Round-trip parsing of data structure format (YAML or whatnot) preserving comments, for writing configuration

Question

I have been using YAML as configuration file format in several applications, and all went well except one thing: when my program needs to write/modify a config variable in a YAM

Answer 1

One approach to this is using "lenses". See Augeas for one implementation.

Answer 2

Yeah, you and everyone who thought wow, yaml sounds cool, simply put, it doesn't exist, yet

update: you probably want to use Config::General, its apache config format (xmlish)

No, PPI is not general purpose tool, if you want BNF-ness, you want to use Marpa

Of all INI/JSON/YAML/XML, XML probably has the best editor support for non-programmers (sounds crazy)

Answer 3

If you are using block structured YAML and Python is acceptable, you can use the Python package¹ ruamel.yaml which is a derivative of PyYAML and supports round trip preservation of comments:

import sys
import ruamel.yaml

inp = """\
# example
name:
  # details
  family: Smith   # very common
  given: Alice    # one of the siblings
"""

yaml = ruamel.yaml.YAML()

code = yaml.load(inp)
code['name']['given'] = 'Bob'

yaml.dump(code, sys.stdout)

with result:

# example
name:
  # details
  family: Smith   # very common
  given: Bob      # one of the siblings

Note that the end-of-line comments are still aligned.

Instead of normal list and dict objects the code consists of wrapped versions² on which the comments attached.

¹ _{Install with pip install ruamel.yaml. Works on Python 2.6/2.7/3.3+. Disclaimer: I am the author of that package.}
² _{ordereddict is used in case of a mapping, to preserve ordering}