Good python equivalent to C goto

[Kurien Mathew]

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

[Thomas Mlynarczyk]

Re: Good python equivalent to C goto

Kurien Mathew schrieb:

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;
}

while loopCondition:
if not( cond1 or cond2 or cond3 ):
stmt1
stmt2
stmt3
stmt4

Not tested.
Greetings,
Thomas

--
Ce n'est pas parce qu'ils sont nombreux à avoir tort qu'ils ont raison!
(Coluche)

[Fredrik Lundh]

Re: Good python equivalent to C goto

Kurien Mathew wrote:

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;
}

seems as if you *don't* want to execute the first part if any of the
conditions are true. in other words,

while loopCondition:
if not (condition1 or condition2 or condition3):
stmt1
stmt2
stmt3
stmt4

</F>

[Christian Heimes]

Re: Good python equivalent to C goto

Kurien Mathew wrote:

Hello,
>
Any suggestions on a good python equivalent for the following C code:
>

There are various ways to write your example in Python. For example

while loopCondition:
condition = 1
while condition:
if condition1:
break
if condition2:
break
if condition3:
break
stmt1
stmt2
condition = 0
else:
stmt3
stmt4

The else block of while isn't execute if you break out of while.

You can emulate multiple gotos with exceptions. In general you shouldn't
try to mimic C in Python code. C like Python code is often hard to read
and slower than well designed Python code.

[Michael Torrie]

Re: Good python equivalent to C goto

Dennis Lee Bieber wrote:

Nasty code even for C... I've never used goto in C... Options:
convert the statements of next into a function, and put in an else
clause...

I think the parent post's pseudocode example was too simple to show the
real benefits and use cases of goto in C. Obviously his simple example
was contrived and could easily be solved with proper if tests. Your
idea of using a function is correct for python, though, but not for C

However, what you say about else clauses gets extremely hairy when you
have to deal with multiple nested if statements. The most common use
case in C for goto is when you have cleanup code that must run before
leaving a function. Since any number of things could happen while
processing a function (at any level of the if statement) that would
trigger an exit, having a whole bunch of else statements gets very, very
tedious and messy. Often involves a lot of code duplication too. Goto
is direct and much cleaner and more readable.

A function in C wouldn't work for this because of scoping rules.

However it's not necessary in python to do any of this, since you can
define nested functions that have access to the parent scope. Anytime
you need to clean up, just call the nested cleanup function and then return.

[Michael Torrie]

Re: Good python equivalent to C goto

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;
}

I think the most direct translation would be this:

def whateverfunc():

def next_func():
stmt3
stmt4

while loopCondition:
if condition1:
next_func()
return
if condition2:
next_func()
return
if condition3:
next_func()
return
stmt1
stmt2

[Michael Torrie]

Re: Good python equivalent to C goto

Michael Torrie wrote:

I think the most direct translation would be this:

Nevermind I forgot about the while loop and continuing on after it.
Guess the function doesn't quite fit this use case after all.

[Carl Banks]

Re: Good python equivalent to C goto

On Aug 17, 12:35 am, Michael Torrie <torr...@gmail. comwrote:

However it's not necessary in python to do any of this, since you can
define nested functions that have access to the parent scope.  Anytime
you need to clean up, just call the nested cleanup function and then return.

That is unnecessary and dangerous in Python.

98% of the time the Python interpreter cleans up what you would have
had to clean up by hand in C.

The rest of the time you should be using a try...finally block to
guarantee cleanup, or a with block to get the interpreter do it for
you.

Calling a nested function to perform cleanup is prone to omission
errors, and it doesn't guarantee that cleanup will happen if an
exception is raised.


Carl Banks

[info@orlans-amo.be]

Re: Good python equivalent to C goto

as an oldtimer, I know that in complex code the goto statement is
still the easiest to code and understand.

I propose this solution using exception.

The string exception is deprecated but is simpler for this example.



# DeprecationWarn ing: raising a string exception is deprecated

def Goto_is_not_dea d(nIn):
try:
if (nIn == 1): raise 'Goto_Exit'
if (nIn == 2): raise 'Goto_Exit'

print 'Good Input ' + str(nIn)
raise 'Goto_Exit'

except 'Goto_Exit':
print 'any input ' + str(nIn)

if __name__ == '__main__':
Goto_is_not_dea d(2)
Goto_is_not_dea d(3)

[Marc 'BlackJack' Rintsch]

Re: Good python equivalent to C goto

On Sat, 16 Aug 2008 23:20:52 +0200, Kurien Mathew wrote:

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;
}

(Don't) use the `goto` module: http://entrian.com/goto/ :-)

from goto import goto, label

# ...

while loop_condition:
if condition1 or condition2 or condition3:
goto .next
print 'stmt1'
print 'stmt2'
label .next
print 'stmt3'
print 'stmt4'


Ciao,
Marc 'BlackJack' Rintsch

[Grant Edwards]

Re: Good python equivalent to C goto

On 2008-08-16, Dennis Lee Bieber <wlfraed@ix.net com.comwrote:

On Sat, 16 Aug 2008 23:20:52 +0200, Kurien Mathew <kmathew@envivi o.fr>
declaimed the following in comp.lang.pytho n:
>

>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;
> }
>>

>
Nasty code even for C... I've never used goto in C...

I do sometimes, but it's for exception handling, and in Python
one uses try/raise/except. The example above isn't the best
way to show this, but perhaps something like the code below

while loopCondition:
try:
if condition 1:
raise MyException
if condition 2:
raise MyException
if condition 3:
raise MyException
stmt1;
stmt2;
stmt3;
except: MyException
pass
stmt4;
stmt5;



--
Grant Edwards grante Yow! People humiliating
at a salami!
visi.com

[Matthew Fitzgibbons]

Re: Good python equivalent to C goto

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

[info@orlans-amo.be]

Re: Good python equivalent to C goto

On Aug 17, 8:09 pm, Matthew Fitzgibbons <eles...@nienna .orgwrote:

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(Exc eption):
pass

def Goto_is_not_dea d(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_dea d(0)
Goto_is_not_dea d(2)
Goto_is_not_dea d(3)

[Matthew Fitzgibbons]

Re: Good python equivalent to C goto

info@orlans-amo.be wrote:

On Aug 17, 8:09 pm, Matthew Fitzgibbons <eles...@nienna .orgwrote:

>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(Exc eption):
pass
>
def Goto_is_not_dea d(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_dea d(0)
Goto_is_not_dea d(2)
Goto_is_not_dea d(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

[Paul Hankin]

Re: Good python equivalent to C goto

On Aug 16, 11:20 pm, Kurien Mathew <kmat...@envivi o.frwrote:

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;
  }

Extract complex test as a function. Assuming conditions 1, 2 and 3 are
difficult enough not to put them all one one line, put them in a
function which describes what they're testing.

def should_do_12(ar gs):
if condition1:
return False
if condition2:
return False
if condition3:
return False
return True

while loop_condition:
if should_do_12(ar gs):
stmt1
stmt2
stmt3
stmt4

This is probably the right way to write it in C too.

--
Paul Hankin