function argument dependent on another function argument?

[Rob Williscroft]

Aaron Brady wrote in
news:6a10378f-addb-4d56-bc1b-0c382b3cb957 at t26g2000prh.googlegroups.com
in comp.lang.python: 

> On Jan 18, 9:36 am, Paul Rubin <http://phr...@NOSPAM.invalid> wrote:
>> Steven D'Aprano <st... at REMOVE-THIS-cybersource.com.au> writes:
>> > def foo(self, x, y=None):
>> >     if y is None:
>> >         y = self.a
>>
>> > I don't find that clumsy in the least. I find it perfectly readable
>> > and 
>  a
>> > standard idiom.
>>
>> That has the same problem as the earlier version.  If the person
>> passes None, they get self.a.  I prefer:
>>
>>     sentinel = object()
>>     ...
>>
>>     def foo(x, y=sentinel):
>>       if y is sentinel:
>>           y = self.a
> 
> It is too bad that it is so much work to detect whether 'y' was passed
> in the function call directly.  However, sentinel is just as good (or
> nearly); at worst, you need one sentinel per argument per function,

One per Module should be good enough. The only reason None doesen't
suffice is that it has other legitimate uses.  Though to be honest
I would always use None as the sentinel if it wasn't a legitimate
argument.

> which is possible to create, which has a specific meaning.  If you are
> making systematic function calls, e.g. with a dictionary or list, you
> can just use the sentinel in the dictionary.

IIUYC then, one sentinel is still only needed as the missing argument
is indicated by *both* position and value or by name and value (in the
case of a keyword-dictionary), so seperate distinct sentinel objects
aren't required, for example:

SENTINEL = object()

def f( a, b, c = SENTINEL, d = SENTINEL ):
  print( "values: %r" % ( ( a, b, c, d ), ) )
  if c is SENTINEL:
    print( "c is missing" )
  if d is SENTINEL:
    print( "d is missing" )

f( *( 1, 2, SENTINEL, SENTINEL ) )

f( **dict( a = 1 , b = 2, d = 4 ) )

f( **dict( a = 1 , b = 2, d = 4, c = SENTINEL ) )

Rob.
-- 
http://www.victim-prime.dsl.pipex.com/