How can I add a blank line before some data using

Question

I can't seem to figure out how I can add a blank line between data using Ruamel.yaml.

Suppose I have data:

---
a: 1

b: 2

I need to add to this so that I will have:

---
a: 1

b: 2

c: 3

I understand that the blank line is implemented as a CommentToken:

Comment(comment=None,
  items={'data': [None, None, CommentToken(value=u'\n\n'), None], 'b': [None, None, CommentToken(value=u'\n\n'), None]})

What I don't know is how to manipulate that structure.

Answer 1

That Comment object is not from the input that you give, as data is not a key in your mapping, that should be a:

import ruamel.yaml

yaml_strs = [
"""\
---
a: 1

b: 2
""",
"""\
---
a: 1

b: 2

c: 3
"""]

for yaml_str in yaml_strs:
    data = ruamel.yaml.round_trip_load(yaml_str)
    print(data.ca)

gives:

Comment(comment=None,
  items={'a': [None, None, CommentToken(), None]})
Comment(comment=None,
  items={'a': [None, None, CommentToken(), None], 'b': [None, None, CommentToken(), None]})

comparing the above comments should give you an idea of what to try:

import sys
import ruamel.yaml

yaml_str = """\
---
a: 1

b: 2
"""

data = ruamel.yaml.round_trip_load(yaml_str)
data['c'] = 3
ct = data.ca.items['a'][2]
data.ca.items['b'] = [None, None, ct, None]
ruamel.yaml.round_trip_dump(data, sys.stdout)

which gives:

a: 1

b: 2

c: 3

The CommentToken ct can also be constructed from scratch:

ct = ruamel.yaml.tokens.CommentToken('\n\n', ruamel.yaml.error.CommentMark(0), None)

as is, e.g. done in ruamel.yaml.comments.CommentedBase.yaml_set_start_comment().

The 0 parameter to CommentMark() is how far the comment is indented, which is not important in case of empty lines, but still needs to be provided.