Q: names with underscores and style

[Thomas A. Bryan]

I'm writing an abstract class, call it MyClass.  
It's meant to be used like this:

class SubClass(MyClass):
  # implementation...
obj = SubClass()
obj.run()

The run() method of MyClass calls other methods of MyClass.  
My question is whether all of the methods other than run() 
should be named with a leading underscore (since no one but 
the programmer doing the subclassing should have to see them) 
or should I begin with a letter the names of run() *and* all 
of the methods that I intend for programmers to override.

Basically, I'm trying to figure out when to use a leading 
underscore for a method/member name.  Is there some rule of
thumb for underscores in Python names?

---Tom