"index" method only for mutable sequences??

7stud bbxx789_05ss at yahoo.com
Fri Apr 6 05:20:27 CEST 2007

C.L. wrote:
> I was looking for a function or method that would return the index to the first
> matching element in a list. Coming from a C++ STL background, I thought it might
> be called "find". My first stop was the Sequence Types page of the Library
> Reference (http://docs.python.org/lib/typesseq.html); it wasn't there. A search
> of the Library Reference's index seemed to confirm that the function did not
> exist. A little later I realized it might be called "index" instead. Voila.
> My point is that the docs list and describe it as a method that only exists for
> MUTABLE sequences. Why only for mutables? The class of objects I would expect it
> to cover would be all ordered sequences, or, to phrase it a little more
> pointedly, anything that supports ordered INDEXing. My understanding is that
> dict's don't fall into that class of objects since their ordering is not
> documented or to be depended on. However, tuple's do support ordered indexing,
> so why don't tuple's have an index method?
> P.S.: I know I haven't yet gotten an answer to my "why" question yet, but,
> assuming it's just an oversight or an example of design without the big picture
> in mind, an added benefit to fixing that oversight would be that the "index"
> method's documentation could be moved from the currently odd seeming location on
> the "Mutable Sequence Types" page to a place someone would look for it logically.
> P.P.S.: As much as the elementary nature of my question would make it seem, this
> isn't my first day using Python. I've used it on and off for several years and I
> LOVE Python. It is only because of my love for the language that I question its
> ways, so please don't be overly defensive when I guess that the cause for this
> possible oversight is a lack of design.
> Corey Lubin

Looking around google a little bit, people have been asking that same
questions since at least 1992.  Here is what the BDFL has to say:

Guido van Rossum (Guido.van.Rossum at cwi.nl)
Wed, 04 Dec 91 18:48:34 +0100

>In reply to: Steven D. Majewski: "Why no index for tuples or strings ?"
>Most of the functions that operate on mutable sequences but NOT on
>immutable ones are obviously there because they DO CHANGE the sequence.
>BUT: why no string.index() or tuple.index() ?
>Is this just an oversight ?
>If not, what is the reason?

Umm, there isn't a real good reason. One thing I can say in my
defense is that string and tuple objects have no methods at all, all
operations on these are done with built-in operations like "+" and
"[...]", so adding an "index" method would be a bit of a change in the

For tuples, I suspect such a function would rarely be used; I think
that is most cases where x.index() would be useful, x is generally a
list, whose contents varies in time, rather than a tuple (which cannot
change easily).

For strings, there is a built-in module "string" which exports a
function "index" which searches for substrings, so you can say

string.index('one two three', 'two')

--Guido van Rossum, CWI, Amsterdam <guido at cwi.nl>

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