how to dynamically create an instance of a class in python?

Assuming you have already imported the relevant classes using something like

from [app].models import *

all you will need to do is

klass = globals()["class_name"]
instance = klass()

This is often referred to as reflection or sometimes introspection. Check out a similar questions that have an answer for what you are trying to do:

Does Python Have An Equivalent to Java Class forname

Can You Use a String to Instantiate a Class in Python

You can often avoid the string processing part of this entirely.

import foo.baa 
import foo.AA
import foo

classes = [ foo.baa.a, foo.daa.c, foo.AA ]

def save(theClass, argument):
   aa = theClass()

save(random.choice(classes), arg)

Note that we don't use a string representation of the name of the class.

In Python, you can just use the class itself.