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

food:
  fruit: "apple"
  veggy: "pepper"

And you want to modify the file, appending grain: rice. 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 append grain: rice. This uses yaml.load to load the YAML file into a dictionary and then appends grain: rice to the dictionary and then creates a new YAML file containing the updated dictionary.

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

yaml = ruamel.yaml.YAML()

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

dictionary['food']['grain'] = "rice"

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