[Python-Dev] Re: PEP309 re-written

[Peter Harris]

David Abrams wrote:

>> If anyone can think of any really elegant hacks that are naturally
>> expressed by partial function application I'd like to see them
>  
>
>
>There are millions, but AFAICT they're all more natural with lambda,
>so...
>
>        "I agree that lambda is usually good enough, just not
>        always."
>
>Examples, please?
>  
>
Well, you can use partial to special-case classes as shorthand object 
factories.  So:

C = partial(Canvas,my_window,width=100,height=100,bg='white')

...gives you a callable C that is a factory for Tkinter Canvases 
parented to my_window,
100 pixels on a side, and with a white background.  How this differs 
from lambda is that
you can override keyword parameters from the supplied defaults:

c1 = C()

c2 = C(width=200)

c3 = C(bg='red')

>        "And I want the possibility of useful introspection and
>        subclassing"
>
>Can you elaborate on that, please?
>
>  
>
You could maybe sub-class partial to override __call__ , in case you 
want to do anything fancy like pre-supplying arguments at arbitrary 
positions.

You can inspect or indeed change C.fn, C.kw or C.args in the example 
above. I'm not speculating on why you might want to, but there's nothing 
stopping you. :)

Peter Harris