How to read a component in YAML file so that I can edit it's key value using ruamel.yaml?

Question

This is my YAML file ( input.yaml ):

team_member:
  name: Max
  hobbies:
    - Reading

team_leader:
  name: Stuart
  hobbies:
    - dancing

I want to edit this YAML file to add more values in key 'hobbies', example:

team_member:
  name: Max
  hobbies:
    - Reading
    - Painting

team_leader:
  name: Stuart
  hobbies:
    - Dancing
    - Fishing

I tried to implement the code Anthon to fit my situation but it didn't helped at all, because the indention level of that YAML file is different from mine.
Example:

import sys
import ruamel.yaml

yaml = ruamel.yaml.YAML()
# yaml.preserve_quotes = True
with open('input.yaml') as fp:
    data = yaml.load(fp)
for elem in data:
    if elem['name'] == 'Stuart':
         elem['hobbies'] = ['Fishing']
         break  # no need to iterate further
yaml.dump(data, sys.stdout)

I get error "TypeError('string indices must be integers',)", I know this code might be completely wrong, but I am new to ruamel.yaml.

How to code this?

Answer 1

The thing missing form the error message displayed is the line number (I assume that it is 9). That points to the line

    if elem['name'] == 'Stuart':

And if that doesn't give you a clue, the approach that I recommend in such cases is starting to add some print functions, so that you know what you are working on. The for loop looks like:

for elem in data:
    print('elem', elem)
    if elem['name'] == 'Stuart':
         print('elem->hobbies', elem['hobbies'])
         elem['hobbies'] = ['Fishing']

this prints

 elem team_member

before the exception is thrown, and I hope that will make you realize your are not iterating over the elem ents (items) of a list, but over the key s of a dict (constructed from the root level mapping in your YAML). And the value associated with the key is the object having a key name and a key hobbies .

So change the variable elem to key to make clear what you're handling and then proceed to work with value , the value associated with that key instead of elem within that loop¹:

for key in data:
    value = data[key]
    if value['name'] == 'Stuart':
         print('value->hobbies', value['hobbies'])
         value['hobbies'] = ['Fishing']

This gives:

value->hobbies ['dancing']
team_member:
  name: Max
  hobbies:
  - Reading

team_leader:
  name: Stuart
  hobbies:
  - Fishing

So we got rid of the exception, but the result is not exactly what you want. The element dancing for the key 'hobbies' is gone, because you assign a new (list) value to that key, whereas what you should do is append a single item to the list. We can also get rid of the print function by now:

for key in data:
    value = data[key]
    if value['name'] == 'Stuart':
         value['hobbies'].append('Fishing')

This will get you two items in the final sequence in the file. There is a few more things to address:

• the capitalization of dancing incorrect. To correct that, add a line handling the list if there is only one element
• the code for the name Max , needs to be added (and that is why you need to get rid of the break in your code)
• the empty line, is considered a comment on the last element of the first sequence, that comment needs to be moved
• your indentation of sequences is non-default

The final code would be like:

from pathlib import Path
import ruamel.yaml

path = Path('input.yaml')
yaml = ruamel.yaml.YAML()
yaml.indent(sequence=4, offset=2)  # for the non-default indentation of sequences

data = yaml.load(path)
for key in data:
    value = data[key]
    if value['name'] == 'Stuart':
         if len(value['hobbies']) == 1:
             value['hobbies'][0] = value['hobbies'][0].capitalize()
         value['hobbies'].append('Fishing')
    elif value['name'] == 'Max':
         last_item_index = len(value['hobbies']) - 1
         value['hobbies'].append('Painting')
         comments = value['hobbies'].ca
         if not comments.items[last_item_index][0].value.strip():
             # move empty comment lines from previous last item to new last item
             comments.items[last_item_index + 1] = comments.items.pop(last_item_index)

yaml.dump(data, path)

Which gives something quite close to what you wanted to get

team_member:
  name: Max
  hobbies:
    - Reading
    - Painting

team_leader:
  name: Stuart
  hobbies:
    - Dancing
    - Fishing

---

¹ _{Alternative for the first two lines: for key, value in data.items()}

Answer 2

Thanks Anthon your code worked I have to edit this code as follows:

import sys
import ruamel.yaml
from pathlib import Path

yaml = ruamel.yaml.YAML()
path = Path('input.yaml')
yaml.indent(sequence=4, offset=2)  # for the non-default indentation of sequences
with open(path) as fp:
    data = yaml.load(fp)
for key in data:
    value = data[key]
    if value['name'] == 'Stuart':
         if len(value['hobbies']) == 1:
             value['hobbies'][0] = value['hobbies'][0].capitalize()
         value['hobbies'].append('Fishing')
    elif value['name'] == 'Max':
         last_item_index = len(value['hobbies']) - 1
         value['hobbies'].append('Painting')
         comments = value['hobbies'].ca
         if not comments.items[last_item_index][0].value.strip():
             # move empty comment lines from previous last item to new last item
             comments.items[last_item_index + 1] = comments.items.pop(last_item_index)
yaml.dump(data, path)