Editing YAML file using python not giving exact output

Question

I'm editing YAML file but it isn't giving the exact output as I want. There is a difference between indentation and square brackets.

import yaml

add_info = {'network':
        {'version': 2,
         'renderer': 'networkd',
         'ethernets':
             {'enx1':
                {
                    'addresses': ['193.254.1.8/24'],
                    'nameservers': {'addresses':
                                    ['193.254.1.5, 8.8.8.8']},
                    'routes': [{
                        'to': '193.254.1.0/24',
                        'via': '193.254.1.5'
                    }]
                }
            }
        }
    }

with open('/etc/netplan/01-network-manager-test.yaml', 'w') as f:
    data1 = yaml.dump(add_info, f, sort_keys=False)
    print(data1)

Current Output I'm getting from above code.

network:
  version: 2
  renderer: networkd
  ethernets:
    enx1:
      addresses:
      - 193.254.1.8/24
      nameservers:
        addresses:
        - 193.254.1.5, 8.8.8.8
      routes:
      - to: 193.254.1.0/24
        via: 193.254.1.5

This is the exact Output I want:

network:
  version: 2
  renderer: networkd
  ethernets:
    enx1:
      addresses:
      - 193.254.1.6/24
      nameservers:
          addresses: [193.254.1.5, 8.8.8.8]
      routes:
          - to: 193.254.1.0/24
            via: 193.254.1.5

Answer 1

You should not be using PyYAML. It only supports a subset of YAML 1.1 (which was superseded in 2009) and doesn't give you much fine control.

Start with what you want to get and see how well that round-trips in ruamel.yaml:

import sys
import ruamel.yaml

yaml_str = """\
network:
  version: 2
  renderer: networkd
  ethernets:
    enx1:
      addresses:
      - 193.254.1.6/24
      nameservers:
          addresses: [193.254.1.5, 8.8.8.8]
      routes:
          - to: 193.254.1.0/24
            via: 193.254.1.5
"""
    
yaml = ruamel.yaml.YAML()
# yaml.indent(mapping=4, sequence=4, offset=2)
yaml.preserve_quotes = True
data = yaml.load(yaml_str)
# print(data)
yaml.dump(data, sys.stdout)

which gives:

network:
  version: 2
  renderer: networkd
  ethernets:
    enx1:
      addresses:
      - 193.254.1.6/24
      nameservers:
        addresses: [193.254.1.5, 8.8.8.8]
      routes:
      - to: 193.254.1.0/24
        via: 193.254.1.5

You do have control seperately over indentation of mappings and sequences, but your sequences are not indented consistently: the sequence that is the value for addresses has an indent of 2 with an offset for the sequence indicator (-) within that of 0, and for the sequence that is the value of routes is indented 6 with an offset of 4; your mappings are not indented consistently either (using 2 and 4 positions). ruamel.yaml doesn't give you that fine control.

If you want to start with your datastructure you need to provide the valued for addresses not as list of a single string with embedded comma, but as the subclass of a list that ruamel.yaml uses (you could find which by inspecting data after loading) and mark it to be represented as flow style:

import sys
import ruamel.yaml

def FS(*args):
    seq = ruamel.yaml.CommentedSeq(args)
    seq.fa.set_flow_style()
    return seq

add_info = {'network':
        {'version': 2,
         'renderer': 'networkd',
         'ethernets':
             {'enx1':
                {
                    'addresses': ['193.254.1.8/24'],
                    'nameservers': {'addresses':
                                    FS('193.254.1.5', '8.8.8.8')},  # <- changed
                    'routes': [{
                        'to': '193.254.1.0/24',
                        'via': '193.254.1.5'
                    }]
                }
            }
        }
    }

yaml = ruamel.yaml.YAML()
yaml.dump(add_info, sys.stdout)

which also gives:

network:
  version: 2
  renderer: networkd
  ethernets:
    enx1:
      addresses:
      - 193.254.1.8/24
      nameservers:
        addresses: [193.254.1.5, 8.8.8.8]
      routes:
      - to: 193.254.1.0/24
        via: 193.254.1.5