Dynamically adding methods to a class in Python

Question

I'm trying to add methods to a class based on a list.

class _Roles(object):
    """
    set the roles for dev, staging and production
    """
    def __init__(self):
        from types import MethodType
        steps = ['dev','stage','prod']
        for step in steps:
            def env_setter(self):
                print step
            method = MethodType(env_setter,self,self.__class__)
            setattr(self,step,method)

The problem is that when I call _Roles.dev(), _Roles.stage(), or _Roles.prod(), I always get printed the last step that is prod instead of getting dev for dev() and so on. what's the reason for this?

Answer 1

Just use setattr :

class Foo:
    def __init__(self, v):
        self.v = v
        
def my_new_method(self):
    print("self.v =", self.v)

setattr(Foo, 'print_v', my_new_method)

Foo(5).print_v()

Output :

self.v = 5