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

[Bengt Richter]

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