Unable to dump as "pure" YAML - yaml

Question

ruamel.yaml==0.15.37
Python 3.6.2 :: Continuum Analytics, Inc.
Current code:
from ruamel.yaml import YAML
import sys
yaml = YAML()
kube_context = yaml.load('''
apiVersion: v1
clusters: []
contexts: []
current-context: ''
kind: Config
preferences: {}
users: []
''')
kube_context['users'].append({'name': '{username}/{cluster}'.format(username='test', cluster='test'), 'user': {'token': 'test'}})
kube_context['clusters'].append({'name': 'test', 'cluster': {'server': 'URL:443'}})
kube_context['contexts'].append({'name': 'test', 'context': {'user': 'test', 'cluster': 'test'}})
yaml.dump(kube_context, sys.stdout)
My yaml.dump() is producing output that contains the list and dict objects, instead of being fully expanded.
Current output:
apiVersion: v1
clusters: [{name: test, cluster: {server: URL:443}}]
contexts: [{name: test, context: {user: test, cluster: test}}]
current-context: ''
kind: Config
preferences: {}
users: [{name: test/test, user: {token: test}}]
What do I need to do in order to have yaml.dump() output fully expanded?
Expected output:
apiVersion: v1
clusters:
- name: test
cluster:
server: URL:443
contexts:
- name: test
context:
user: test
cluster: test
current-context: ''
kind: Config
preferences: {}
users:
- name: test/test
user:
token: test

Answer 1

ruamel.yaml, when using the default YAML() or YAML(typ='rt') will preserve the flow- or block style of sequences and mappings. There is no way to make a block style empty sequence or empty mapping and your [] and {} are therefore tagged as flow style when loaded.
Flow style can only contain flow style (whereas block style can contain block style or flow style) (YAML 1.2 spec 8.2.3):
YAML allows flow nodes to be embedded inside block collections (but not vice-versa).
Because of that, the dict/mapping data that you insert in the (flow-style) list/sequence will also be represented as flow-style.
If you want everything to be block style (what you call "expanded" mode), you can explicitly set that by calling the .set_block_style() method on the .fa attribute (which is only available on the collections, hence the try/except):
from ruamel.yaml import YAML
import sys
yaml = YAML()
kube_context = yaml.load('''
apiVersion: v1
clusters: []
contexts: []
current-context: ''
kind: Config
preferences: {}
users: []
''')
kube_context['users'].append({'name': '{username}/{cluster}'.format(username='test', cluster='test'), 'user': {'token': 'test'}})
kube_context['clusters'].append({'name': 'test', 'cluster': {'server': 'URL:443'}})
kube_context['contexts'].append({'name': 'test', 'context': {'user': 'test', 'cluster': 'test'}})
for k in kube_context:
try:
kube_context[k].fa.set_block_style()
except AttributeError:
pass
yaml.dump(kube_context, sys.stdout)
this gives:
apiVersion: v1
clusters:
- name: test
cluster:
server: URL:443
contexts:
- name: test
context:
user: test
cluster: test
current-context: ''
kind: Config
preferences: {}
users:
- name: test/test
user:
token: test
Please note that it is not necessary to set yaml.default_flow_style = False in the default round-trip-mode; and that although block-style has been set for the value of key preferences, it is represented flow style as there is no other way to represent an empty mapping.

Answer 2

The output is „pure“ YAML. You want the nodes to be presented in block style (indentation-based) as opposed to the current flow style ([]{}-based). Here's how to do that:
yaml = YAML(typ="safe")
yaml.default_flow_style = False
(Note Athon's comment on the typ below; you need to set it to safe or unsafe so that the RoundTripLoader does not set the style of the empty sequences)