for: else: - any practical uses for the else clause?

[metaperl.etc@gmail.com]

A very old thread:

http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gst&q=for+else&rnum=9#1542d2041257c47e

discusses the optional "else:" clause of the for statement.

I'm wondering if anyone has ever found a practical use for the else
branch?

[skip@pobox.com]

Re: for: else: - any practical uses for the else clause?


metaperlI'm wondering if anyone has ever found a practical use for the
metaperlelse branch?

Yeah, I use it from time to time:

for foo in bar:
if foo matches some condition:
print "sail to tahiti!"
break
else:
print "abandon ship!"

Skip

[Amaury Forgeot d'Arc]

Re: for: else: - any practical uses for the else clause?

metaperl.etc@gm ail.com a écrit :

A very old thread:

http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gst&q=for+else&rnum=9#1542d2041257c47e

>
discusses the optional "else:" clause of the for statement.
>
I'm wondering if anyone has ever found a practical use for the else
branch?
>

I use it regularly in contructs like:

for value in someList:
if someCondition(v alue):
print "a matching item was found"
break
else:
print "no matching item"
return False

# ... continue with value

--
Amaury

[Bruno Desthuilliers]

Re: for: else: - any practical uses for the else clause?

metaperl.etc@gm ail.com a écrit :

A very old thread:

http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gst&q=for+else&rnum=9#1542d2041257c47e

>
discusses the optional "else:" clause of the for statement.
>
I'm wondering if anyone has ever found a practical use for the else
branch?
>

<aol>
Just like Skip and Amaury...
</aol>

[Fuzzyman]

Re: for: else: - any practical uses for the else clause?


Amaury Forgeot d'Arc wrote:

metaperl.etc@gm ail.com a écrit :

A very old thread:

http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gst&q=for+else&rnum=9#1542d2041257c47e

discusses the optional "else:" clause of the for statement.

I'm wondering if anyone has ever found a practical use for the else
branch?

>
I use it regularly in contructs like:
>
for value in someList:
if someCondition(v alue):
print "a matching item was found"
break
else:
print "no matching item"
return False
>
# ... continue with value

Me too...

Same with while loops.

Fuzzyman

http://www.voidspace.org.uk/python/index.shtml

--
Amaury

[Ben Sizer]

Re: for: else: - any practical uses for the else clause?

skip@pobox.com wrote:

metaperlI'm wondering if anyone has ever found a practical use for the
metaperlelse branch?
>
Yeah, I use it from time to time:
>
for foo in bar:
if foo matches some condition:
print "sail to tahiti!"
break
else:
print "abandon ship!"

As a C++ programmer (which I'm sure undermines my argument before
you've even read it...), this feels 'backwards' to me. Although I am no
purist, the 'else' typically implies failure of a previous explicit
condition, yet in this case, it's executed by default, when the
previous clause was successfully executed. It would seem more natural
if the else clause was triggered by 'bar' being empty, or even if the
loop was explicitly broken out of, though I'm sure that would make the
construct much less useful.

--
Ben Sizer

[Sion Arrowsmith]

Re: for: else: - any practical uses for the else clause?

Ben Sizer <kylotan@gmail. comwrote:

>skip@pobox.c om wrote:

>Yeah, I use it from time to time:
>>
> for foo in bar:
> if foo matches some condition:
> print "sail to tahiti!"
> break
> else:
> print "abandon ship!"

>
>As a C++ programmer (which I'm sure undermines my argument before
>you've even read it...), this feels 'backwards' to me. Although I am no
>purist, the 'else' typically implies failure of a previous explicit
>condition, yet in this case, it's executed by default, when the
>previous clause was successfully executed. It would seem more natural
>if the else clause was triggered by 'bar' being empty, [ ... ]

It does:

>>for foo in []:

.... print foo
.... else:
.... print 'else'
....
else

I think it's clearer to see by comparing while with if:

if False:
do nothing
else:
do something

while False:
do nothing
else:
do something

and getting to the for behaviour from while is trivial.

That said, I've not had much call for it and was kind of surprised to
find myself writing a genuine for ... else the other week. But it was
the obvious way to do the task at hand, and I was happy it was there.

--
\S -- siona@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[metaperl]

Re: for: else: - any practical uses for the else clause?

Actually right after posting this I came up with a great usage. I use
meld3 for my Python based dynamic HTML generation. Whenever I plan to
loop over a tree section I use a for loop, but if there is no data to
iterate over, then I simply remove that section from the tree or
populate it with a "no data" message.

