Feature Requests item #1275677, was opened at 2005-08-29 08:49
Message generated for change (Comment added) made by rhettinger
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>Category: Python Interpreter Core
>Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Antoine Pitrou (pitrou)
Assigned to: Raymond Hettinger (rhettinger)
Summary: add a get() method to sets
Initial Comment:
Hi,
I would like to propose a new method for the builtin
set objects. Currently we have a pop() method which
pops an element from the set. What I often need,
though, is a method that gets an arbitrary element
without removing it (because adding / removing stuff is
dealt with in
another part of the program).
Right now the simplest way to do that is :
value = iter(my_set).next()
There are two problems with this:
1. it's ugly and not very intuitive
2. it is not atomic; this means if another thread
updates the set, I can get a "RuntimeError: dictionary
changed size during iteration" (btw, the message is
slightly wrong, it should be "set" instead of "dictionary")
Although the first problem is rather minor (but
annoying nevertheless), the second one is a real
showstopper in some cases - yes, I did encounter it for
real.
There is a way to avoid the second problem :
value = list(my_set)[0]
But of course, not only it is still ugly, but it is
also highly inefficient when the set is big. So in the
end I am forced to use an explicit lock around the
aforementioned construct (value = iter(my_set).next())
as well as around any other piece of code that can
update the set from another thread. This is tedious and
error-prone, and probably a bit inefficient.
So for the bottom line: it would be in some cases very
useful to have an atomic get() method in addition to
the pop() method on sets. (it could probably be applied
to frozensets and dicts too)
The implementation would probably not be very
difficult, since it's the same as pop() with the
removal feature removed. ;) But I'm not familiar with
the Python internals.
What do you think ?
Regards
Antoine.
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>Comment By: Raymond Hettinger (rhettinger)
Date: 2005-08-29 12:10
Message:
Logged In: YES
user_id=80475
We've looked at a choose() method a couple of times and
rejected it. Since the method gets an arbitrary element, it
might as well get the first one it sees. But that is of
little use in a loop (when do you ever need to get the same
object over and over again?).
Also, a choose method() would need to raise a KeyError if
the set if emtpy and/or provide a default argument like
dict.get() does. This complicates the heck out of using it.
Put the two together and the idea loses any charm.
Also, for mutable sets, a better approach is to pop()
elements from the set and add them back when you're done
with them.
I'm not too concerned about atomicity. For one, that is
almost never a good guide to API design. Second, it is
implementation dependent (i.e. no guarantees for PyPy or
Jython). Three, it generally indicates a problem in your
design (if the set could mutate smaller during a thread
accessing the set, then you have a race condition where the
set could shrink to zero or not). Four, the right way to
deal with atomicity issues is to use locks or control access
via Queue.
I do understand that basic motivation (I have a set, now how
do I a representative element) but find the proposal
lacking. It just doesn't do much for us.
BTW, please post your use case (in a condensed form that
gets to the essentials of why you think this method is needed)..
----------------------------------------------------------------------
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