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

[Sybren Stuvel]

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.next()
   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
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Re: for: else: - any practical uses for the else clause?

[MonkeeSage]

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

+1

I just wish it had a more intuitive name like after: or then:, as
else: seems like a choice between the loop and the other block (but
really the choice is between StopIteration and break).

Regards,
Jordan

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

[Paul Rubin]

Sybren Stuvel [EMAIL PROTECTED] writes:
 I must say that the for/else construct is a LOT more readable than the
 rewritten alternatives.

They are all pretty ugly.  I prefer the genexp version with a
hypothetical is_empty function:

 all_heights = (block.height for block in stack if block.is_marked())
 if is_empty(all_heights):
raise SomeError(No marked block)
 heights = sum(all_heights)

Python generators don't really work the right way for this though.

I've been fooling around with Haskell, so these kinds of constructions
seems more natural to me than explicit for loops.
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Re: for: else: - any practical uses for the else clause?

[Peter Otten]

Sybren Stuvel wrote:

 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.next()
   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.

I like

def blocks_til_mark(stack):
for block in stack:
if block.is_marked():
return
yield block
raise SomeError
height = sum(block.height for block in blocks_til_mark(stack))

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

[Peter Otten]

Paul Rubin wrote:

 Sybren Stuvel [EMAIL PROTECTED] writes:
 I must say that the for/else construct is a LOT more readable than the
 rewritten alternatives.
 
 They are all pretty ugly.  I prefer the genexp version with a
 hypothetical is_empty function:
 
  all_heights = (block.height for block in stack if block.is_marked())
  if is_empty(all_heights):
 raise SomeError(No marked block)

Such a function would have to rebind the generator:

def check_empty(items):
items = iter(items)
try:
first = items.next()
except StopIteration:
return False
return chain([first], items)

all_heights = check_empty(all_heights)
if not all_heights:
raise SomeError

  heights = sum(all_heights)
 
 Python generators don't really work the right way for this though.

You can make it work, but the result tends to be messy:

from itertools import chain

def raiseiter(exc, *args):
raise exc(*args)
yield None # unreachable
def stop():
raise StopIteration

height = sum(block.height for block in chain(stack, raiseiter(SomeError)) if
(not block.is_marked()) or stop())

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

[Paul Rubin]

Peter Otten [EMAIL PROTECTED] writes:
 I like
 
 def blocks_til_mark(stack):
 for block in stack:
 if block.is_marked():
 return
 yield block
 raise SomeError
 height = sum(block.height for block in blocks_til_mark(stack))

Oh my, I realize now that I mis-read the original, and my examples
were all wrong.  Shows how the explicit loop isn't necessarily so
clear ;-).  Note that unlike the original, the loop above fails to set
height = 0 in the case where the exception gets raise.

I'd maybe just scan the stack twice.  The rescan is needed only if
there can be blocks with height = 0.  Otherwise, you can just raise
SomeError if height == 0:

height = sum(b.height for b in 
 itertools.takewhile(lambda: not block.is_marked(), stack))

if height == 0 and True not in (b.is_marked() for b in stack):
 raise SomeError
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Re: for: else: - any practical uses for the else clause?

[Paul Rubin]

Peter Otten [EMAIL PROTECTED] writes:
   all_heights = (block.height for block in stack if block.is_marked())
   if is_empty(all_heights):
  raise SomeError(No marked block)
 
 Such a function would have to rebind the generator:

Yeah, that's what I mean about generators not working the right way.

 You can make it work, but the result tends to be messy:

I think it's better underneath, but still syntactically ugly, to just
defer the generator creation:

 all_heights = lambda: 
 (block.height for block in stack if block.is_marked())
 if is_empty(all_heights ()):
 raise SomeError(No marked block)
 height = sum(all_heights ())

Now sum and is_empty see two separate generators, so is_empty is
straightforward:

 def is_empty(gen):
try:
   gen.next()
   return True
except StopIteration:
   return False
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Re: for: else: - any practical uses for the else clause?

