# short-circuiting any/all ?

Discussion in 'Python' started by kj, Mar 22, 2010.

1. ### kjGuest

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

kj, Mar 22, 2010

2. ### Tim GoldenGuest

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

Tim Golden, Mar 22, 2010

3. ### Jean-Michel PichavantGuest

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

Jean-Michel Pichavant, Mar 22, 2010
4. ### Tim WintleGuest

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

Tim Wintle, Mar 22, 2010
5. ### nnGuest

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))"

nn, Mar 22, 2010
6. ### Raymond HettingerGuest

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

Raymond Hettinger, Mar 22, 2010
7. ### kjGuest

In <> 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

kj, Mar 22, 2010
8. ### Tim GoldenGuest

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

Tim Golden, Mar 22, 2010
9. ### kjGuest

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

kj, Mar 22, 2010
10. ### Steven D'ApranoGuest

On Mon, 22 Mar 2010 22:19:57 +0000, 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

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

Steven D'Aprano, Mar 23, 2010
11. ### kjGuest

In <> Steven D'Aprano <> writes:

>On Mon, 22 Mar 2010 22:19:57 +0000, 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

kj, Mar 23, 2010
12. ### Jean-Michel PichavantGuest

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 )

JM

Jean-Michel Pichavant, Mar 23, 2010