On Jan 22, 2010, at 11:56 AM, Roald de Vries wrote:
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.
Question out of general interest in the language:
If I would want to generate such functions in a for-loop, what would I
have to do? This doesn't work:
class Move(object):
def __call__(self, direction):
return direction
move = Move()
for f in ['up', 'down', 'right', 'left']:
move.__dict__[f] = lambda: move(f)
... because now 'move.up()' returns 'left' because thats the current
value of f. Is there a way to 'expand' f in the loop? Or a reason that
you never should use this?
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