Roald de Vries wrote:
<div class="moz-text-flowed" style="font-family: -moz-fixed">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.
Once pointed down that road, how about:
class Move(object):
def __call__(self, direction):
print(direction)
return 0
@property
def up(self):
return self('up')
move = Move()
Now you can just say
move.up
with no parentheses.
When I've wanted a DSL (Domain Specific Language) in the past, I've used
Forth. It has so little syntax of its own, it's not hard to design your
own simple syntax for a particular problem. And one of the things you
can do is to define keywords that intercept the compile process of the
following token (or line, or whatever).
I am not claiming it's quick to become that proficient in Forth,
however. The learning curve has a couple of steep sections, and this
sort of thing is one of them.
DaveA
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