what is self._base?

[Benjamin Tai]

Hi,

I have come across the following statement in a constructor (Programming
Python by Mark Lutz). After extending a type in C (stacktype), it is
wrapped by a Python class (Stack).

import stacktype
class Stack:
  def __init__(self, start=None):
    self._base = start or stacktype.Stack()

However I can't find any documents/explanation for the following:

1) What is  "_base"
2) What is  "start=None"
3) Why can't I just write  "self = stacktype.Stack()"


Any help for this newbie would be appreciated.

Ben