PEP 424 – A method for exposing a length hint
- Alex Gaynor <alex.gaynor at gmail.com>
- Standards Track
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
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
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
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
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
Being able to pre-allocate lists based on the expected size, as
__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.
This document has been placed into the public domain.
Last modified: 2022-03-09 16:04:44 GMT