why does the % string format operator only take tuples?

[Roy Smith]

"Alex Martelli" <aleaxit at yahoo.com> wrote:
> lists are homogeneous, tuples need not be

But python is perfectly happy to have heterogeneous lists:

>>> foo = [1, 'two', type(3)]
>>> foo
[1, 'two', <type 'int'>]

On the other hand, if you consider that to be a convention, it does at 
least explain why, for example, the DB-API has fetchall() returning a list 
of tuples: each individual row is heterogeneous (tuple), but the collection 
of rows themselves is homogeneous (list).

I've been wondering about that for three years.  Is there anything else you 
guys havn't told me? :-)