Updating the Yaml file, with new fields using ruamel - yaml

Question

I am trying to update the yaml file using ruamel python.
proc=subprocess.Popen(['kubectl','get','pod','web3','-o','yaml','--export'], stdout=subprocess.PIPE)
rein=proc.stdout.read()
result, indent, block_seq_indent = ruamel.yaml.util.load_yaml_guess_indent(rein, preserve_quotes=True)
So far I have tried :
result['spec'].append('nodeSelector')
which gives ERROR :
result['spec'].append('nodeSelector')
AttributeError: 'CommentedMap' object has no attribute 'append'
Also tried like this :
result['spec']['nodeSelector']['kubernetes.io/hostname']='kubew1'
gives :
result['spec']['nodeSelector']['kubernetes.io/hostname']='kubew1'
File "/usr/local/lib/python3.6/dist-packages/ruamel/yaml/comments.py", line 752, in __getitem__
return ordereddict.__getitem__(self, key)
KeyError: 'nodeSelector'
My Intial Yaml File is :
apiVersion: v1
kind: Pod
metadata:
creationTimestamp: null
labels:
app: demo
name: web
name: web3
selfLink: /api/v1/namespaces/default/pods/web3
spec:
containers:
- image: aexlab/flask-sample-one
imagePullPolicy: Always
name: web
ports:
- containerPort: 5000
name: http
protocol: TCP
resources: {}
terminationMessagePath: /dev/termination-log
terminationMessagePolicy: File
volumeMounts:
- mountPath: /var/run/secrets/kubernetes.io/serviceaccount
name: default-token-7bcc9
readOnly: true
dnsPolicy: ClusterFirst
enableServiceLinks: true
And Expected fields I want to add inside 'spec' is :
nodeSelector:
kubernetes.io/hostname: kubew1
Any Ideas how to achieve this with ruamel library.

Answer 1

In your YAML file your root level collection is a mapping and the value for the key spec in that mapping is itself a mapping. Both of those mappings get loaded as dict-like objects using ruamel.yaml named CommentedMap.
As with normal dicts you can add key-value pairs, deleted keys (and their values), and update values for a key, but there is no .append() method, as there is with a list (i.e. appending an extra item to a list).
Your output is a bit terse, but of course you cannot just add nodeSelector to anything (list/sequence nor dict/mapping) and expect that to add kubernetes.io/hostname: kubew1 (a mapping in its own right) automatically.
Your try of:
result['spec']['nodeSelector']['kubernetes.io/hostname'] = 'kubew1'
cannot work because there is no dict result['spec']['nodeSelector'] where you can add the key kubernetes.io/hostname.
You would either first have to create a key with an emtpy dict as value:
result['spec']['nodeSelector'] = {}
result['spec']['nodeSelector']['kubernetes.io/hostname'] = 'kubew1'
or do
result['spec']['nodeSelector'] = {'kubernetes.io/hostname': 'kubew1'}
Please note that the above has nothing much to do with ruamel.yaml, that is just basic Python data structure manipulation. Also note that there are over 100 libraries in the ruamel namespace, out of which ruamel.yaml is just one of several published as open source, so using ruamel is not very clear statement, although of course the context often provides enough information on which library you actually use.