GOTO (was Re: Appeal for python developers)

[Andrew Dalke]

beliavsky wrote:
> Goto is useful in breaking out of a nested loop and when there is a
> clean-up section of a function that should be executed for various
> error conditions.

But much less useful in languages like Python which have exception
handling.

At rare times I've needed something like

for i in xrange:
  for j in yrange:
    for k in zrange:
      if lookup(i,j,k) == target:
        completely break out of the loop
else:
  raise Exception("not found")
print "I have it!", i, j, k

There are four common solutions to this problem

1) use a goto statement to break out of the loop

for i in xrange:
  for j in yrange:
    for k in zrange:
      if lookup(i,j,k) == target:
        goto FOUND
else:
  raise Exception("not found")

FOUND:
  print "I have it!", i, j, k


2) wrap things inside of an exception

try:
  for i in xrange:
    for j in yrange:
      for k in zrange:
        if lookup(i,j,k) == target:
          raise Found
  else:
    raise Exception("not found")
except Found:
  pass
print "I have it!", i, j, k

3) have a mutiple-level break statement

for i in xrange:
  for j in yrange:
    for k in zrange:
      if lookup(i,j,k) == target:
        break 3
else:
  raise Exception("not found")

print "I have it!", i, j, k

4) introduce a new function, possibly embedded

def _search():
  for i in xrange:
    for j in yrange:
      for k in zrange:
        if lookup(i,j,k) == target:
          return i, j, k
  raise Exception("not found")

i,j,k = _search()
print "I have it!", i, j, k

Both the exception and function definitions solutions can be
done now and I don't think the goto solution adds extra clarity
or elegance, so there's little gain at the expense of some
rather well known pitfalls.


> In another newsgroup I once asked "who needs GOTO" and got some good
> answers -- the thread can be found by Googling 'beliavsky at aol.com
> "explicit GOTO"'. Goto's are less dangerous when they are in the
> forward direction, to code appearing later.

I only found two google hits, both in a Fortran newsgroup.  Other
posts by you suggest you often program in that language.  Fortran
doesn't have exceptions, so gotos are the best solution for how
to do certain types of error handling.  The same holds for C,
as you can tell by reading the Python source code.

				Andrew
				dalke at dalkescientific.com