PEP: 424 Title: A method for exposing a length hint Version: $Revision$
Last-Modified: $Date$ Author: Alex Gaynor <alex.gaynor@gmail.com>
Status: Final Type: Standards Track Content-Type: text/x-rst Created:
14-Jul-2012 Python-Version: 3.4 Post-History: 15-Jul-2012

Abstract

CPython currently defines a __length_hint__ method on several types,
such as various iterators. This method is then used by various other
functions (such as list) to presize lists based on the estimate returned
by __length_hint__. Types which are not sized, and thus should not
define __len__, can then define __length_hint__, to allow estimating or
computing a size (such as many iterators).

Specification

This PEP formally documents __length_hint__ for other interpreters and
non-standard-library Python modules to implement.

__length_hint__ must return an integer (else a TypeError is raised) or
NotImplemented, and is not required to be accurate. It may return a
value that is either larger or smaller than the actual size of the
container. A return value of NotImplemented indicates that there is no
finite length estimate. It may not return a negative value (else a
ValueError is raised).

In addition, a new function operator.length_hint hint is added, with the
following semantics (which define how __length_hint__ should be used):

    def length_hint(obj, default=0):
        """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 under-estimate by an
        arbitrary amount. The result will be an integer >= 0.
        """
        try:
            return len(obj)
        except TypeError:
            try:
                get_hint = type(obj).__length_hint__
            except AttributeError:
                return default
            try:
                hint = get_hint(obj)
            except TypeError:
                return default
            if hint is NotImplemented:
                return default
            if not isinstance(hint, int):
                raise TypeError("Length hint must be an integer, not %r" %
                                type(hint))
            if hint < 0:
                raise ValueError("__length_hint__() should return >= 0")
            return hint

Rationale

Being able to pre-allocate lists based on the expected size, as
estimated by __length_hint__, can be a significant optimization. CPython
has been observed to run some code faster than PyPy, purely because of
this optimization being present.

Copyright

This document has been placed into the public domain.



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