Good python equivalent to C goto

info at orlans-amo.be info at orlans-amo.be
Sun Aug 17 23:22:16 CEST 2008


On Aug 17, 9:23 pm, Matthew Fitzgibbons <eles... at nienna.org> wrote:
> i... at orlans-amo.be wrote:
> > On Aug 17, 8:09 pm, Matthew Fitzgibbons <eles... at nienna.org> wrote:
> >> Kurien Mathew wrote:
> >>> Hello,
> >>> Any suggestions on a good python equivalent for the following C code:
> >>> while (loopCondition)
> >>> {
> >>>     if (condition1)
> >>>         goto next;
> >>>     if (condition2)
> >>>         goto next;
> >>>     if (condition3)
> >>>         goto next;
> >>>     stmt1;
> >>>     stmt2;
> >>> next:
> >>>     stmt3;
> >>>     stmt4;
> >>>  }
> >>> Thanks
> >>> Kurien
> >>> --
> >>>http://mail.python.org/mailman/listinfo/python-list
> >> I would not be too happy if I saw C code like that in my repository.
> >> This is equivalent:
>
> >> while (loopCondition) {
> >>      if (!(condition1 || condition2 || condition3)) {
> >>          stmt1;
> >>          stmt2;
> >>      }
> >>      stmt3;
> >>      stmt4;
>
> >> }
>
> >> In Python:
>
> >> while (loopCondition):
> >>      if not (condition1 or condition2 or condition3):
> >>          stmt1
> >>          stmt2
> >>      stmt3
> >>      stmt4
>
> >> If stmt3 and stmt4 are error cleanup code, I would use try/finally.
>
> >> while loopCondition:
> >>      try:
> >>          if condition1:
> >>              raise Error1()
> >>          if condition2:
> >>              raise Error2()
> >>          if condition3:
> >>              raise Error3()
> >>          stmt1
> >>          stmt2
> >>      finally:
> >>          stmt3
> >>          stmt4
>
> >> This will also bail out of the loop on and exception and the exception
> >> will get to the next level. If you don't want that to happen, put an
> >> appropriate except block before the finally.
>
> >> -Matt- Hide quoted text -
>
> >> - Show quoted text -
>
> > class Goto_Target(Exception):
> >     pass
>
> > def Goto_is_not_dead(nIn):
> >     try:
> >         if (nIn == 1): raise Goto_Target
> >         if (nIn == 2): raise Goto_Target
>
> >         inv = 1.0 / nIn
> >         print 'Good Input ' + str(nIn) + ' inv=' + str(inv)
>
> >     except Goto_Target:
> >         pass
> >     except Exception, e:
> >         print 'Error Input ' + str(nIn) + ' ' + str(e)
> >     finally:
> >         print 'any input ' + str(nIn)
>
> > if __name__ == '__main__':
> >     Goto_is_not_dead(0)
> >     Goto_is_not_dead(2)
> >     Goto_is_not_dead(3)
> > --
> >http://mail.python.org/mailman/listinfo/python-list
>
> I think this is needlessly ugly. You can accomplish the same with a
> simple if-else. In this case you're also masking exceptions other than
> Goto_Target, which makes debugging _really_ difficult. If you need to
> have the cleanup code executed on _any_ exception, you can still use
> try-finally without any except blocks. Your code is equivalent to this:
>
> def goto_is_dead(n):
>      try:
>          if n == 0 or n == 1 or n == 2:
>              # if you're validating input, validate the input
>              print "Error Input %s" % n
>          else:
>              print "Good Input %s inv= %s" % (n, (1. / n))
>      finally:
>          print "any input %s" % n
>
> if __name__ == '__main__':
>      goto_id_dead(0)
>      goto_id_dead(2)
>      goto_id_dead(3)
>
> More concise, readable, and maintainable.
>
> -Matt- Hide quoted text -
>
> - Show quoted text -

as mentioned 'in complex code the goto statement is  still the easiest
to code and understand'.
The examples are very small and do not require that at all. I agree
it's ugly.
Just to show a way to do it.

A very few functions where I use goto in C or C# are a few hundred
lines of code, difficult to split in smaller functions.
A  lot of common data.
One coming to my mind is a complex validation function for the user
input of a complex transaction.
If any test fails, goto the cleaning part and issue error message.

The goto code is the simpler way to do it.
We are not talking about simple if-else, but let say 20 if-else.
Many nested if-else  are more difficult to understand and do not fit
better the semantics.



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