How do I mock a class in a Python unit test?

I have a class:

class A:
    __init__(self):
        self.b = B()

   def is_authorized(self)
      name = self.b.get_name()

      if name == 'admin':
          return True
      else:
          return False

I'd like to write a unit test to test the is_authorized method. The problem is that it requires an instance of the B class which is very complicated to construct in isolation as it requires network connections and other stuff. How can I mock this class and provide something that just has the get_name method. So that I can create the A class and test the method.