Tuple Semantics - Rationale'?

[Quinn Dunkan]

On Wed, 11 Jul 2001 21:40:52 GMT, Nick Perkins <nperkins7 at home.com> wrote:
>
>"Quinn Dunkan" <quinn at yak.ugcs.caltech.edu> wrote in message
>"""
>Large complicated data structures of nested tuples might be a mistake.
>Tuples
>and python's simple pattern matching can be handy for small ad hoc types,
>but
>don't forget about classes.
>"""
>
>Yes,
>...but I have found one good use for such 'tuple gymnastics'.
>( perhaps this is what you mean by 'pattern matching' )

No, what I mean by that is what python calls "tuple unpacking".  For instance:

t = ((green, (10, 20), 1),
     (blue, (20, 30), 0),
     (green, (40, 20), 1))

for (color, (x, y), active) in t:
    ...

def display((color, (x, y), active)):
    ...

map(display, t)

might be quicker and clearer than throwing together some class for the same.
Although some might disagree about that 'def' line (and I might be one of them
<wink>).

>When you want to have a dictionary that uses some type of object as a key,
>but you want 'identical' objects to considered 'the same', you can pack the
>relevant bits of an instance's data into a tuple, and use it as the dict
>key.

In this case I'd write __hash__ and __cmp__ methods and stuff the objects
directly into the dict.

>example:
>Suppose you want to keep track of the color any given x,y point...
>
>>>> class point:
>...  def __init__(self,x,y):
>...   self.x = x
>...   self.y = y
>...  def as_tuple(self):
>...   return (self.x,self.y)
>...
>>>> p1 = point(3,4)
>>>> p2 = point(3,4)
>>>> color = {}
>>>> color[p1]='red'
>>>> color[p2]
>Traceback (most recent call last):
>  File "<interactive input>", line 1, in ?
>KeyError: <__main__.point instance at 01373A6C>

>>> class Point:
...     def __init__(self, x, y):
...         self.x = x
...         self.y = y
...     def __hash__(self):
...         return hash((self.x, self.y))
...     def __cmp__(self, other):
...         return cmp((self.x, self.y), (other.x, other.y))
...
>>> p1 = Point(3, 4)
>>> p2 = Point(3, 4)
>>> color = {}
>>> color[p1] = 'red'
>>> color[p2]
'red'
>>>