Dynamic instantiation from string name of a class in dynamically imported module?

In python, I have to instantiate certain class, knowing its name in a string, but this class ‘lives’ in a dynamically imported module. An example follows:

loader-class script:

import sys
class loader:
  def __init__(self, module_name, class_name): # both args are strings
    try:
      __import__(module_name)
      modul = sys.modules[module_name]
      instance = modul.class_name() # obviously this doesn't works, here is my main problem!
    except ImportError:
       # manage import error

some-dynamically-loaded-module script:

class myName:
  # etc...

I use this arrangement to make any dynamically-loaded-module to be used by the loader-class following certain predefined behaviours in the dyn-loaded-modules…