Wow - Alex Martelli's 'Black Magic' Pycon notes http://www.python.org/pycon/2005/papers/36/pyc05_bla_dp.pdf
include this gem: > Functions 'r descriptors > def adder(x, y): return x + y > add23 = adder.__get__(23) > add42 = adder.__get__(42) > print add23(100), add42(1000) > 123 1042
This means that you can do (left) currying without a separate curry function (Of course, google reveals that the idea has been discussed before, http://mail.python.org/pipermail/python-dev/2003-October/038933.html)
Although it's less flexible than a general curry function, 'method currying' is much faster, e.g., compare two functions for tail-filtering an iterator:
def filtertail(op, iterable):
"""Recursively filter the tail of an iterator, based on its head
Useful for succinct (though not very fast) implementations
of sieve of eratosthenes among other"""
iterator = iter(iterable)
while 1:
head = iterator.next()
yield head
iterator = it.ifilter(curry(op,Missing,head), iterator)def filtertail2(op, iterable):
"""An alternative to filtertail, using Alex Martelli's observation
that functions are descriptors. Will not work for built-in
functions that lack a __get__ method"""
iterator = iter(iterable)
opcurry = op.__get__
while 1:
head = iterator.next()
yield head
iterator = it.ifilter(opcurry(head), iterator)using these generator functions, a Sieve of Eratosthenes can be written as:
primes = list(filtertail(operator.mod, xrange(2,N))) or primes = list(filtertail2(lambda head, tail: tail % head, xrange(2,N)))
but the second version, using 'method currying' is 4 times the speed, despite not using the stdlib operator.mod function
def timethem(N):
import time
t1 = time.clock()
p = list(filtertail(op.mod, xrange(2,N)))
t2 = time.clock()
p = list(filtertail2(lambda head, tail: tail % head, xrange(2,N)))
t3 = time.clock()
return t2-t1, t3-t2>>> timethem(10000) (3.8331997502475588, 0.79605759949936328) >>> timethem(100000) (240.68151008019186, 61.818026872130304) >>>
of course, neither version is anywhere near the most efficient Python implementation - this is a comparison of currying, not sieving.
BTW, here's the curry function I used (it could probably be faster; I'm not sure what/where the future stdlib version is)
Missing = Ellipsis def curry(*cargs, **ckwargs): fn, cargs = cargs[0], cargs[1:] if cargs[0] is Missing: while cargs[0] is Missing: # rightcurry cargs = cargs[1:] def call_fn(*fargs, **fkwargs): d = ckwargs.copy() d.update(fkwargs) return fn(*(fargs+cargs),**d) name = "%s(...,%s)" % (fn.__name__, ",".join(repr(i) for i in cargs)) else: def call_fn(*fargs, **fkwargs): d = ckwargs.copy() d.update(fkwargs) return fn(*(cargs + fargs), **d) name = "%s(%s,...)" % (fn.__name__, ",".join(repr(i) for i in cargs)) call_fn.func_name = name call_fn.curry = True return call_fn
Michael
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