[Klaas]

Re: for: else: - any practical uses for the else clause?

metaperl wrote:

Actually right after posting this I came up with a great usage. I use
meld3 for my Python based dynamic HTML generation. Whenever I plan to
loop over a tree section I use a for loop, but if there is no data to
iterate over, then I simply remove that section from the tree or
populate it with a "no data" message.

else: does not trigger when there is no data on which to iterate, but
when the loop terminated normally (ie., wasn't break-ed out). It is
meaningless without break.

Python 2.4.3 (#1, Mar 29 2006, 15:37:23)
[GCC 4.0.2 20051125 (Red Hat 4.0.2-8)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>for x in []:

.... print 'nothing'
.... else:
.... print 'done'
....
done

-Mike

[Klaas]

Re: for: else: - any practical uses for the else clause?

Klaas wrote:

else: does not trigger when there is no data on which to iterate, but
when the loop terminated normally (ie., wasn't break-ed out). It is
meaningless without break.

Sorry, this was worded confusingly. "else: triggers when the loop
terminates normally, not simply in the case that there is no iterated
data".

-Mike

[Mike Klaas]

Re: for: else: - any practical uses for the else clause?

On 9/29/06, Johan Steyn <johan.steyn@gm ail.comwrote:

On 29 Sep 2006 11:26:10 -0700, Klaas <mike.klaas@gma il.comwrote:
>

else: does not trigger when there is no data on which to iterate, but
when the loop terminated normally (ie., wasn't break-ed out). It is
meaningless without break.

>
The else clause *is* executed when there is no data on which to iterate.
Your example even demonstrates that clearly:

Yes--there is a missing "just" in that sentence.

-Mike

[BJörn Lindqvist]

Re: for: else: - any practical uses for the else clause?

On 9/29/06, Johan Steyn <johan.steyn@gm ail.comwrote:

I agree that it is meaningless without a break statement, but I still find
it useful when I want to determine whether I looped over the whole list or
not. For example, if I want to see whether or not a list contains an odd
number:
>
for i in list:
if i % 2 == 1:
print "Found an odd number."
break
else:
print "No odd number found."
>
Without the else clause I would need to use an extra variable as a "flag"
and check its value outside the loop:

You can use generator comprehension:

if (i for i in list if i % 2 == 1):
print "Found an odd number."
else:
print "No odd number found."

I *think* any() should also work:

if any(i % 2 == 1 in list):
....

And so on. For every use of the for/else clause there exists a better
alternative. Which sums up my opinion about the construct -- if you
are using it, there's something wrong with your code.

--
mvh Björn

[Matthew Woodcraft]

Re: for: else: - any practical uses for the else clause?

bjourne@gmail.c om wrote:

And so on. For every use of the for/else clause there exists a better
alternative. Which sums up my opinion about the construct -- if you
are using it, there's something wrong with your code.

How do you transform this?

height = 0
for block in stack:
if block.is_marked ():
print "Lowest marked block is at height", height
break
height += block.height
else:
raise SomeError("No marked block")

-M-

[Paul Rubin]

Re: for: else: - any practical uses for the else clause?

Matthew Woodcraft <mattheww@chiar k.greenend.org. ukwrites:

How do you transform this?
>
height = 0
for block in stack:
if block.is_marked ():
print "Lowest marked block is at height", height
break
height += block.height
else:
raise SomeError("No marked block")

Untested:

all_heights = [block.height for block in stack if block.is_marked ()]
if all_heights:
height = sum(all_heights )
else:
raise SomeError("No marked block")

Alternatively (lower memory usage for large list):

all_heights = (block.height for block in stack if block.is_marked ())
try:
height = all_heights.nex t()
height += sum(all_heights )
except StopIteration:
raise SomeError("No marked block")

[Sybren Stuvel]

Re: for: else: - any practical uses for the else clause?

Paul Rubin enlightened us with:

>height = 0
>for block in stack:
> if block.is_marked ():
> print "Lowest marked block is at height", height
> break
> height += block.height
>else:
> raise SomeError("No marked block")

>
all_heights = [block.height for block in stack if block.is_marked ()]
if all_heights:
height = sum(all_heights )
else:
raise SomeError("No marked block")
>
Alternatively (lower memory usage for large list):
>
all_heights = (block.height for block in stack if block.is_marked ())
try:
height = all_heights.nex t()
height += sum(all_heights )
except StopIteration:
raise SomeError("No marked block")

I must say that the for/else construct is a LOT more readable than the
rewritten alternatives.

Sybren
--
Sybren Stüvel
Stüvel IT - http://www.stuvel.eu/