Issue with Python single and double underscore variables

Question

I am unsure what I am doing wrong; or why this is the case.

I've the following code:

class Expression (Node):
  """
  ...
  """

  def __init__ (self):
    self.__expressionType = None


  def expressionType (self):
    return self.__expressionType


class Number (Expression):
  """
  Number .
  """

  def __init__ (self, value):
    """
    Value is an entry of type Constant.
    """
    Expression.__init__(self)
    assert isinstance (value, KST.Constant)
    self.__constant = value
    self.__expressionType = value.elementType()

For a number object say n = Number(KST.Constant(..)), I am always returned None for the following statement—

 n.expressionType()

Now if I change the double underscores to single ones, it all works. I understand the difference between private and semi-private variables but why this is happening — I've no idea. Also, I've used "__" in a number of other places and it all seems to work fine.

Answer 1

Attribute names with double underscores are "mangled" to make it harder to have conflicting name in subclasses.

So use single underscores.