Re: usage of try except for review.

[Steven D'Aprano]

On Mon, 29 Feb 2016 07:13 pm, Ganesh Pal wrote:


> def run_cmd_and_verify(cmd, timeout=1000):
> try:
> out, err, ret = run(cmd, timeout=timeout)
> assert ret ==0,"ERROR (ret %d): " \
> " \nout: %s\nerr: %s\n" % (ret, out, err)

Do not use assert for error checking.

http://import-that.dreamwidth.org/676.html


Instead, you should write this:


out, err, ret = run(cmd, timeout=timeout)
if ret != 0:
raise RuntimeError(
  "ERROR (ret %d): \nout: %s\nerr: %s\n" % (ret, out, err))


> except Exception as e:
> logging.error("Failed to run %s got %s" % (cmd, e))
> return False
> return True
> 
> 
> def run_test():
> """
> Mount
> """
> pdb.set_trace()

Do not use the debugger as part of production code. The debugger is for
debugging. When you have debugged the section of code, remove the debugger
calls.


> for cmd in ["mount /nfs_mount1", "mount /cifs_mount1"]:
> try:
> if not run_cmd_and_verify(cmd, timeout=3600):
> return False
> except:
>logging.error("Failure while running command %")
> logging.info("Setup and Creation Done !!!")

Do not use bare except clauses like this unless you know what you are doing.

https://realpython.com/blog/python/the-most-diabolical-python-antipattern/



> cmd = "run_scan"
> out, err, ret = run(cmd)
> for cmd in ["create_data.py -nfs ",
> "validate.py -30 "]:
> try:
> if not run_cmd_and_verify(cmd, timeout=3600):
>return False
> except:
> logging.error("")
> return False
> logging.info("Mount IS START.Done !!!")
> 
> def main():
> if not run_test():
> sys.exit("Exiting Main")
> 
> 
> if __name__ == '__main__':
> main()


I would re-write this script as something like this:



# Untested

def run_cmd(cmd, timeout=1000):
out, err, ret = run(cmd, timeout=timeout)
if ret != 0:
raise RuntimeError(
  "ERROR (ret %d): \nout: %s\nerr: %s\n" % (ret, out, err))


def run_test():
for cmd in ["mount /nfs_mount1", "mount /cifs_mount1"]:
run_cmd(cmd, timeout=3600)
logging.info("Setup and Creation Done !!!")
cmd = "run_scan"
out, err, ret = run(cmd)
# Do you care about the result of the scan? Then you should log it.
for cmd in ["create_data.py -nfs ", "validate.py -30 "]:
run_cmd(cmd, timeout=3600)
logging.info("Mount IS START.Done !!!")
 

if __name__ == '__main__':
try:
run_test()
except Exception as err:
logging.error("run_test failed", exc_info=True)
sys.exit(1)



> Question 1:
> 
> 
> 
> 1. Have I used try and expect block correctly  ? , In my case I have
> the except block that's  is not needed it just gives an  message I
> have still included for the sake of try block

If the except block is not needed, then you should not use try.


> 2.  If a failure’s are encountered  the error by assert condition the
> errors are now displayed on the screen , how do I redirect it to log
> file using logging error

Don't use assert like that.



> 3. my function def has 1000 but Iam using 3600 in the calling fnx etc
> , Time out value are overwritten ?

I don't understand the question.



-- 
Steven

-- 
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Re: usage of try except for review.

[Steven D'Aprano]

On Tue, 1 Mar 2016 04:38 am, Chris Angelico wrote:

> On Tue, Mar 1, 2016 at 4:34 AM, Ganesh Pal  wrote:
>> On Mon, Feb 29, 2016 at 10:10 PM, Dennis Lee Bieber
>>  wrote:
>>
>>> Ask yourself: Will my program still work if I remove all the
>>> assert
>>> statements. If the answer is "No", then you should not be using an
>>> assert.
>>
>> You meant if the answer is "NO" then I should be using asset ?
> 
> No, Dennis was correct. You should assume that "assert" can
> potentially be replaced with "pass" and your program will continue to
> work.


http://import-that.dreamwidth.org/676.html


-- 
Steven

-- 
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Re: usage of try except for review.

[Ganesh Pal]

> I have tried down the code to

Read "I have tried down the code to "  as I have trimmed down the code  as below

Ganesh
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Re: usage of try except for review.

[Ganesh Pal]

> No, Dennis was correct. You should assume that "assert" can
> potentially be replaced with "pass" and your program will continue to
> work.

Thanks Chris for clarifying Dennis point of view  ,


>>try:
>>if not run_cmd_and_verify(cmd, timeout=3600):
>
> Since your version of rcav() always trapped exceptions internally, 
> this
> call will never raise an exception, so using a try: block is meaningless
>>return False
>
> And your conditional basically comes down to:
>
> if False:
> return False:


What would be the alternative for try expect  in the  FOR loop section
of the program ,  will this work fine i.e expect with pass , or any
better alternative for this ?

