collect data using threads

[Kent Johnson]

Peter Hansen wrote:
> Qiangning Hong wrote:
> 
>> A class Collector, it spawns several threads to read from serial port.
>> Collector.get_data() will get all the data they have read since last
>> call.  Who can tell me whether my implementation correct?
> 
> [snip sample with a list]
> 
>> I am not very sure about the get_data() method.  Will it cause data lose
>> if there is a thread is appending data to self.data at the same time?
> 
> 
> That will not work, and you will get data loss, as Jeremy points out.
> 
> Normally Python lists are safe, but your key problem (in this code) is 
> that you are rebinding self.data to a new list!  If another thread calls 
> on_received() just after the line "x = self.data" executes, then the new 
> data will never be seen.

Can you explain why not? self.data is still bound to the same list as x. At least if the execution sequence is 
x = self.data
                    self.data.append(a_piece_of_data)
self.data = []

ISTM it should work.

I'm not arguing in favor of the original code, I'm just trying to understand your specific failure mode.

Thanks,
Kent