Let's say you have a YAML file named original.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/original.yml", "r") as file:
  dictionary = yaml.load(file)

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

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

Of, if you are feeling brave, you could overwrite the original YAML file with the updated dictionary.

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

yaml = ruamel.yaml.YAML()

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

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

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

Let's say you YAML file contains a list of items.

foods:
- type: fruit
  item: banana
- type: fruit
  item: apple

Here is how you could append another item to the YAML list.

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

yaml = ruamel.yaml.YAML()

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

dictionary['foods'].append({'type': 'fruit', 'item': 'grapes' })

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

Be aware that if you have a long value, by default, the value will be wrapped.

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

yaml = ruamel.yaml.YAML()

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

dictionary['foods'].append({'type': 'fruit', 'item': 'asdfasfas dasfasdfasdfdasf adsfasdfafdasf asdfasdfasdfdasf asdfasdfdasfadsfasf asdfasdfdasfasfdas asdfasdfdasfdfas' })

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

In this example, new.yml would have something like this.

foods:
- type: fruit
  item: banana
- type: fruit
  item: apple
- type: fruit
  item: asdfasfas dasfasdfasdfdasf adsfasdfafdasf asdfasdfasdfdasf asdfasdfdasfadsfasf
    asdfasdfdasfasfdas asdfasdfdasfdfas

The yaml.width option can be used to set a long enough width to prevent the wrapping.

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

yaml = ruamel.yaml.YAML()
yaml.width = 1000

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

dictionary['foods'].append({'type': 'fruit', 'item': 'asdfasfas dasfasdfasdfdasf adsfasdfafdasf asdfasdfasdfdasf asdfasdfdasfadsfasf asdfasdfdasfasfdas asdfasdfdasfdfas' })

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