Kevin D. Smith wrote: > On 2009-05-07 23:48:43 -0500, Chris Rebert <c...@rebertia.com> said: > >> On Thu, May 7, 2009 at 2:23 PM, Kevin D. Smith <kevin.sm...@sas.com> >> wrote: >>> I need the behavior of heapq.merge to merge a bunch of results from a >>> database. I was doing this with sorted(itertools.chain(...), key= >> ...), but >>> I would prefer to do this with generators. My issue is that I need >> the key >>> argument to sort on the correct field in the database. heapq.merge >> doesn't >>> have this argument and I don't understand the code enough to know if >>> it's possible to add it. Is this enhancement possible without >>> drasticall >> y >>> changing the current code? >> >> I think so. Completely untested code: >> >> def key(ob): >> #code here >> >> class Keyed(object): >> def __init__(self, obj): >> self.obj = obj >> def __cmp__(self, other): >> return cmp(key(self.obj), key(other.obj)) >> >> def keyify(gen): >> for item in gen: >> yield Keyed(item) >> >> def stripify(gen): >> for keyed in gen: >> yield keyed.obj >> >> merged = stripify(merge(keyify(A), keyify(B), keyify(C))) #A,B,C being >> the iterables > > Ah, that's not a bad idea. I think it could be simplified by letting > Python do the comparison work as follows (also untested). > > def keyify(gen, key=lamda x:x): > for item in gen: > yield (key(item), item) > > def stripify(gen): > for item in gen: > yield item[1]
This is not as general. It makes sorting unstable and can even fail if the items are unorderable: >>> def decorate(gen, key): ... for item in gen: ... yield key(item), item ... >>> def undecorate(gen): ... for item in gen: ... yield item[-1] ... >>> sorted(decorate([1j, -1j, 1+1j], key=lambda c: c.real)) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: no ordering relation is defined for complex numbers One way to fix it: >>> def decorate(gen, key): ... for index, item in enumerate(gen): ... yield key(item), index, item ... >>> sorted(decorate([1j, -1j, 1+1j], key=lambda c: c.real)) [(0.0, 0, 1j), (0.0, 1, -1j), (1.0, 2, (1+1j))] >>> list(undecorate(_)) [1j, -1j, (1+1j)] Peter -- http://mail.python.org/mailman/listinfo/python-list