I have tried down the code to


#!/usr/bin/env python


"""
"""
import os
import shlex
import subprocess
import sys
import time
import logging
from utils import run

def run_cmd_and_verify(cmd, timeout=1000):

try:
out, err, ret = run(cmd, timeout=timeout)
assert ret ==0,"ERROR (ret %d): " \
" \nout: %s\nerr: %s\n" % (ret, out, err)
except Exception as e:
logging.error("Failed to run %s got %s" % (cmd, e))
return False
return True

def run_test():
"""
Mount
"""
# For Loop section of the program
for cmd in ["mount /nfs_mount1", "mount /cifs_mount1"]:
try:
if not run_cmd_and_verify(cmd, timeout=3600):
 logging.error("mount Failed")
return False
except:
pass
logging.info("Setup and Creation Done !!!")

def main():
if not run_test():
sys.exit("Exiting Main")

if __name__ == '__main__':
main()


Regards,
Ganesh
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Re: usage of try except for review.

[Chris Angelico]

On Tue, Mar 1, 2016 at 4:34 AM, Ganesh Pal  wrote:
> On Mon, Feb 29, 2016 at 10:10 PM, Dennis Lee Bieber
>  wrote:
>
>> Ask yourself: Will my program still work if I remove all the assert
>> statements. If the answer is "No", then you should not be using an assert.
>
> You meant if the answer is "NO" then I should be using asset ?

No, Dennis was correct. You should assume that "assert" can
potentially be replaced with "pass" and your program will continue to
work.

ChrisA
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Re: usage of try except for review.

[Ganesh Pal]

On Mon, Feb 29, 2016 at 10:10 PM, Dennis Lee Bieber
 wrote:

> Ask yourself: Will my program still work if I remove all the assert
> statements. If the answer is "No", then you should not be using an assert.

You meant if the answer is "NO" then I should be using asset ?

> Can your "run()" raise an exception? Since you never show it to us we
> can't tell. And if it can, which ones? However, checking a return code is
> not an exceptional condition -- that's expected logic.
>

The run api is like a wrapper to sub-process module it does a Popen
run's the command and returns stdout ,err and ret. The only exception
it can raise is a timeout error.

>>try:
>>if not run_cmd_and_verify(cmd, timeout=3600):
>
> Since your version of rcav() always trapped exceptions internally, 
> this
> call will never raise an exception, so using a try: block is meaningless
>>return False
>
> And your conditional basically comes down to:
>
> if False:
> return False:
>
>
>>except:
>>   logging.error("Some meaningful message")
>>logging.info("Setup and Creation Done !!!")
>
> But you fall off and return Null on success...

Iam using the try expect block to loop over  the list do I have
alternatives  but would prefer the for loop because in the actual
program there are 100 of command and putting them in a list is quite
easy and running over with the same operation saves many lines of code
,  Cam I modify it something like a try except with pass in the except
? or any suggestions


for cmd in ["mount /nfs_mount1", "mount /cifs_mount1"]:
try:
if not run_cmd_and_verify(cmd, timeout=3600):
return False
except:
pass
logging.info("Setup and Creation Done !!!")

Regards,
Ganesh
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Re: usage of try except for review.

[Ganesh Pal]

Iam really sorry , I will have to resend my question again , because
the earlier post had mistakes and formatting was bad , so apologies
for top posting  will try to avoid such mistakes in future.


Iam on python 2.6 and Linux , need your suggestion on the usage of try
and except in this program

#!/usr/bin/env python


"""
"""
import os
import shlex
import subprocess
import sys
import time
import logging
import run
import pdb

def run_cmd_and_verify(cmd, timeout=1000):

try:
out, err, ret = run(cmd, timeout=timeout)
assert ret ==0,"ERROR (ret %d): " \
" \nout: %s\nerr: %s\n" % (ret, out, err)
except Exception as e:
logging.error("Failed to run %s got %s" % (cmd, e))
return False
return True

def run_test():
"""
Mount

"""
pdb.set_trace()

for cmd in ["mount /nfs_mount1", "mount /cifs_mount1"]:
try:
if not run_cmd_and_verify(cmd, timeout=3600):
return False
except:
   logging.error("Some meaningful message")
logging.info("Setup and Creation Done !!!")
#
cmd = "run_scan"
out, err, ret = run(cmd)

for cmd in ["create_data.py -nfs ",
"validate.py -30 "]:
try:
if not run_cmd_and_verify(cmd, timeout=3600):
   return False
except:
logging.error("some meaningful message")
return False
logging.info("Mount IS START.Done !!!")

def main():

if not run_test():
sys.exit("Exiting Main")

if __name__ == '__main__':
main()

Question 1:

(a)  Have I used try and expect block correctly  ?  in run_test()  I
have  expect  block which displays some error message  instead of pass
, is it fine ?

 (b)  Have I returned True and  False correctly , looks fine for me

try:
if not run_cmd_and_verify(cmd, timeout=3600):
return False
except:
   logging.error("inside except")
   return False

Question 2.

(a) If a failure’s are encountered  the error by assert condition the
errors are now displayed on the screen , how do I redirect it to log
file using logging error
 because the moment assert ret==0 becomes true the program exits

def run_cmd_and_verify(cmd, timeout=1000):
try:
out, err, ret = run(cmd, timeout=timeout)
assert ret ==0,"ERROR (ret %d): " \

" \nout: %s\nerr: %s\n" % (ret, out, err)
except Exception as e:
logging.error("Failed to run %s got %s" % (cmd, e))
return False
return True

#script_10.py
Failed to run  mount /nfs got ERROR (ret 1):
out:
host-44-3 exited with status 1
err:
host-44-3: mount_efs:  on /nfs: efs is already mounted

3. my function def has 1000 but Iam using 3600 in the calling fnx etc
, Time out value are overwritten ?
4. Any further improvement particularly on try -except ?


Regards,
Ganesh
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