Behavior of mutable class variables

[Terry Reedy]

<tkpmep at hotmail.com> wrote in message 
news:1178744257.369030.95310 at w5g2000hsg.googlegroups.com...
| Here's what I expect to happen each time simulation( ) is called: the
| class variable NStocks for the class Stock is initialized to an empty
| list,

Why would you expect that ;-)
A class statement is usually executed exactly once, as in your code.
The body of a class statement usually consists of several name bindings:
some are explicit, like your NStocks assignment statement;
some are implicit, like you __init__ function definition.
The resulting dictionary is used to create the class object, which is 
mostly a wrapper around the dict created by the class statement body.

tjr