Feature #9145
openQueue#pop(true) return nil if empty instead of raising ThreadError
Description
I propose the non-blocking form of Queue#pop behave like Array#pop and return nil when empty.
Current behavior is to raise a ThreadError, with a message indicating the queue is empty.
For example:
q = Queue.new
begin
loop do
next_item = q.pop(true)
end
rescue ThreadError
queue is empty...or maybe something bad happened¶
end
Instead, this could be
q = Queue.new
while next_item = q.pop(true)
end
Alternatively, raise an exception that is a subclass of ThreadError with a more specific name, such as "QueueEmpty". This would be a small improvement while remaining compatible with existing code.
Updated by Glass_saga (Masaki Matsushita) almost 11 years ago
- Category changed from lib to ext
- Status changed from Open to Feedback
I think we can't change default behavior of Queue#pop(true) because some code expects ThreadError to be raised.
However, it may be possible to introduce new keyword argument like following:
q = Queue.new
while next_item = q.pop(true, exception: false) # it doesn't raise ThreadError and returns nil.
do something¶
end
Updated by normalperson (Eric Wong) almost 11 years ago
"Glass_saga (Masaki Matsushita)" glass.saga@gmail.com wrote:
I think we can't change default behavior of Queue#pop(true) because some code expects ThreadError to be raised.
However, it may be possible to introduce new keyword argument like following:q = Queue.new
while next_item = q.pop(true, exception: false) # it doesn't raise ThreadError and returns nil.do something¶
end
+1 to that. All non-blocking methods (I/O or not) should support
exception: false to avoid (expensive/noisy-on-$DEBUG=$true) exceptions.
Updated by jsc (Justin Collins) almost 11 years ago
Glass_saga (Masaki Matsushita) wrote:
I think we can't change default behavior of Queue#pop(true) because some code expects ThreadError to be raised.
However, it may be possible to introduce new keyword argument like following:q = Queue.new
while next_item = q.pop(true, exception: false) # it doesn't raise ThreadError and returns nil.do something¶
end
That would work for me.
Updated by Anonymous almost 11 years ago
On 11/23/2013 08:30 PM, Glass_saga (Masaki Matsushita) wrote:
I think we can't change default behavior of Queue#pop(true) because some code expects ThreadError to be raised.
However, it may be possible to introduce new keyword argument like following:q = Queue.new
while next_item = q.pop(true, exception: false) # it doesn't raise ThreadError and returns nil.do something¶
end
Or what about a new method, Queue#pop?, which is always non-blocking and
non-raising. It would behave like:
class Queue
def pop?
pop(true)
rescue ThreadError
nil
end
end
q = Queue.new
q << 1
q << 2
q << 3
while x=q.pop?
p x
end
END
output:
1
2
3
Updated by drbrain (Eric Hodel) almost 11 years ago
Note that the current behavior allows you to distinguish between a nil in the queue (returns nil) and no value in the queue (raises ThreadError)
Updated by Glass_saga (Masaki Matsushita) over 7 years ago
- Status changed from Feedback to Closed
Currently, Queue#pop takes non_block flag.
Updated by normalperson (Eric Wong) over 7 years ago
glass.saga@gmail.com wrote:
Issue #9145 has been updated by Glass_saga (Masaki Matsushita).
Status changed from Feedback to Closed
Currently, Queue#pop takes non_block flag.
No, I don't think this should be closed.
I think Justin's point was:
Currently, it is impossible to know if a queue is closed
(permanent condition) or if it is empty (temporary condition).
So at the very least, a different exception should be raised:
Justin Collins wrote:
> Alternatively, raise an exception that is a subclass of
> ThreadError with a more specific name, such as "QueueEmpty".
> This would be a small improvement while remaining compatible
> with existing code.
On a side note, relying on exceptions for flow control has all
the same performance and $DEBUG noise problems it did with
IO#*_nonblock [ruby-core:38666] [Feature #5138]
But thinking of an efficient API for that is tricky :<
Updated by Glass_saga (Masaki Matsushita) over 7 years ago
- Status changed from Closed to Open
Updated by uwe@kubosch.no (Uwe Kubosch) almost 7 years ago
How about a block form where the block is called with the popped element? The method would return false if called with non_block set to true if the queue is empty.
q = Queue.new
q << 1
q << 2
q << 3
while q.pop(true){|x| p x}