Symbols as parameters?

[Roald de Vries]

Hi Martin,

On Jan 21, 2010, at 8:43 AM, Martin Drautzburg wrote:
> Hello all,
>
> When passing parameters to a function, you sometimes need a paramter
> which can only assume certain values, e.g.
>
>        def move (direction):
>                ...
> If direction can only be "up", "down", "left" or "right", you can  
> solve
> this by passing strings, but this is not quite to the point:
>
>        - you could pass invalid strings easily
>        - you need to quote thigs, which is a nuisance
>        - the parameter IS REALLY NOT A STRING, but a direction
>
> Alternatively you could export such symbols, so when you "import *"  
> you
> have them available in the caller's namespace. But that forces you
> to "import *" which pollutes your namespace.
>
> What I am really looking for is a way
>
>        - to be able to call move(up)
>        - having the "up" symbol only in the context of the function  
> call
>
> So it should look something like this
>
> ... magic, magic ...
> move(up)
> ... unmagic, unmagic ...
> print up
>
> This should complain that "up" is not defined during the "print" call,
> but not when move() is called. And of course there should be as little
> magic as possible.
>
> Any way to achieve this?

You could do something like this:

class Move(object):
     def __call__(self, direction):
         print(direction)
         return 0

     def up(self):
         return self('up')

move = Move()

Now move.up() means move('up'), and you can obviously do similar  
things for other directions.