"in"consistency?

[Terry Reedy]

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).

>>>> [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.

> 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.

Terry J. Reedy