PEP: 424 Title: A method for exposing a length hint Version: $Revision$ Last-Modified: $Date$ Author: Alex Gaynor Status: Final Type: Standards Track Content-Type: text/x-rst Created: 14-July-2012 Python-Version: 3.4 Post-History: https://mail.python.org/pipermail/python-dev/2012-July/120920.html 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. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: