[Python-Dev] _length_cue() (original) (raw)

Armin Rigo arigo at tunes.org
Wed Feb 8 15:20:34 CET 2006

Hi all,

Last september, the len method of iterators was removed -- see discussion at:

http://mail.python.org/pipermail/python-dev/2005-September/056879.html

It was replaced by an optional undocumented method called _length_cue(), which would be used to guess the number of remaining items in an iterator, for performance reasons.

I'm worried about the name. There are now exactly two names that behave like a special method without having the double-underscores around it. The first name is 'next', which is kind of fine because it's for iterator classes only and it's documented. But now, consider: the CPython implementation can unexpectedly invoke a method on a user-defined iterator class, even though this method's name is not '*' and not documented as special! That's new and that's bad.

IMHO for safety reasons we need to stick double-underscores around this name too, e.g. length_cue(). It's new in 2.5 and not documented anyway so this change won't break anything. Do you agree with that?

BTW the reason I'm looking at this is that I'm considering adding another undocumented internal-use-only method, maybe getitem_cue(), that would try to guess what the nth item to be returned will be. This would allow the repr of some iterators to display more helpful information when playing around with them at the prompt, e.g.:

enumerate([3.1, 3.14, 3.141, 3.1415, 3.14159, 3.141596]) <enumerate (0, 3.1), (1, 3.14), (2, 3.141),... length 6>

A bientot,

Armin

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