[Python-Dev] Fwd: Range contains and objects with index methods (original) (raw)

Mark Dickinson dickinsm at gmail.com
Mon Dec 27 13:46:21 CET 2010

.. and here's my original reply to Nick, which was also intended to go to the list. Sorry, folks.

Mark

---------- Forwarded message ---------- From: Mark Dickinson <dickinsm at gmail.com> Date: Mon, Dec 27, 2010 at 10:27 AM Subject: Re: [Python-Dev] Range contains and objects with index methods To: Nick Coghlan <ncoghlan at gmail.com>

On Mon, Dec 27, 2010 at 12:15 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

Starting in Python 3.2, range() supports fast containment checking for integers (i.e. based on an O(1) arithmetic calculation rather than an O(N) iteration through the entire sequence).

Currently, this fast path ignores objects that implement index - they are relegated to the slow iterative search. This seems wrong to me [...]

Is seems to me that there are two separate issues here. Namely, (1) Determine the semantics of 'x in range(...)' for an object x that implements index, and (2) implement the containment check efficiently.

At the moment, it looks as though the index method is ignored entirely for the containment check, so you're proposing a change in semantics.  If/when that change in semantics is made, fixing up the code to do the containment check efficiently seems like it should be a straightforward task.

With the proposed change in semantics, the simple definition (x in range(...) iff x == y for some element y of range(...)) no longer holds.  I'm not convinced (but not unconvinced either) that it's worth breaking that simplicity.

Mark

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