[Peter Otten]

Paul Rubin wrote:

 Peter Otten [EMAIL PROTECTED] writes:
   all_heights = (block.height for block in stack if
   block.is_marked()) if is_empty(all_heights):
  raise SomeError(No marked block)
 
 Such a function would have to rebind the generator:
 
 Yeah, that's what I mean about generators not working the right way.
 
 You can make it work, but the result tends to be messy:
 
 I think it's better underneath, but still syntactically ugly, to just
 defer the generator creation:
 
  all_heights = lambda:
  (block.height for block in stack if
  block.is_marked())

You still need the stop() trick to omit the heights after the marked block.

  if is_empty(all_heights ()):
  raise SomeError(No marked block)
  height = sum(all_heights ())
 
 Now sum and is_empty see two separate generators, so is_empty is
 straightforward:
 
  def is_empty(gen):
 try:
gen.next()
return True
 except StopIteration:
return False

Alternatively you can turn all_heights into a list (comprehension) which
makes the test trivial. 

I think it will be interesting to see how Python 3000's emphasis on
iterators will affect overall code complexity.

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

[Paul Rubin]

Peter Otten [EMAIL PROTECTED] writes:
   all_heights = lambda:
   (block.height for block in stack if
   block.is_marked())
 
 You still need the stop() trick to omit the heights after the marked block.

Yeah, that code was based on my earlier mis-read of the original post.
How's this:

def blocks_until_mark():
   return itertools.takewhile(lambda block:\
  not block.is_marked(), \
  stack)

height = sum(b.height for b in blocks_until_mark())
if is_empty(blocks_until_mark()):
raise SomeError(No marked block)

 Alternatively you can turn all_heights into a list (comprehension) which
 makes the test trivial. 

Yes, I felt it wasn't in the spirit of the thing to use that memory.

 I think it will be interesting to see how Python 3000's emphasis on
 iterators will affect overall code complexity.

We will need more iterator primitives, which are still evolving.
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Re: for: else: - any practical uses for the else clause?

[Carl Banks]

[EMAIL PROTECTED] wrote:
 I'm wondering if anyone has ever found a practical use for the else
 branch?

Say you have code that looks like this:

if command.startswith(set):
do_set_action(command)
elif command.startswith(input):
do_input_action(command)
elif command.startswith(print):
do_print_action()
else:
do_general_action()

Now, during refactoring, we note that there's a lot of similarity in
all those if clauses that we can exploit to simplify the code.  Let's
ignore the final else clause for now, since it's not similar to the if
and elif clauses (it has no test).  We define a mapping of prefixes to
actions, and rewrite the if...elif as a for loop:

command_map = (
(set,do_set_action),
(input,do_input_action),
(print,do_print_action)
)

for keyword,action in command_map:
if command.startswith(keyword):
action(command)
break

Ok, this works great.  Now let's come back to that else clause: how do
we translate default case to the for loop?  Well, I guess we could set
some sort of flag indicating we got a match  But wait!  We don''t
have to!  We can just leave the else clause there as-is, and it'll
work.  The for loop here is quite literally a drop-in replacement for
the if...elif...elif, even going so far as to allow the same else
clause.

for keyword,action in command_list:
if command.startswith(keyword):
action(command)
break
else:
do_general_action()


Moral of the story: there are two ways to do a linear search (or linear
sequence of tests): either an unrolled sequence of if...elif...elif
clauses, or a rolled up for loop with a break.  Either way you do it,
you can tack on an else clause for when you don't find anything.


