Velocity Reviews > short-circuiting any/all ?

# short-circuiting any/all ?

kj
Guest
Posts: n/a

 03-22-2010

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

Tim Golden
Guest
Posts: n/a

 03-22-2010
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...

<code>
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

</code>

TJG

Jean-Michel Pichavant
Guest
Posts: n/a

 03-22-2010
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

Tim Wintle
Guest
Posts: n/a

 03-22-2010
On Mon, 2010-03-22 at 14:45 +0000, 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

nn
Guest
Posts: n/a

 03-22-2010

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))"

Raymond Hettinger
Guest
Posts: n/a

 03-22-2010
On Mar 22, 7:45*am, kj <(E-Mail Removed)> 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

kj
Guest
Posts: n/a

 03-22-2010
In <(E-Mail Removed)> nn <(E-Mail Removed)> 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

Tim Golden
Guest
Posts: n/a

 03-22-2010
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

kj
Guest
Posts: n/a

 03-22-2010
In <(E-Mail Removed)> Tim Golden <(E-Mail Removed)> 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

Steven D'Aprano
Guest
Posts: n/a

 03-23-2010
On Mon, 22 Mar 2010 22:19:57 +0000, kj wrote:

> In <(E-Mail Removed)> Tim Golden
> <(E-Mail Removed)> 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

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