Re: what thread-synch mech to use for clean exit from a thread

[MRAB]

On 15/07/2013 04:04, Steven D'Aprano wrote:

On Mon, 15 Jul 2013 10:27:45 +0800, Gildor Oronar wrote:

A currency exchange thread updates exchange rate once a minute. If the
thread faield to update currency rate for 5 hours, it should inform
main() for a clean exit. This has to be done gracefully, because main()
could be doing something delicate.

I, a newbie, read all the thread sync tool, and wasn't sure which one to
use. In fact I am not sure if there is a need of thread sync, because
there is no racing cond. I thought of this naive way:

class CurrencyExchange():
    def __init__(in_case_callback):
       this.callback = in_case_callback

You need to declare the instance parameter, which is conventionally
called "self" not "this". Also, your class needs to inherit from Thread,
and critically it MUST call the superclass __init__.

So:

class CurrencyExchange(threading.Thread):
     def __init__(self, in_case_callback):
         super(CurrencyExchange, self).__init__()
         self.callback = in_case_callback

But I'm not sure that a callback is the right approach here. See below.

    def __run__():

Likewise, you need a "self" parameter.

       while time.time() - self.rate_timestamp < 5*3600:
          ... # update exchange rate
          if success:

The "==" in this line should, of course, be "=":

             self.rate_timestamp == time.time()
          time.sleep(60)
       this.callback() # rate not updated 5 hours, a crisis

I think that a cleaner way is to just set a flag on the thread instance.
Initiate it with:

     self.updates_seen = True

in the __init__ method, and then add this after the while loop:

     self.updates_seen = False

def main():
    def callback()
       Go_On = False

I don't believe this callback will work, because it will simply create a
local variable call "Go_On", not change the non-local variable.

In Python 3, you can use the nonlocal keyword to get what you want, but I
think a better approach is with a flag on the thread.

    agio = CurrencyExchange(in_case = callback)
    agio.start()

    Go_On = True
    while Go_On:
       do_something_delicate(rate_supplied_by=agio)

Change to:

     while agio.updates_seen:
         do_something_delicate...

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