Re: short-circuiting any/all ?
kj wrote: Arguably, Knuth's "premature optimization is the root of all evil" applies even to readability (e.g. "what's the point of making code optimally readable if one is going to change it completely next day?") The guy who will change it will have to read it. The only waste would be if the code would never be read again. If there were the equivalent of a profiler for code clutter, I guess I could relax my readability standards a bit... ~K Don't relax, just keep up :o) JM -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
In Steven D'Aprano writes: >On Mon, 22 Mar 2010 22:19:57 +, kj wrote: >In any case, the once-off cost of creating or importing a function is >usually quite cheap. As usual, the best advise is not to worry about >optimization until you have profiled the code and learned where the >actual bottlenecks are. Write what reads best, not what you guess might >be faster, until you really know you need the speed and that it is an >optimization and not a pessimation. My preference for map in this case is not due to performance considerations, but to avoid unnecessary code-clutter. I just find, e.g., x = map(int, y) slightly easier on the eyes than x = [int(z) for z in y] This tiny improvement in readability gets negated if one needs to define a function in order to use map. Hence, e.g., I prefer x = [_[0] for _ in y] over x = map(lambda _: _[0], y) and certainly over def _first(seq): return seq[0] x = map(_first, y) Arguably, Knuth's "premature optimization is the root of all evil" applies even to readability (e.g. "what's the point of making code optimally readable if one is going to change it completely next day?") If there were the equivalent of a profiler for code clutter, I guess I could relax my readability standards a bit... ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
On Mon, 22 Mar 2010 22:19:57 +, kj wrote: > In Tim Golden > writes: > >>On 22/03/2010 18:30, kj wrote: >>> Thanks! I'm glad to know that one can get the short circuiting using >>> a map-type idiom. (I prefer map over comprehensions when I don't need >>> to define a function just for the purpose of passing it to it.) > >>In what way does "map" over "comprehensions" save you defining a >>function? > >>any (map (is_invalid, L)) >>any (is_invalid (i) for i in L) > > I was talking in the *general* case. map at the very least requires a > lambda expression, which is a one-time function defintion. But keep in mind that instead of this: map(lambda x,y: x+y, somelist) you can do this: import operator map(operator.add, somelist) In any case, the once-off cost of creating or importing a function is usually quite cheap. As usual, the best advise is not to worry about optimization until you have profiled the code and learned where the actual bottlenecks are. Write what reads best, not what you guess might be faster, until you really know you need the speed and that it is an optimization and not a pessimation. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
In Tim Golden writes: >On 22/03/2010 18:30, kj wrote: >> Thanks! I'm glad to know that one can get the short circuiting >> using a map-type idiom. (I prefer map over comprehensions when I >> don't need to define a function just for the purpose of passing it >> to it.) >In what way does "map" over "comprehensions" save you defining a function? >any (map (is_invalid, L)) >any (is_invalid (i) for i in L) I was talking in the *general* case. map at the very least requires a lambda expression, which is a one-time function defintion. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
On 22/03/2010 18:30, kj wrote: Thanks! I'm glad to know that one can get the short circuiting using a map-type idiom. (I prefer map over comprehensions when I don't need to define a function just for the purpose of passing it to it.) In what way does "map" over "comprehensions" save you defining a function? any (map (is_invalid, L)) any (is_invalid (i) for i in L) TJG -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
In <291d82b7-b13b-4f49-901c-8194f3e07...@e7g2000yqf.googlegroups.com> nn writes: >If you are in Python 3 "any(map(is_invalid, L))" should short circuit. >If you are in Python 2 use "from itertools import imap; >any(imap(is_invalid, L))" Thanks! I'm glad to know that one can get the short circuiting using a map-type idiom. (I prefer map over comprehensions when I don't need to define a function just for the purpose of passing it to it.) And thanks also to the other repliers for pointing out that the comprehension version does what I was asking for. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
On Mar 22, 7:45 am, kj wrote: > I have a list of items L, and a test function is_invalid that checks > the validity of each item. To check that there are no invalid > items in L, I could check the value of any(map(is_invalid, L)). > But this approach is suboptimal in the sense that, no matter what > L is, is_invalid will be executed for all elements of L, even though > the value returned by any() is fully determined by the first True > in its argument. In other words, all calls to is_invalid after > the first one to return True are superfluous. Is there a > short-circuiting counterpart to any(map(is_invalid, L)) that avoids > these superfluous calls? > > OK, there's this one, of course: > > def _any_invalid(L): > for i in L: > if is_invalid(i): > return True > return False > > But is there anything built-in? (I imagine that a lazy version of > map *may* do the trick, *if* any() will let it be lazy.) Yes, that will work: from itertools import imap # lazy version of map any(imap(is_invalid, L) # short-circuits on first True Yet another approach (slightly faster): from itertools import ifilter any(ifilter(is_invalid, L)) Raymond -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
