stupid thread question

[David Bolen]

Michael Vanier <mvanier at endor.bbb.caltech.edu> writes:

> def callback(event):
>     global thread_on
>     key = event.char
> 
>     if key == "s":
>         if not thread_on:
>             stop_flag = 0
              ^^^^^^^^^^^^^
>             thread_on = 1
>             thread.start_new_thread(run, ())
>         else:
>             print "halting thread"
>             stop_flag = 1
              ^^^^^^^^^^^^^
>             thread_on = 0
>     elif key == "q":
>         raise SystemExit

You haven't stop_flag as global ("global stop_flag"), so the
highlighted operations have created a local (to the callback function)
variable, and thus are not changing the global variable, which the
thread function is correctly seeing.

This is definitely a somewhat subtle issue with Python scoping -
scopes are used dynamically but determined statically.  See near the
end of section 9.2 of the tutorial for some info.  See also sections
4.36 and 4.57 of the FAQ.

--
-- David
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