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Calling superclass method with variable args?

Michael P. Reilly arcege at shore.net
Thu Jul 15 17:24:48 EDT 1999 
Steve Johnson <slj at pixar.com> wrote:
: Hi,

: I'm a long-time C++ programmer, but I'm really new to Python, so please
: forgive me if this is a dumb question with a simple or obvious answer.

: A fundamental idea in OOP is the ability to have a subclass augment the
: behavior of a superclass's method by redefine that method, and then
: calling the superclass's version of the method before doing anything
: else.  The key to this idea, and to polymorphism in general, is that the
: two methods have the same argument signature so that the superclass's
: method gets passed the same set of arguments from the caller whether the
: subclass overrides the method or not.

: If these methods have a static argument list, this is an easy thing to
: do in Python. Here's an example:

: class Task:
:     def __init__(self,a,b,c):
:         <whatever>

: class Job(Task):
:     def __init__(self,a,b,c):
:         Task.__init__(self,a,b,c)
:         <whatever>

: aJob = Job(x,y,z);

: But what if the method takes unnamed positional and keyword arguments.
: Then, how do you pass these along to the superclass's version of the
: method such that it gets the arguments in the same form that the
: subclass's method did?  Here's the code in question:

: class Task:
:     def __init__(self,a,b,c,*moreargs,**morekeyargs):
:         <whatever>

: class Job(Task):
:     def __init__(self,a,b,c, *moreargs, **morekeyargs):
:         Task.__init__(self,a,b,c,?????)
:         <whatever>

: aJob = Job(x,y,z,m1,m2,m3,m4,m5);

: My question really is...what do I replace the ????? (or that whole line)
: with to get this fundamental OOP behavior?

: This doesn't work:

:         Task.__init__(self,a,b,c,moreargs,morekeyargs)

: because now Task.__init__ will see exactly 5 arguments, instead of some
: arbitrary number.  This almost works:

:         apply(Task.__init__,(self,a,b,c) + moreargs)

: but the extra keyword arguments don't get passed along. So, I took the
: idea one step further and tried to introduce the keyword arguments back
: into the parameter list that gets passed to apply:

: def reconstruct(attribs):
:     r = ()
:     for key in attribs.keys():
:         r = r + (key + "=" + attribs[key]",)
:         print r
:     return r

: class Task:
:     def __init__(self,a,b,c,*moreargs,**morekeyargs):
:         <whatever>

: class Job(Task):
:     def __init__(self,a,b,c, *moreargs, **morekeyargs):
:         apply(Task.__init__, (self,a,b,c) + moreargs +
: reconstruct(morekeyargs))
:         <whatever>

: aJob = Job(x,y,z,m1,m2,m3,m4,m5);

: but Task.__init__() doesn't recognize the keyword parameters as such,
: instead it thinks they are string arguments of the form "foo=bar".

: Can anyone tell me how to get the classic OOP behavior I'm looking for
: here?

You need to look at apply more, it takes up to three arguments:
  apply(function, args[, keywords])

So you could have:
  def __init__(self, a, b, c, *moreargs, **morekeyargs):
    apply(Task.__init__, (self, a, b, c) + moreargs, morekeyargs)

The URL for the documentation of apply() is:
 http://www.python.org/doc/current/lib/built-in-funcs.html#l2h-100

  -Arcege
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