[Python-Dev] any support for a methodcaller HOF?

[Michael Hudson]

Alex Martelli <aleaxit at gmail.com> writes:

> I was recently reviewing a lot of the Python 2.4 code I have written,
> and I've noticed one thing: thanks to the attrgetter and itemgetter
> functions in module operator, I've been using (or been tempted to use)
> far fewer lambdas, particularly but not exclusively in key= arguments
> to sort and sorted.

Interesting.  Something I'd noticed was that *until* the key= argument
to sort appeared, I was hardly using any lambdas at all (most of the
places I had used them were rendered obsolete by list comprehensions).

> Most of those "lambda temptations" will be
> removed by PEP 309 (functional.partial), and most remaining ones are
> of the form:
>     lambda x: x.amethod(zip, zop)
>
> So I was thinking -- wouldn't it be nice to have (possibly in module
> functional, like partial; possibly in module operator, like itemgetter
> and attrgetter -- I'm partial to functional;-) a methodcaller entry
> akin to (...possibly with a better name...):
>
> def methodcaller(methodname, *a, **k):
>     def caller(self):
>         getattr(self, methodname)(*a, **k)
>     caller.__name__ = methodname
>     return caller
>
> ...?  This would allow removal of even more lambdas.
>
> I'll be glad to write a PEP, but I first want to check whether the
> Python-Dev crowd would just blast it out of the waters, in which case
> I may save writing it...

Hmm.

>>> funcTakingCallback(lamda x:x.method(zip, zop))
>>> funcTakingCallback(methodcaller("method", zip, zop))

I'm not sure which of these is clearer really.  Are lambdas so bad?
(FWIW, I haven't internalized itemgetter/attrgetter yet and still tend
to use lambdas instead those too).

A class I wrote (and lost) ages ago was a "placeholder" class, so if
'X' was an instance of this class, "X + 1" was roughly equivalent to
"lambda x:x+1" and "X.method(zip, zop)" was roughly equivalent to your
"methodcaller("method", zip, zop)".  I threw it away when listcomps
got implemented.  Not sure why I mention it now, something about your
post made me think of it...

Cheers,
mwh


-- 
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  If you give someone Lisp, he has any language he pleases.
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