How to round trip ruamel.yaml strings like "on" - ruamel.yaml

Question

When using ruamel.yaml to round-trip some YAML I see the following issue. Given this input:
root:
matchers:
- select: "response.body#state"
test: all
expected: "on"
I see this output:
root:
matchers:
- select: response.body#state
test: all
expected: on
Note that in YAML, on parses as a boolean true value while off parses as false.
The following code is used to read/write:
# Use the default (round-trip) settings.
yaml = YAML()
if args.source == '-':
src = sys.stdin
else:
src = open(args.source)
doc = yaml.load(src)
process(args.tag, set(args.keep.split(',')), doc)
if args.destination == '-':
dest = sys.stdout
else:
dest = open(args.destination, 'w')
yaml.dump(doc, dest)
The process function is not modifying values. It only removes things with a special tag in the input after crawling the structure.
How can I get the output to be a string rather than a boolean?

Answer 1

You write that:
Note that in YAML, on parses as a boolean true value while off parses as false.
That statement is not true (or better:
has not been true for ten years). If you have an unquoted on in your
YAML, like in your output, that is obviously not the case when using ruamel.yaml:
import sys
import ruamel.yaml
yaml_str = """\
root:
matchers:
- select: response.body#state
test: all
expected: on
"""
yaml = ruamel.yaml.YAML()
data = yaml.load(yaml_str)
expected = data['root']['matchers'][0]['expected']
print(type(expected), repr(expected))
which gives:
<class 'str'> 'on'
This is because in the YAML 1.2 spec on/off/yes/no are no
longer mentioned as having the same meaning as true
resp. false. They are mentioned in the YAML 1.1 spec, but that was
superseded in 2009. Unfortunately there are YAML libraries out in the
wild, that have not been updated since then.
What is actually happening is that the suprefluous quotes in your
input are automatically discarded by the round-trip process. You can
also see that happen for the value "response.body#state". Although
there the character that starts comments (#) is included, to
actually start a comment that character has to be proceded by
white-space, and since it is isn't, the quotes are not necessary.
So your output is fine, but if you are in the unfortunate
situation where you have to deal with other programs relying on
outdated YAML 1.1, then you can e.g. specify that you want to preserve
your quotes on round-trip:
yaml_str = """\
root:
matchers:
- select: "response.body#state"
test: all
expected: "on"
"""
yaml = ruamel.yaml.YAML()
yaml.indent(sequence=4, offset=2)
yaml.preserve_quotes = True
data = yaml.load(yaml_str)
yaml.dump(data, sys.stdout)
as this gives your exact input:
root:
matchers:
- select: "response.body#state"
test: all
expected: "on"
However maybe the better option would be that you actually specify that your
YAML is and has to conforming to the YAML 1.1 specification by making
your intensions, and the output document, explicit:
yaml_str = """\
root:
matchers:
- select: response.body#state
test: all
expected: on
"""
yaml_in = ruamel.yaml.YAML()
yaml_out = ruamel.yaml.YAML()
yaml_out.indent(sequence=4, offset=2)
yaml_out.version = (1, 1)
data = yaml_in.load(yaml_str)
yaml_out.dump(data, sys.stdout)
Notice that the "unquoted" YAML 1.2 input, gives output where on is quoted:
%YAML 1.1
---
root:
matchers:
- select: response.body#state
test: all
expected: 'on'