[Tutor] constructors

[alan.gauld@bt.com]

> On Friday, April 12, 2002, at 08:59  AM, alan.gauld@bt.com wrote:
> > The only snag is that sometimes you can lose context
> > which makes it harder to recover from an exception

> I'm not sure I understand what you mean?  How can I avoid 
> this snag (or rather, what is this snag)?

What I mean is that if the try/except encompasses many 
statements when an exception occurs you jump to the 
end of that block with no way back in (I've always thought 
a 'continue' for exception handlers would be nice!)

Thus if you are reading through a list or file and some
relatively unimportant error(divide by zero say) occurs 
throwing an exception you cannot just go back to reading 
your list/file. The context has been lost...

With explicit error testing on each call you only fail 
the one bit in the middle of the block, you can still 
go back to the start.

You can do this with exceptions too by using very small, 
tightly controlled try/excepts - down to each single 
statement if needed - but it then spoils the improved 
readability of the code. The trick is to make the 
enclosed block 'atomic', that is it only does one thing, 
with no unrelated bits of code. So in my example above:

try:
   for line in file.xreadlines():
      processData(line)
except DivideByZero: # want to just move on to next line but...
   continue          # can't do this, we lost the loop context!
except IOError:
   print 'File error'

So we have to do this;

try:    # enclose file handling bits, catching those exceptions
   for line in file.xreadlines():
      try:   # now enclose the data processing bits
          processData(line)
      except DivideByZero:
          continue   # still have the file/loop context  
except IOError:
   print 'File error'

Which looks less elegant but maintains context.

HTH,

Alan G