ODL subnet REST API, Python, and some error handling

Its been a while since I’ve had a chance to play around on OpenDaylight for a while, so I thought I’d warm up with some Python and API calls. One thing I haven’t done much of with my code so far is handling errors, so in this post I’m going to access the Subnet API, and try to do some error handling as well from Python.

I recently found a nice HTTP client called Requests that simplifies the HTTP requests. You can add it using pip:

pip install requests

And then import it into your Python code:

import requests
from requests.auth import HTTPBasicauth

I’ve also imported the auth stuff for logging into the controller.
Now making a GET requests is simple, here is an example of querying the controller for all the configured subnets:

user = 'admin'
password = 'admin'
servierIP = '10.55.17.20'
container = 'default'
allSubnets = '/controller/nb/v2/subnet/' + container + '/subnet/all'
url = 'http://' + serverIP + ':' + port + allSubnets
r = requests.get(url, auth=(user, password))
print r.json()

Requests can automatically take the response from the controller and give you back the JSON data. Now lets add some error handling by wrapping everything with try/except/else:

errorcodes = {
    400: 'Invalid data',
    401: 'User not authorized',
    409: 'Name conflict',
    404: 'Container name not found',
    500: 'Internal error',
    503: 'Service unavailable'
    }

try:
    r = requests.get(url, auth=(user, password))
    r.raise_for_status()
except requests.exceptions.HTTPError as e:
    print e
    print "Reason : %s" % errorcodes[r.status_code]
else:
    # No errors loading URL
    result = find_subnet(r.json()['subnetConfig'], subnetquery)
    print result

The first thing we do is use r.raise_for_status() which returns null if all goes well, otherwise it will raise an exception. One of the exceptions that can be raised is an HTTPError, in which case we’ll print out the error.

If everything is ok, then I pass the resulting JSON to a find_subnet function which just searches the JSON list for a particular subnet:

# given a list of subnets and a subnet to find, will return the subnet if found
# or None if not found
def find_subnet(subnets, subnetName):
    for subnet in subnets:
        if subnet['subnet'] == subnetName:
            return subnet
    return None

Nothing too crazy here, but just another Python example for those who are interested. Also, please note that the APIs have changed here and there, so be sure to check the URLs you are calling to make sure they are correct. I’ve found that the API docs on the OpenDaylight wiki are not always in sync with the version of controller I’m using. You can find the full script here:
https://github.com/fredhsu/odl-scripts/tree/master/python/subnets

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