Let's say you have a YAML file named that contains the following.

domains:
  foo.example.com:
    alias: "foo"
    region: "us-east-1"
  bar.example.com:
    alias: "bar"
    region: "us-east-1"

And you want to modify the file. This can be done using ruamel.yaml. The pip install command can be used to install the ruamel.yaml package.

pip install ruamel.yaml

And here is how you could create a new file, updating the value of a key.

#!/usr/bin/python
import ruamel.yaml

yaml = ruamel.yaml.YAML()

with open("/path/to/original.yml", "r") as original_file:
  dictionary = yaml.load(open("/path/to/original.yml")

dictionary['domains']['foo.example.com']['region'] = 'us-east-2'
dictionary['domains']['bar.example.com']['region'] = 'us-east-2'

with open("/path/to/new.yml", "w") as new_file:
  yaml.dump(dictionary, new_file)

Let's say line 1 of the YAML file contains three dashes, which is fairly common with Ansible playbooks.

---
domains:
  foo.example.com:
    alias: foo
    region: us-east-1
  bar.example.com:
    alias: bar
    region: us-east-1

In this scenario, yaml.explicit_start = True will need to be set to include line 1 in yaml.dump.

#!/usr/bin/python
import sys
import ruamel.yaml

yaml = ruamel.yaml.YAML()
yaml.explicit_start = True

with open("/path/to/original.yml", "r") as original_file:
  dictionary = yaml.load(open("/path/to/original.yml")

dictionary['domains']['foo.example.com']['region'] = 'us-east-2'
dictionary['domains']['bar.example.com']['region'] = 'us-east-2'

with open("/path/to/new.yml", "w") as new_file:
  yaml.dump(dictionary, new_file)

By default, the double quotes around values will often be removed. yaml.preserve_quotes = True can be used to keep the double quotes.

#!/usr/bin/python
import sys
import ruamel.yaml

yaml = ruamel.yaml.YAML()
yaml.preserve_quotes = True

with open("/path/to/original.yml", "r") as original_file:
  dictionary = yaml.load(open("/path/to/original.yml")

dictionary['domains']['foo.example.com']['region'] = 'us-east-2'
dictionary['domains']['bar.example.com']['region'] = 'us-east-2'

with open("/path/to/new.yml", "w") as new_file:
  yaml.dump(dictionary, new_file)