kj wrote: > I have a list of items L, and a test function is_invalid that checks > the validity of each item. To check that there are no invalid > items in L, I could check the value of any(map(is_invalid, L)). > But this approach is suboptimal in the sense that, no matter what > L is, is_invalid will be executed for all elements of L, even though > the value returned by any() is fully determined by the first True > in its argument. In other words, all calls to is_invalid after > the first one to return True are superfluous. Is there a > short-circuiting counterpart to any(map(is_invalid, L)) that avoids > these superfluous calls? > > OK, there's this one, of course: > > def _any_invalid(L): > for i in L: > if is_invalid(i): > return True > return False > > But is there anything built-in? (I imagine that a lazy version of > map *may* do the trick, *if* any() will let it be lazy.) > > TIA! > > ~K If you are in Python 3 "any(map(is_invalid, L))" should short circuit. If you are in Python 2 use "from itertools import imap; any(imap(is_invalid, L))" -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
On Mon, 2010-03-22 at 14:45 +, kj wrote: > I have a list of items L, and a test function is_invalid that checks > the validity of each item. To check that there are no invalid > items in L, I could check the value of any(map(is_invalid, L)). > But this approach is suboptimal in the sense that, no matter what > L is, is_invalid will be executed for all elements of L, any( is_invalid(a) for a in L ) ... generator expression will be lazily computed. Tim -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
kj wrote: I have a list of items L, and a test function is_invalid that checks the validity of each item. To check that there are no invalid items in L, I could check the value of any(map(is_invalid, L)). But this approach is suboptimal in the sense that, no matter what L is, is_invalid will be executed for all elements of L, even though the value returned by any() is fully determined by the first True in its argument. In other words, all calls to is_invalid after the first one to return True are superfluous. Is there a short-circuiting counterpart to any(map(is_invalid, L)) that avoids these superfluous calls? OK, there's this one, of course: def _any_invalid(L): for i in L: if is_invalid(i): return True return False But is there anything built-in? (I imagine that a lazy version of map *may* do the trick, *if* any() will let it be lazy.) TIA! ~K Sounds like unnecessary optimization. Just write def _any_valid(L): return bool([i for i in L if is_valid(i)]) If you really care about speed, meaning if the user experiences some execution duration increase, then the solution you proposed is fine. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: short-circuiting any/all ?
On 22/03/2010 14:45, kj wrote: I have a list of items L, and a test function is_invalid that checks the validity of each item. To check that there are no invalid items in L, I could check the value of any(map(is_invalid, L)). But this approach is suboptimal in the sense that, no matter what L is, is_invalid will be executed for all elements of L, even though the value returned by any() is fully determined by the first True in its argument. In other words, all calls to is_invalid after the first one to return True are superfluous. Is there a short-circuiting counterpart to any(map(is_invalid, L)) that avoids these superfluous calls? OK, there's this one, of course: def _any_invalid(L): for i in L: if is_invalid(i): return True return False But is there anything built-in? (I imagine that a lazy version of map *may* do the trick, *if* any() will let it be lazy.) Have I missed the point of your question, perhaps? This seems to work as lazily as you'd like... def less_than_five (x): print "testing", x return x < 5 L = range (10) print any (less_than_five (i) for i in L) print all (less_than_five (i) for i in L) # for symmetry TJG -- http://mail.python.org/mailman/listinfo/python-list
short-circuiting any/all ?
I have a list of items L, and a test function is_invalid that checks the validity of each item. To check that there are no invalid items in L, I could check the value of any(map(is_invalid, L)). But this approach is suboptimal in the sense that, no matter what L is, is_invalid will be executed for all elements of L, even though the value returned by any() is fully determined by the first True in its argument. In other words, all calls to is_invalid after the first one to return True are superfluous. Is there a short-circuiting counterpart to any(map(is_invalid, L)) that avoids these superfluous calls? OK, there's this one, of course: def _any_invalid(L): for i in L: if is_invalid(i): return True return False But is there anything built-in? (I imagine that a lazy version of map *may* do the trick, *if* any() will let it be lazy.) TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
