List vs tuples

[Roy Smith]

Isaac To <kkto at csis.hku.hk> wrote:
> And you can use a list here as well.  Indeed, if you want to emulate C
> struct, you probably will use list instead of tuples, since in C, structs
> are mutable.

It's a bit of a misnomer to say that tuples (or lists) emulate C 
structs.  You access the elements of a struct by name, but you access 
the elements of a tuple by index.  A much better emulation of a C struct 
is a degenerate class with no methods.  If I wanted to emulate:

struct myData {
   int x;
   int y;
} datum = {1, 42};

I'd do something like:

class myData:
   pass

datum = myData()
datum.x = 1
datum.y = 42

The reason people sometimes think of tuples as struct-like is because 
they are often used to return multiple values from functions.  To use 
the example often cited, you might do:

x, y = getCoordinates()

where getCoordinates() returns a tuple of two numbers.  It sort of looks 
like a struct because it's got x an y "members", but the x and y names 
are assigned in the unpacking operation, they're not inherent in the 
object returned.  A more "struct-like" way of doing it would be to have 
getCoordinates() return something like the myData object shown above.  
Then you'd have:

point = getCoordinates()

and you could make use of point.x and point.y in your code.

I'm not saying that the idea of returning a tuple and immediately 
unpacking it isn't handy.  It certainly is, and I use it often in the 
code that I write.  But, I think saying that it emulates a C struct 
gives people the wrong idea of what's going on.