#411 Invalid YAML output with empty array when there is a comment between key and values

[Guillermo Julián]

I found a bug when dealing with YAML entries when there is a comment between the key and the array values. When I change the contents of the array to be an empty array, the library outputs an invalid YAML syntax, with the comment between the key name and an empty array ([]).

Code for reproduction (uploaded as file too just in case):

#!/usr/bin/env python3

from ruamel.yaml import YAML, __version__

yaml = YAML()

testfile = '/tmp/test.yml'

contents = """
root:
    # A comment here

    - key: value
"""

print(f"YAML version: {__version__}")

with open(testfile, 'w', encoding='UTF-8') as f:
    f.write(contents)

with open(testfile, 'r', encoding='UTF-8') as f:
    data = yaml.load(f)

print(f'Loaded data: {data}')
data['root'] = []

with open(testfile, 'w', encoding='UTF-8') as f:
    yaml.dump(data, f)

with open(testfile, 'r', encoding='UTF-8') as f:
    print("File contents after modification:")
    print(f.read())

    f.seek(0)

    try:
        new_data = yaml.load(f)
    except Exception as e:
        print(f"Re-read failed: {e}")

Output on my machine:

```
YAML version: 0.17.17
Loaded data: ordereddict([('root', [ordereddict([('key', 'value')])])])
File contents after modification:
root:
# A comment here
[]

Re-read failed: while scanning a simple key
in "/tmp/test.yml", line 3, column 1
could not find expected ':'
in "/tmp/test.yml", line 4, column 1
```

If I manage to get mercurial going on my machine I might give it a try this evening and see if I can fix it and send you a MR.

Seems to be related to https://sourceforge.net/p/ruamel-yaml/tickets/398/ and https://sourceforge.net/p/ruamel-yaml/tickets/397.

[Guillermo Julián]

I got to the causing code but I'm not sure how to fix it properly without breaking things. To me it seems that self.write_pre_comment(self.event) on emitter.py:437 should check if it is an empty sequence before writing it, but I am not sure what should be done with that comment and where to put it.

[Anthon van der Neut]

There is no easy fix, otherwise I would already have pushed out a new version. I'll be looking at this when I have time, but comments between keys and value need work, as does handling flow style collections (which are forced upon us as YAML has no block style empty collections)

[Anthon van der Neut]

This should have been fixed in 0.17.18