On Thu, Dec 15, 2011 at 6:56 PM, Ned Batchelder <ned@nedbatchelder.com> wrote:
On 12/15/2011 8:39 PM, Guido van Rossum wrote:
On Thu, Dec 15, 2011 at 5:16 PM, Terry Reedy <tjreedy@udel.edu> wrote:
On 12/15/2011 3:42 PM, Ned Batchelder wrote:

This is another place where Python is inconsistent. We're told, "lists
are for homogenous sequences of varying length, like a C array; tuples
are for heterogenous aggregations of known length, like a C struct."

I have not been told that for several years, and I am pretty sure you will not find any such thing in the current docs. I consider it pretty much obsolete, as the differences that flowed from that idea are gone. In Python 3, tuples have all the non-mutating sequence methods that list does. The situation was much different in 1.4.

I strongly disagree. Being immutable sequences (i.e. homogeneous) is a minor secondary role for tuples. Their primary role remains to hold a small bunch of heterogeneous values -- like namedtuple, but without needing forethought. A good example are dictionary items -- these are (key, value) pairs where for a given dict, the keys are all of the same type (or of a small set of related types) and ditto for the values, but the key type and the value types are unrelated.
Could you explain why the foo(*args) syntax creates args as a tuple rather than a list?

It's a historical accident. In ABC, functions didn't have multiple arguments -- instead there was a single argument that was a tuple. If you go back way, way in Python's early history (before 1.0 I think) you'll find that arguments kind of worked the same way. Then an idiom developed to accept a variable number of arguments, in order to support default argument values (because Python's tuples, unlike ABC's, were sequences even then). This turned out to be awkward if you wanted 1 or 2 arguments (the 1-arg case had to be dealt with special because you'd receive a plain value instead of a tuple) and then the *args and arg=value syntax was invented. But because previously varargs had been tuples, we kept them tuples. (I think this was also in the days that you couldn't unpack a list -- but I'm not actually sure of that. I do know that tuple unpacking was always in the language.)

--Guido van Rossum (python.org/~guido)