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

Nick Coghlan ncoghlan at gmail.com
Sun Jul 15 07:16:02 CEST 2012

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.length_hint" function with the following semantics:

def length_hint(obj):
    """Return an estimate of the number of items in obj. This is

useful for presizing containers when building from an iterable.

    If the object supports len(), the result will be exact.

Otherwise, it may over or underestimate by an arbitrary amount. The result will be an integer >= 0. """ try: return len(obj) except TypeError: try: get_hint = obj.length_hint except AttributeError: return 0 hint = get_hint() if not isinstance(hint, int): raise TypeError("Length hint must be an integer, not %r" % type(hint)) if hint < 0: raise ValueError("Length hint (%r) must be >= 0" % hint) return hint

There's no reason to make pure Python container implementations reimplement all that for themselves.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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