python-dev summary, July 16-31

[Guido van Rossum]

bellman at lysator.liu.se (Thomas Bellman) writes:

> To me, a tuple isn't something that you iterate over.  A tuple is
> a lightweight record.  The equivalent of a struct i C, but with
> unnamed members.  The difference between lists and tuples isn't
> really that tuples are read-only, but in how they are used.
> 
> A list typically holds a number of homogeneous elements.  All
> elements are to be treated more or less the same.  The length
> of the list is often not fixed.  The elements of the list might
> be of different Python types or classes, 
> 
> The elements of a tuple, on the other hand, are heterogeneous.
> They might be of the same Python type, but they don't mean the
> same thing.  An example is the return value from time.gmtime():
> the elements are all integers, but they have vastly different
> meanings.  Coordinates is another example: the X, Y and Z
> coordinates in a three-dimensional room have distinct meanings,
> and it is seldom meaningful to iterate over the three values in a
> single coordinate triple and doing a certain operation on each of
> them.  In C you would implement a coordinate as
> 
>     struct coord_3d {
> 	double x,y,z;
>     };
> 
> not as
> 
>     typedef double coord_3d[3];
> 
> Sure, there are examples when you want to iterate over the
> elements in the struct/tuple, like when implementing matrix
> multiplication, but those are special cases, not the norm for
> how a tuple *should* be used.

Exactly.

Thanks for an excellent explanation of the difference between tuples
and lists!

--Guido van Rossum (home page: http://www.python.org/~guido/)