how to keep null value in yaml file while dumping

I have YAML file site.yaml:

Kvm_BLOCK:
  ip_address: 10.X.X.X
  property: null
  server_type: zone

loaded and then dumped with:

ruamel.yaml.dump(site_yaml, new_file, Dumper=ruamel.yaml.RoundTripDumper)

it becomes

Kvm_BLOCK:
      ip_address: 10.X.X.X
      property: 
      server_type: zone

how to retain this null value in property block

The null value in YAML 1.2 (constructed as Python's None) can be represented as null, Null, NULL and ~, as specified here.

Additionally:

Nodes with empty content are interpreted as if they were plain scalars with an empty value. Such nodes are commonly resolved to a “null” value.

Therefore your null value is not gone, it is just represented differently by the default representation for null in ruamel.yaml when using RoundTripDump. If you load that output again, you once more get a None as value for the key property

If that is not to your liking you can change the output for all None/null values by doing:

import sys
import ruamel.yaml


yaml_str = """\
Kvm_BLOCK:
  ip_address: 10.X.X.X
  property: null
  server_type: zone
"""


def my_represent_none(self, data):
    return self.represent_scalar(u'tag:yaml.org,2002:null', u'NULL')

yaml = ruamel.yaml.YAML()
yaml.representer.add_representer(type(None), my_represent_none)

data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)

which will dump:

Kvm_BLOCK:
  ip_address: 10.X.X.X
  property: NULL
  server_type: zone

You can get finer grained control by creating different classes in Python (NULL, Null, null, etc. ) and have different representers for each of them (much in the same way that the string subclasses in ruamel.yaml.scalarstring.py are used to represent a string in different ways (double quoted, single quoted, literal block style scalar). The problem is that you cannot subclass NoneType so this is not so easily done transparently as with the string scalars.