Symbols as parameters?
Alf P. Steinbach
alfps at start.no
Thu Jan 21 09:30:40 CET 2010
* Martin Drautzburg:
> 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 ...
> ... 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?
>>> def move( direction ):
... print( "move " + str( direction ) )
>>> move( "up" )
>>> class using_directions:
... up = 42
... move( up )
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'up' is not defined
Of course it's an abuse of the language. :-)
So I wouldn't recommend it, but it's perhaps worth having seen it.
Cheers & hth.,
PS: I hope this was not a homework question.
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