On 2020-03-19 20:08, duncan smith wrote:
Hello,
I have generator code along the following lines,
Q = queue.Queue()
Q.put(x)
while not Q.empty():
x = Q.get()
if <some condition that depends on x>:
yield x
else:
Q.put(<some value that depends on x>)
If I change it to,
Q = []
Q.append(x)
for x in Q:
if <some condition that depends on x>:
yield x
else:
Q.append(<some value that depends on x>)
then it runs approximately 3 times more quickly. I seem to remember
(from somewhere) that the language specification doesn't guarantee that
the latter will continue to work, but that it's unlikely that future
implementations will break it. Apart from that, is there any good reason
why I shouldn't use the list? Is there another approach that might avoid
the overhead of using a queue? Results from profiling the two
implementations below in case it makes anything clearer. TIA.
[snip]
A number of points:
1. I'm not sure it's a good idea to mutate a list while iterating over it.
2. The example with the list retains all of the results, whereas the one
with the queue consumes them.
3. Queues are thread-safe because they're used for communication between
threads, but lists aren't thread-safe; that might be something to do
with the difference.
4. If it doesn't need to be thread-safe, why not try deques instead?
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