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On 07/24/2010 05:08 AM, Robert Collins wrote:
> I know that there is some work underway at the moment to defer the
> point where we call len() on collections. I'd like feedback on an even
> more ambitious proposal:
>
> - not calling len() ever
What supports len() that is not a collection?
> I need input here - where do people use len(), why do they use len(),
> what would the impact of nuking it be? We need this input to build
> better interfaces - ones that scale and perform well.
I use len when testing for the empty list. I don't think that
evaluating the boolean value of a list is precise enough, especially
since lists are sometimes None, and that can have radically different
semantics from [].
I recently used len when calculating the median duration of a build.
I also used it to decide how many recent recipe builds to show, based on
the number of pending builds.
> Some inputs that lead me to proposing this goal:
> - len() is a precise interface
>
> - highly precise counting is extremely expensive.
Are you sure? I believe that highly-precise counting *may* be
expensive, but I believe that there are counterexamples where it's quite
cheap, e.g. python lists.
> - the results of such counting are also stale almost immediately:
> API's query in separate transactions each time
>
> - its not useful for users [200000 open bugs vs 200001 is a
> near-valueless distinction]
How often is it sensible to deal with that many bugs at once anyhow?
I hold that there are huge distinctions between zero and one, and pretty
significant distinctions between values in the range of 1-10. After
that, the significance of a single increment gets pretty small.
> So far, I've thought of two replacement interfaces:
> - estimate_size(collection) => {0..99, hundreds, thousands, millions...}
> This would be used for providing UI feedback on collections
>
> - closed_since|changed_since parameters on various searches, so that
> the use of len() to generate trend lines is able to be done - we can
> precisely identify recent work without precisely identifying total
> unfiltered collection size.
>
> What do you think?
I don't think your rationale justifies avoiding len entirely. I also
could have sworn that ResultSet didn't support len anyhow-- if I want
the number of results, I call ResultSet.count.
Aaron
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