Home Original page

Passing variable-length keyworded arguments list to base class constructor?

Alex Martelli alex at magenta.com
Mon Jul 3 05:35:43 EDT 2000 
Jerome Quelin <jerome.quelin at insalien.org> wrote in message
news:962367281.1198008296 at news.libertysurf.fr...
> "Alex Martelli" <alex at magenta.com> wrote:
> >> I was wondering how to pass variable-length keyworded arguments list
> >> to my base-class constructor? Look at the example code below:
> >Doesn't apply work for __init__ just as for any other function?
> Sure it works for __init__ too, I've already tried this approch, but it
doesn't
> work either. In fact, I'm asking the question for __init__ because I run
into
> this problem when calling constructors, but the question is the same for
every
> inherited and overloaded method.
>
> When using the *args syntax, I get an error when calling my constructor
because
> I'm using keywords arguments.

Ok, so *args is out.


> When using the **args syntax, python fetches all keyworded arguments in
_one_
> dictionnary in my constructor, and I want to keep the arguments in
multiple
> key/value pairs (and not flattened in one dictionnary) since the parent
> constructor is waiting for keyworded arguments too.

But that's what apply does (in part): turn a "flattened in one dictionary"
set of name/value pairs into "keyworded arguments" for the function being
applied.


> If I use the **args syntax and then call the base constructor with:
> Parent.__init__(self, args)
> I got an error since it doesn't wait for a dictionnary as an argument (and
args
> is a dictionnary in this case) but for key=value arguments, with key and
value
> taken from the args dictionnary.

So you should instead call apply(Parent.__init__,(self,),args), shouldn't
you?

Toy example:
class Parent:
    def __init__(self, foo=23, bar=45):
        print foo, bar

class Derived(Parent):
    def __init__(self, **kwargs):
        apply(Parent.__init__, (self,), kwargs)

>>> esem.Derived(foo=99)
99 45
<esem.Derived instance at 16a21c0>
>>>


Of course you can alter the kwargs dictionary as required in your
Derived.__init__ method for whatever special needs you may have;
e.g., suppose you want to change the *default* for bar from 45 to
73 in Derived (only), you might code:

class Derived(Parent):
    def __init__(self, **kwargs):
        if not kwargs.has_key('bar'):
            kwargs['bar']=73
        apply(Parent.__init__, (self,), kwargs)

and now:

>>> esem.Derived(foo=99)
99 73
<esem.Derived instance at 16a6810>
>>>


Alex
More information about the Python-list mailing list