Single leading underscore means that the method is for internal use inside method only. Also, if you will perform such import:
from A import *
This wouldn't import methods with the leading underscore. Nevertheless, if you need to use this method, you can import it explicitly and use. But it is not a good habit.
In other words, a single underscore is like a caution that this method is internal and should be used only inside the class.
Double underscore is a stronger indicator that the method should be used only inside the given class. Double underscore performs so-called name mangling. To call such a method you will need to use a weird construction: