"in"consistency?

David C. Ullrich dullrich at sprynet.com
Tue Jul 8 17:58:44 CEST 2008


In article <mailman.137.1215480143.20628.python-list at python.org>,
 Terry Reedy <tjreedy at udel.edu> wrote:

> David C. Ullrich wrote:
> 
> >>>> 'ab' in 'abc'
> > True
> 
> 'a' in 'abc' works according to the standard meaning of o in collection.
> 
> 'ab' in 'abc' could not work by that standard meaning because strings, 
> as virtual sequences, only contain characters (length 1 strings).  Among 
> built-in collections, this limitation is unique to strings (and bytes, 
> in 3.0).  So in 2.3, 'in' was given a useful extension of meaning that 
> is also unique to strings (and bytes).

Ah, I didn't realize that this was new. Thanks - at least this
means I was right about the way it worked formerly.

> >>>> [1,2] in [1,2,3]
> > False
> 
> [1,2] can be an member of tuples, lists, dicts and other general 
> collections.  [1,2] in collection therefore has that meaning, that it is 
> a single element of collection.  Extending the meaning would conflict 
> with this basic meaning.

Well of course.

> > Is there a reason for the inconsistency? I would
> > have thought "in" would check for elements of a
> > sequence, regardless of what sort of sequence it was...
> 
> It is not an inconsistency but an extension corresponding to the 
> limitation of what an string element can be.

It's an inconsistency. That doesn't mean it's a bad thing or that
I want my money back. It may well be a reasonable inconsistency -
strings _can_ work that way while it's clear lists had better not.
But it's an inconsistency.
 
> Terry J. Reedy

-- 
David C. Ullrich



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