Good python equivalent to C goto

[info@orlans-amo.be]

Re: Good python equivalent to C goto

On Aug 17, 9:23 pm, Matthew Fitzgibbons <eles...@nienna .orgwrote:

i...@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- 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.

[Fredrik Lundh]

Re: Good python equivalent to C goto

info@orlans-amo.be wrote:

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.

let's see...

$ cd ~/svn/python25
$ grep goto */*.c | wc
2107 7038 86791

but given that the Python language doesn't have a goto statement, can we
perhaps drop this subtopic now?

</F>

[Matthew Fitzgibbons]

Re: Good python equivalent to C goto

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.
--

http://mail.python.org/mailman/listinfo/python-list

>

I'm sorry, but if you have any single function that's a few hundred
lines of code, you need to do some serious refactoring. How do you even
begin to test that? A goto is just a hack that hides underlying
problems. If I see a function of more than about twenty lines or having
more than two or three execution paths, I start thinking of ways to
break it down.

There are a lot of approaches. Paul Hankin put all the conditionals in a
helper function. Similarly, you could do something like this (for more
fun make the validators pluggable):

# each validator is a function that takes the input the be validated
# and returns True for good input; otherwise False
_foo_validators = [_check_this, _check_that, _check_the_othe r] # etc.

def foo(input):
for v in _validators:
if not v(input):
_on_bad_input(i nput)
break
else:
_on_good_input( input)
_on_all_input(i nput)

Alternatively, you can often turn complex input validation into a state
machine.

-Matt

[Chuck Rhode]

Re: Good python equivalent to C goto

On Sun, 17 Aug 2008 09:08:35 -0500, Grant Edwards wrote:

In Python one uses try/raise/except.

At the risk of introducing an anachronism and in deference to
Mr. "ElementTre e" Lundh, I now invoke Godwin's Law (1990):

Isn't *try/except* kinda sorta like the musty, old *come from*
construct proposed for *fortran*?

Here there be typos (abject apologies):

o Clark, R. Lawrence. "A Linguistic Contribution to GOTO-less
Programming." _Datamation_ Dec. 1973. 18 Aug. 2008
<http://www.fortranlib. com/gotoless.htm>.

--
... Be Seeing You,
... Chuck Rhode, Sheboygan, WI, USA
... Weather: http://LacusVeris.com/WX
... 71° — Wind W 9 mph

[Robin Becker]

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;
}
>

such a pity that the goto module

http://mail.python.org/pipermail/python-announce-list/2004-April/002982.html

never got into core python :)
--
Robin Becker

[Reedick, Andrew]

RE: Good python equivalent to C goto

-----Original Message-----
From: python-list-bounces+jr9445= att.com@python. org [mailto:python-
list-bounces+jr9445= att.com@python. org] On Behalf Of Kurien Mathew
Sent: Saturday, August 16, 2008 5:21 PM
To: python-list@python.org
Subject: Good python equivalent to C goto

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;
}

=============== ============
Use a flag and a one loop for loop


while loopCondition:
flag = False
for i in range(1):
if condition1:
break
if condition2:
break
if condition3:
break
stmt1
stmt2
flag = True
if not flag:
stmt3
stmt4


=============== ============
Or just a straight flag

while ( loopCondition ):
flag = False

if (condition1)
flag = True # goto next;
if (not flag and condition2)
flag = True # goto next;
if (not flag and condition3)
flag = True # goto next;

if not flag:
stmt1
stmt2
else
stmt3
stmt4



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[Paul McGuire]

Re: Good python equivalent to C goto

On Aug 17, 1:09 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 -

Close, but there is no reason for the conditions to raise anything,
they can just use the continue statement:

i = 20
while i 0:
try:
if i % 2:
continue
if i % 3:
continue
print i, "is even and a multiple of 3"
finally:
i -= 1

Prints:
18 is even and a multiple of 3
12 is even and a multiple of 3
6 is even and a multiple of 3

I think this is closest to the OP's stated requirements.

-- Paul

[Matthew Fitzgibbons]

Re: Good python equivalent to C goto

Paul McGuire wrote:

On Aug 17, 1:09 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 -

>
Close, but there is no reason for the conditions to raise anything,
they can just use the continue statement:
>
i = 20
while i 0:
try:
if i % 2:
continue
if i % 3:
continue
print i, "is even and a multiple of 3"
finally:
i -= 1
>
Prints:
18 is even and a multiple of 3
12 is even and a multiple of 3
6 is even and a multiple of 3
>
I think this is closest to the OP's stated requirements.
>
-- Paul
--

http://mail.python.org/mailman/listinfo/python-list

>

I'm aware my example where I raised exceptions was functionally
different ("This will bail out of the loop on an exception and the
exception will get to the next level.") Just thinking in terms of
telling the caller something went wrong. The other examples were
functionally equivalent, however.

Still, this is the best way so far. :) I never thought of using continue
with finally. It gets added to my Python tricks file for future use.

-Matt