[Python-Dev] Re: PEP309 re-written (original) (raw)

David Abrahams dave at boost-consulting.com
Mon Apr 5 09:55:38 EDT 2004

Peter Harris <scav at blueyonder.co.uk> writes:

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,mywindow,width=100,height=100,bg='white') ...gives you a callable C that is a factory for Tkinter Canvases parented to mywindow, 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')

C = lambda *args, **kw:
Canvas(my_window, width=100, height=100, bg='white', *args, **kw)

OK, partial is a little shorter in that case. I'm not sure that example justifies a PEP, though.

def _merge(d1, d2):
    d1.update(d2)
    return d1

def partial(f, *args, **kw):
     return lambda *args2, **kw2: f(args + args2, merge(kw2,kw))

??

"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.

A perfect example of what lambda is already good at.

-- Dave Abrahams Boost Consulting www.boost-consulting.com

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