How to create an object instance from a string??

[Tian]

How can I create an instance of an object from a string?

For example, I have a class Dog:
class Dog:
  def bark(self):
    print "Arf!!!"


I have a string:
classname = "Dog"

How can I create a instance of Dog from classname?
Is there any such methods like those in Java?

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