python ruamel.yaml package, how to get header comment lines?

Question

I want to get YAML file comments on header lines, like

# 11111111111111111
# 11111111111111111
# 22222222222222222
# bbbbbbbbbbbbbbbbb
---
start:
....

And I used the ca attribute on the loaded data, butfound there are no these comments on it. Is there any other way to get these comments?

Answer 1

Currently (ruamel.yaml==0.17.17) the comments that occur before the document start token (---) are not passed on from the DocumentStartToken to the DocumentStartEvent, so these comments are effectively lost during parsing. Even if they were passed on, it is non-trivial to preserve them as the DocumentStartEvent is silently dropped during composition.

You can either put the comments after the end of directives indicator (---) which allows you to get at the comments using the .ca attribute without a problem, or remove that indicator altogether as it is superfluous (at least in your example). Alternatively you will have to write a small wrapper around the loader:

import sys
import pathlib
import ruamel.yaml

fn = pathlib.Path('input.yaml')

def load_with_pre_directives_comments(yaml, path):
    comments = []
    text = path.read_text()
    if '\n---\n' not in text and '\n--- ' not in text:
         return yaml.load(text), comments
    for line in text.splitlines(True):
        if line.lstrip().startswith('#'):
            comments.append(line)
        elif line.startswith('---'):
            return yaml.load(text), comments
            break

yaml = ruamel.yaml.YAML()
yaml.explicit_start = True
data, comments = load_with_pre_directives_comments(yaml, fn)
print(''.join(comments), end='')
yaml.dump(data, sys.stdout)

which gives:

# 11111111111111111
# 11111111111111111
# 22222222222222222
# bbbbbbbbbbbbbbbbb
---
start: 42