for what are for/while else clauses

[Diez B. Roggisch]

Hi,

today I rummaged through the language spec to see whats in the for ... else:
for me. I was sort of disappointed to learn that the else clauses simply
gets executed after the loop-body - regardless of the loop beeing entered
or not.

So where is an actual use case for that feature?

I imagined that the else-clause would only be executed if the loop body
wasn't entered, so I could write this

for r in result:
print r
else:
print "Nothing found, dude!"

instead of

if len(result):
for r in result
print r
else:
print "Nothing found, dude!"

waiting for enlightment,

Diez

[Magnus Lyck?]

"Fredrik Lundh" <fr*****@python ware.com> wrote in message news:<ma******* *************** **************@ python.org>...

the break statement has nothing to do with the else clause; the else
is executed when there's no more item in the sequence. if you break
or raise or return or yield-and-never-resume or call-and-never-return
your way out of the loop doesn't matter.

On the other hand, I only think it's in the case of a break
in block that...

while condition:
block
else:
print "Loop finished"

....will behave differently than...

while condition:
block
print "Loop finished"

So, unless you use a break in the block, the else statement
is just noise: an extra line of code and additional whitespace
for the following statement(s).

It's only together with the python idiom of breaking out of
loops with ifs (which is considered bad form in some (non
python) circles that the else part of the for and while statements
make sense.

[Aahz]

In article <25************ **************@ posting.google. com>,
Magnus Lyck? <ma****@thinkwa re.se> wrote:

On the other hand, I only think it's in the case of a break
in block that...

while condition:
block
else:
print "Loop finished"

...will behave differently than...

while condition:
block
print "Loop finished"

So, unless you use a break in the block, the else statement is just
noise: an extra line of code and additional whitespace for the
following statement(s).

While technically correct, I don't think you could claim that it's "just
noise" in this example:

try:
for ORlist in includes:
try:
for filter in ORlist:
for field in curr_fields:
for item in record[field]:
if match(item, filter):
raise Found
else:
raise NotFound
except Found:
continue
except NotFound:
continue
else:
result.append(r ecord)
--
Aahz (aa**@pythoncra ft.com) <*> http://www.pythoncraft.com/

Weinberg's Second Law: If builders built buildings the way programmers wrote
programs, then the first woodpecker that came along would destroy civilization.

[Magnus Lyck?]

aa**@pythoncraf t.com (Aahz) wrote in message news:<bp******* ***@panix2.pani x.com>...

In article <25************ **************@ posting.google. com>,
Magnus Lyck? <ma****@thinkwa re.se> wrote:

On the other hand, I only think it's in the case of a break
in block that...

while condition:
block
else:
print "Loop finished"

...will behave differently than...

while condition:
block
print "Loop finished"

So, unless you use a break in the block, the else statement is just
noise: an extra line of code and additional whitespace for the
following statement(s).

While technically correct, I don't think you could claim that it's "just
noise" in this example:

try:
for ORlist in includes:
try:
for filter in ORlist:
for field in curr_fields:
for item in record[field]:
if match(item, filter):
raise Found
else:
raise NotFound
except Found:
continue
except NotFound:
continue
else:
result.append(r ecord)

First of all, I'm not sure it's a good idea to use
exceptions like a poor (rich?) man's GOTO like this. :)

If you persist, I'm not so sure your code is clearer than:

try:
for ORlist in includes:
try:
for filter in ORlist:
for field in curr_fields:
for item in record[field]:
if match(item, filter):
raise Found
raise NotFound
except Found:
continue
except NotFound:
continue
else:
result.append(r ecord)

With the else clause on the for loop, just after the if
statement, you are bound to confuse people. I would guess
that most people who casually looked at that code would
think that you had made an indentation mistake. Some would
probably "correct" it.

[Michele Simionato]

ma****@thinkwar e.se (Magnus Lyck?) wrote in message news:<25******* *************** ****@posting.go ogle.com>...

First of all, I'm not sure it's a good idea to use
exceptions like a poor (rich?) man's GOTO like this. :)

If you persist, I'm not so sure your code is clearer than:

try:
for ORlist in includes:
try:
for filter in ORlist:
for field in curr_fields:
for item in record[field]:
if match(item, filter):
raise Found
raise NotFound
except Found:
continue
except NotFound:
continue
else:
result.append(r ecord)

With the else clause on the for loop, just after the if
statement, you are bound to confuse people. I would guess
that most people who casually looked at that code would
think that you had made an indentation mistake. Some would
probably "correct" it.

I, for one, could get confused. BTW, I don't remember a single case in
all my code where I have used so much indentation. I would write some
helper function and avoid nesting as much as possible.

Michelee