question od default args

[Trent Mick]

On 26-Jun-2002 Gonçalo Rodrigues wrote:
> Hi,
> 
> When I want default args I usually do
> 
> def (arg = None):
>     ...
> 
> But what if I want to make None a reasonable argument too? That is, I
> want to know if the user has in fact passed an argument, and if not to
> do something special, with None (or whatever object) being a reasonable
> arg to be passed.


You could try something like this:


>>> class _NoArgSpecified:
...     pass
...
>>> def foo(arg=_NoArgSpecified):
...     if arg is _NoArgSpecified:
...         print "no arg specified"
...     else:
...         print "arg:", repr(arg)
...
>>> foo()
no arg specified
>>> foo(None)
arg: None
>>> foo(1)
arg: 1


Mind you, the user *can* still cheat, but you shouldn't have to worry
about that.

>>> foo(_NoArgSpecified)
no arg specified
>>>


Trent


-- 
Trent Mick
TrentM at ActiveState.com