Missing values in tuple assignment

[Terry Reedy]

Jim Garrison wrote:
> Use case: parsing a simple config file line where lines start with a
> keyword and have optional arguments.  I want to extract the keyword and
> then pass the rest of the line to a function to process it. An obvious
> use of split(None,1)
> 
>     cmd,args= = line.split(None,1);
>     if cmd in self.switch: self.switch[cmd](self,args)
>     else: self.errors.append("unrecognized keyword '{0)'".format(cmd))
> 
> Here's a test in IDLE:
> 
>  >>> a="now is the time"
>  >>> x,y=a.split(None,1)
>  >>> x
>  'now'
>  >>> y
>  'is the time'
> 
> However, if the optional argument string is missing:
> 
>  >>> a="now"
>  >>> x,y=a.split(None,1)
>  Traceback (most recent call last):
>    File "<pyshell#42>", line 1, in <module>
>      x,y=a.split(None,1)
>  ValueError: need more than 1 value to unpack
> 
> I understand the problem is not with split() but with the assignment
> to a tuple.  Is there a way to get the assignment to default the
> missing values to None?

In 3.0 (and 2.6?), the following is close:

 >>> a,*b = (1,2)
 >>> a,b
(1, [2])
 >>> a,*b = (1,)
 >>> a,b
(1, [])