What meaning of this ""hello %s you are %s years old" % x"

[Dan Stromberg]

On Sun, Jul 27, 2014 at 11:49 AM, fl <rxjwg98 at gmail.com> wrote:
> In Python, when should you use lists and when tuples?
>
> Sometimes you don't have a choice, for example if you have
>
> "hello %s you are %s years old" % x
> then x must be a tuple.
>
> But if I am the one who designs the API and gets to choose the data types, then
> what are the guidelines?

You should use a tuple when you need something immutable (readonly),
like a dictionary key.  Immutable objects are good for hashing.

You should use a list when you need something mutable (read/write),
like appending over and over.  Mutable objects are not good for
hashing, because their hash value could change.