Threads and variable assignment

[elbertlev at hotmail.com]

Theoretically you have to use lock, while accesing the isgood instance,
but... practically noting bad can happen IN THIS PARTICULAR CASE, if
you don't. Python uses Global Interpreter Lock. In other  words only
one thread is running at any particular moment. Thread scheduling is
preemptive, but "atomic" actions are not interrupted. Bollean asigment
is an atomic action.

WELCOME TO THE POWER OF MULTITHREADING :)