newbie: confused with example in Learning Python 2nd Edition: can anyone give a hint

[Steven Rumbalski]

Porky Pig Jr wrote:

> Here is an example of Stack class which got me totally confused:
> 
>>>> class Stack:
> ...     def __init__(self, data):
> ...             self._data = list(data)
> ...             self.push = data.append
> ...             self.pop = data.pop

It should have confused you.  It was wrong.

>From the errata at
http://www.oreilly.com/catalog/lpython2/errata/lpython2.confirmed:

{457}  class Stack: code;
self.push and self.pop should both reference self._data, not just data

So the corrected code should look like:
>>> class Stack:
 ...     def __init__(self, data):
 ...             self._data = list(data)
 ...             self.push = _data.append
 ...             self.pop = _data.pop

And now the class should act like you expect.  Before it was modifying the
list (data) it was passed in the constructor.  Now it modifies its own copy
(_data).

By the way, a brief note on errata.  IMO it is counter-productive to dwell
on errata (although a quick skim doesn't hurt).  Trying to notice each
error distracts from understanding.  Usually the brain glosses over the
error and reads what was intended rather than what was said.