[Tutor] knowing when a list is updated by a thread

[bob gailer]

Kent Johnson wrote:
> Vaibhav.bhawsar wrote:
>> i have this code to print every new element in a list only when the 
>> list length changes (while the list is updated by a thread running 
>> elsewhere)...I was wondering if there is a pythonic way to do this? 
>> how does one know when there is a new element in the list?
>>
>> prevlength = 0
>>     while geoCode.running:
>>         places = geoCode.getResults() #returns a list with most up to 
>> date elements..the list grows as the thread that updates it
>>         if len(places) > prevlength:
>>             print places[prevlength]
>>             prevlength = len(places)
>
> Ouch. This loop will chew up CPU time unless there is some kind of 
> delay in getResults(). If the intent of the loop is to print every new 
> value placed in the list, there is no guarantee that it will work.
>
> If you are using the list to communicate results between two threads 
> you should see if queue.Queue meets your needs. It is thread safe and 
> has blocking access so you don't have to use a busy loop.
>
Evey time someone recommends Queue I think "oh boy this will really help 
me". Then I go to the Library Reference, read the Queue docs and think 
"oh boy who can help me understand this". Even the sample code is 
confusing.

Is there some other documentation or example?


-- 
Bob Gailer
919-636-4239 Chapel Hill, NC