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

[Grant Edwards]

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

On Sat, 16 Aug 2008 23:20:52 +0200, Kurien Mathew <km*****@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]

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]

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]

in**@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]

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

[info]

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

[e-mail] 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]

in**@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]

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]

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]

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/pyt...il/002982.html

never got into core python :)
--
Robin Becker