Carl Banks

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

[BJörn Lindqvist]

 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)

def get_height_of_first_marked_bock(stack):
height = 0
for block in stack:
if block.is_marked():
return height
height += block.height
raise SomeError(No marked block)

height = get_height_of_first_marked_block(stack)
print Lowest marked block is at height, height

Yes, this transformation is one line longer, but the control flow is
much easier to understand. In general, using the for/else clause mixes
the retrieval and the usage of the element. Consider this:

for item in list:
if somecond(item):
A) do something with item
break
else:
B) exceptional code when somecond() doesnt match anything in list

The code that you write in the positions A and B really are misplaced.
They arent part of the iteration of list. The two tasks, find item and
do something with item should be separated.

def find_item(list):
for item in list:
if somecond(item):
return item
return None # OR raise an exception

item = find_item(list)
if item:
A) do something with item
else:
B) exceptional code

This is, IMHO, much better style.

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

[MonkeeSage]

BJörn Lindqvist wrote:
 The code that you write in the positions A and B really are misplaced.
 They arent part of the iteration of list. The two tasks, find item and
 do something with item should be separated.

I think it is useful to have them joined. Consider a contrived example:

for i in (1,2,3,0,5,6):
  try:
print 10 / i
  except:
print 'Error in data'
break
else:
  print 'Data processed cleanly'

Yes, you could use a flag variable instead:

done = 1
for i in (1,2,3,0,5,6):
  try:
print 10 / i
  except:
print 'Error in data'
done = 0
break
if done:
  print 'Data processed cleanly'

...but that is not as clean imo.

Regards,
Jordan

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

[metaperl]

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.

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

[Klaas]

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

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

[Johan Steyn]

On 29 Sep 2006 11:26:10 -0700, Klaas [EMAIL PROTECTED] wrote:
else: does not trigger when there is no data on which to iterate, butwhen 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: 


 for x in []:... print 'nothing'... else:... print 'done'
...done
The else clause is executed because the loop still terminates normally with an empty list - albeit without having looped at all.

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:

found = False
for i in list:
 if i % 2 == 1:
 print Found an odd number.
 found = True
 break
if not found:
 print No odd number found.

OK, so using else only saves me 2 lines and a variable - not much to write home about, but I still like it.

Johan.

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

[Klaas]

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

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

[Mike Klaas]

On 9/29/06, Johan Steyn [EMAIL PROTECTED] wrote:
 On 29 Sep 2006 11:26:10 -0700, Klaas [EMAIL PROTECTED] 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.

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

[BJörn Lindqvist]

On 9/29/06, Johan Steyn [EMAIL PROTECTED] wrote:
  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.

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

[Matthew Woodcraft]

[EMAIL PROTECTED] 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-

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

[Paul Rubin]

Matthew Woodcraft [EMAIL PROTECTED] writes:
 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.next()
  height += sum(all_heights)
except StopIteration:
  raise SomeError(No marked block)
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Re: for: else: - any practical uses for the else clause?

[Sion Arrowsmith]

Ben Sizer [EMAIL PROTECTED] wrote:
[EMAIL PROTECTED] 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.

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

[Ben Sizer]

[EMAIL PROTECTED] wrote:
 metaperl I'm wondering if anyone has ever found a practical use for the
 metaperl else 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.

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

[metaperl . etc]

A very old thread:
http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gstq=for+elsernum=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?

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

[skip]

metaperl I'm wondering if anyone has ever found a practical use for the
metaperl else 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
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Re: for: else: - any practical uses for the else clause?

[Amaury Forgeot d'Arc]

[EMAIL PROTECTED] a écrit :
 A very old thread:
 http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gstq=for+elsernum=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(value):
 print a matching item was found
 break
 else:
 print no matching item
 return False

 # ... continue with value

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

[Bruno Desthuilliers]

[EMAIL PROTECTED] a écrit :
 A very old thread:
 http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gstq=for+elsernum=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
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Re: for: else: - any practical uses for the else clause?

[Fuzzyman]

Amaury Forgeot d'Arc wrote:
 [EMAIL PROTECTED] a écrit :
  A very old thread:
  http://groups.google.com/group/comp.lang.python/browse_frm/thread/2c5022e2b7f05525/1542d2041257c47e?lnk=gstq=for+elsernum=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(value):
  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

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