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

Fri Feb 3 18:51:31 CET 2006

On Thu, 2 Feb 2006 14:43:51 -0800, Alex Martelli <aleaxit at gmail.com> wrote:
>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. 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.
>Yes, but what semantics do you really want? The above, as I'm sure you know, is
not a direct replacement for the lambda:
 >>> import dis
 >>> foo = lambda x: x.amethod(zip, zop)
 >>>
 >>> def methodcaller(methodname, *a, **k):
 ... def caller(self):
 ... getattr(self, methodname)(*a, **k)
 ... caller.__name__ = methodname
 ... return caller
 ...
 >>> bar = methodcaller('amethod', zip, zop)
 Traceback (most recent call last):
 File "<stdin>", line 1, in ?
 NameError: name 'zop' is not defined
 >>> zop = 'must exist at methodcaller call time'
 >>> bar = methodcaller('amethod', zip, zop)
 >>> dis.dis(foo)
 1 0 LOAD_FAST 0 (x)
 3 LOAD_ATTR 1 (amethod)
 6 LOAD_GLOBAL 2 (zip)
 9 LOAD_GLOBAL 3 (zop)
 12 CALL_FUNCTION 2
 15 RETURN_VALUE
 >>> dis.dis(bar)
 3 0 LOAD_GLOBAL 0 (getattr)
 3 LOAD_FAST 0 (self)
 6 LOAD_DEREF 2 (methodname)
 9 CALL_FUNCTION 2
 12 LOAD_DEREF 0 (a)
 15 LOAD_DEREF 1 (k)
 18 CALL_FUNCTION_VAR_KW 0
 21 POP_TOP
 22 LOAD_CONST 0 (None)
 25 RETURN_VALUE
>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...
-0 ;-)
Regards,
Bengt Richter