[Python-Dev] PEP 0424: A method for exposing a length hint (original) (raw)

Steven D'Aprano steve at pearwood.info
Sun Jul 15 10:21:13 CEST 2012

Nick Coghlan wrote:

On Sun, Jul 15, 2012 at 9:18 AM, Benjamin Peterson <benjamin at python.org> wrote:

Open questions ==============

There are two open questions for this PEP: * Should list expose a kwarg in it's constructor for supplying a length hint. * Should a function be added either to builtins or some other module which calls _lengthhint_, like builtins.len calls _len_. Let's try to keep this as limited as possible for a public API. Length hints are very useful for any container implementation, whether those containers are in the standard library or not. Just as we exposed operator.index when index was added, we should expose an "operator.lengthhint" function with the following semantics: [...]

As given, length_hint gives no way of distinguishing between iterables and non-iterables:

py> length_hint([]) 0 py> length_hint(42) 0

nor does it give iterable objects a way to indicate that either they don't know their length, or that they are infinite.

I suggest:

Presumably anything that wishes to create a list or other sequence from an object with a hint of -1 could then raise an exception immediately.

-- Steven

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