python catching exception

Question

i am running a curl command to check the status of a website:

try:
    connectionTest = subprocess.Popen([r"curl --interface xx.xx.xx.xx http://www.yahoo.com"], shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
    cstdout,cstderr = connectionTest.communicate()
    if cstdout:
        #print cstdout
        status = "OK"
    elif cstderr:
        #print cstderr
        status = "PROBLEM"
except:
    e = sys.exc_info()[1]
    print "Error: %s" % e

The code works fine except for the try:except statement as its not catching the exception properly, below is the output of the script when the interface is down, now i would like to catch that first line in the except statement... instead it being spawned...is this possible??

SIOCSIFFLAGS: Cannot assign requested address

PROBLEM

Show us the whole stacktrace. What makes you think there is a problem? If your except clause is not being triggered, there is no exception. — Marcin, Mar 26 '12 at 11:34
simply raise an exception if there is a problem in the output of the curl command. — Not_a_Golfer, Mar 26 '12 at 11:38

score 5 · Answer 1 · answered Mar 26 '12 at 11:42

There is no exception thrown. You can check return code and throw an exception when it is not zero:

import sys, subprocess
try:
    connectionTest = subprocess.Popen([r"curl --interface 1.1.1.1 http://www.yahoo.com"], shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
    cstdout,cstderr = connectionTest.communicate()
    if connectionTest.returncode:
        raise Exception("Curl returned %s"%connectionTest.returncode)
    if cstdout:
        #print cstdout
        status = "OK"
    elif cstderr:
        #print cstderr
        status = "PROBLEM"
except:
    e = sys.exc_info()[1]
    print "Error: %s" % e

score 3 · Answer 2 · answered Mar 26 '12 at 11:37

if any exception happens in subprocess, it will not be thrown. check stderr and raise appropriate exception

try:
    connectionTest = subprocess.Popen([r"curl --interface xx.xx.xx.xx http://www.yahoo.com"], shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
    cstdout,cstderr = connectionTest.communicate()
    if cstdout:
        #print cstdout
        status = "OK"
    elif cstderr:
        #print cstderr
        status = "PROBLEM"
        raise some exception
except:
    e = sys.exc_info()[1]
    print "Error: %s" % e

score 0 · Answer 3 · edited May 23 '17 at 12:14

Are you sure you need to call out to curl?

import urllib2
try:
    urllib2.urlopen("http://www.yahoo.com")
except urllib2.URLError as err:
    print(err)
except urllib2.HTTPError as err:
    print(err)

Secondly, it sounds like your interface address is dodgy, rather than your code. Check the --interface flag argument for correctness (ie: run the curl command outside of python and check that it works as expected.)

As a general point, you will never be able to catch a child process error inside of your python program (Which is what I think you are asking). You will have to rely on exit codes and stdout/err output, which is why a solution that relies on Python's included batteries will be much more robust.

If you need to specify the interface manually, you have several other options:

PyCurl:

import pycurl
import StringIO
c = pycurl.Curl()
c.setopt(pycurl.URL, "http://www.python.org/")
c.setopt(pycurl.CURLOPT_INTERFACE, "eth0")
# [...]
try:
    c.perform()
except: # Handle errors here.
  pass

Change the source interface manually in python for urllib2

python catching exception

3 Answers3