Preserve quotes and also add data with quotes in Ruamel

In order to preserve quotes (and literal block style) for string scalars, ruamel.yaml¹—in round-trip-mode—represents these scalars as SingleQuotedScalarString, DoubleQuotedScalarString and PreservedScalarString. The class definitions for these very thin wrappers can be found in scalarstring.py. When serializing such instances are written "as they were read", although sometimes the representer falls back to double quotes when things get difficult, as that can represent any string.

To get this behaviour when adding new key-value pairs (or when updating an existing pair), you just have to create these instances yourself:

import sys
from ruamel.yaml import YAML
from ruamel.yaml.scalarstring import SingleQuotedScalarString, DoubleQuotedScalarString

yaml_str = """\
---
a: '1'
b: "2"
c: 3
"""

yaml = YAML()
yaml.preserve_quotes = True
yaml.explicit_start = True
data = yaml.load(yaml_str)
data['foo'] = SingleQuotedScalarString('bar')
data.yaml_add_eol_comment('# <- single quotes added', 'foo', column=20)
yaml.dump(data, sys.stdout)

gives:

---
a: '1'
b: "2"
c: 3
foo: 'bar'          # <- single quotes added

the yaml.explicit_start = True recreates the (superfluous) document start marker. Whether such a marker was in the original file or not is not "known" by the top-level dictionary object, so you have to re-add it by hand.

Please note that without preserve_quotes, there would be (single) quotes around the values 1 and 2 anyway to make sure they are seen as string scalars and not as integers.

---

¹ _{Of which I am the author.}

Since Ruamel 0.15, set the preserve_quotes flag like this:

from ruamel.yaml import YAML
from pathlib import Path

yaml = YAML(typ='rt') # Round trip loading and dumping
yaml.preserve_quotes = True
data = yaml.load(Path("in.yaml"))
yaml.dump(data, Path("out.yaml"))