[Python-Dev] I'd like list.pop to accept an optional second argument giving a defaultvalue

1999年7月23日 03:23:27 -0400

In a moment of insanity, Guido gave me carte blanche to suggest new list
methods, and list.pop & list.extend were the result. I considered spec'ing
list.pop to take an optional "default on bad index" argument too, but after
playing with it didn't like it (always appeared just as easy & clearer to
use "if list:" / "while list:" etc).
Jim has a novel use I hadn't considered:
> With pop, you can essentially test whether the list is
> empty and get a value if it isn't in one atomic operation:
>> try:
> foo=queue.pop(0)
> except IndexError:
> ... empty queue case
> else:
> ... non-empty case, do something with foo
>> Unfortunately, this incurs exception overhead. I'd rather do
> something like:
>> foo=queue.pop(0,marker)
> if foo is marker:
> ... empty queue case
> else:
> ... non-empty case, do something with foo

It's both clever and pretty. OTOH, the original try/except isn't expensive
unless the "except" triggers frequently, in which case (the queue is often
empty) a thread is likely better off with a yielding Queue.get() call.
So this strikes me as useful only for thread micro-optimization, and a kind
of optimization most users should be steered away from anyway. Does anyone
have a real use for this outside of threads? If not, I'd rather it not go
in.
For threads that need an optimized non-blocking probe, I'd write it:
 gotone = 0
 if queue:
 try:
 foo = queue.pop(0)
 gotone = 1
 except IndexError:
 pass
 if gotone:
 # use foo
 else:
 # twiddle thumbs
For the IndexError to trigger there, a thread has to lose its bytecode slice
between a successful "if queue" and the queue.pop, and not get another
chance to run until other threads have emptied the queue.