Ruamel.yaml: How To Access Merge Keys And Comments In Loaded Ordereddict

I have a Python program that is parsing a number of YAML files, some containing comments, anchors, references, and merge keys that I'd like preserved when I load the YAML file into

Solution 1:

Yes you can access the comments, etc. Your mappings (python dict) will be loaded in an instance of CommentedMap and your sequences (python list) in an instance of CommentedSeq. These are subclasses of ordereddict and CommentedBase, resp. of list and CommentedBase.

The CommentedBase has several attributes to which comments, merge, anchor and flow-style information are attached. It has also several methods that set/get these values which rely on some map/sequence specific helper functions.

import sys
from ruamel.yaml import YAML

yaml_str = """\
a: 1   # comment 1
b: 2
c: 3
d: 4
e: 5
"""

yaml = YAML()
data = yaml.load(yaml_str)
data.yaml_add_eol_comment('comment 2', key='c')
data.yaml_add_eol_comment('#  comment 3', key='e', column=8)
yaml.dump(data, sys.stdout)

will give you:

a:1# comment 1b:2c:3# comment 2d:4e:5#  comment 3

Please note that:

• if you don't specify a starting column, the column of the next previous comment is taken.
• a leading # and space will be inserted if a comment string doesn't already start with a that combination of characters.
• there will be at least one space between the scalar and the comment.

The interface is underdocumented, primarily because of laziness of the library author. You best have a look at the tests for comments and for anchors to get some examples. The interface also will also require some changes on the attribute content level e.g. to allow of attachment of EOL comments to keys as well as to key+value combinations. The following YAML doesn't roundtrip as you would expect/correctly:

abc:      # thisis the key
    9989  # thisis the value

So be sure to wrap the functionality that you need so there is a single point where you can make changes if the interface in ruamel.yaml changes in a backwards incompatible way.