optional arguments

[Martin von Loewis]

Silvio Arcangeli <sarcangeli at montrouge.sema.slb.com> writes:

> class Connection:
> 	def __init__(self, ip=def_ip, port=def_port)
> 		...
> 
[...]
> how can I tell wheter no arguments were passed from the user or whether 
> they were passed but they were just like the default values?

There are two options. One is, give them default values that nobody
else could ever pass, like

class NoValue:pass
_novalue = NoValue()
del NoValue

class Connection:
    def __init__(self, ip=_novalue, port=_novalue):
       if ip is _novalue:
         ip = def_ip
         # other actions to perform if ip was not given

Now, this still could be cheated, if somebody really meant
to. Therefore, most people are happy with using None as the default
value:

class Connection:
    def __init__(self, ip=None, port=None):
       if ip is None:
         ip = def_ip
         # other actions to perform if ip was not given

If you really *must* find out what arguments have been passed, use a
varargs syntax:

class Connection:
    def __init__(self, *args):
       try:
         ip = args[0]
       except IndexError:
         ip = def_ip
         # other actions to perform if ip was not given
        
BTW, if you require ip and port always to be passed together, you
might follow the convention of the socket module, i.e. an IPv4 address
is always passed as a pair (host, port):

def_addr = (def_ip, def_port)
class Connection:
    def __init__(self, addr = None):
      if addr:
        ip, port = addr
      else:
        ip, port = def_addr

Regards,
Martin