
 print(self.name,"says hello")
class A:
 def __init__(self, name, opt=0):
 self.name = name
 if opt == 1:
 self.hello = types.MethodType(hello1, self)
 else:
 self.hello = types.MethodType(hello2, self)
A('a').hello() # ('a', 'says hello')
A('a', 1).hello() # ('a', 'says hi')