Dumping multiline string gives incorrect result

[rjekker]

(This is related to issue #240). Dumping a multiline string adds newlines, so the dumped data is different from the actual data. Consider the following:

Input.yaml

---
- |
  Hi!
  Second Line
  Another Line

My program:

import yaml
with open("input.yaml") as db:
    data = yaml.load(db)
    print(yaml.dump(data, default_flow_style=False))

The output:

- 'Hi!

  Second Line

  Another Line

  '

This is clearly incorrect: this simple program changes my data even though I am simply reading and dumping it

[perlpunk]

It's not a bug. Both strings are actually the same. Represented in doublequotes:
"Hi!\nSecond Line\nAnother Line\n"
In single quotes, several lines get folded, so newlines vanish. Only if you have empty lines in between, the newline does not get folded.

May I recommend my article about quoting strings?
http://blogs.perl.org/users/tinita/2018/03/strings-in-yaml---to-quote-or-not-to-quote.html

I agree that in this case it would be nicer to output with the same block scalar style |, but figuring out which string style to use when dumping is not trivial.

[schollii]

How about capturing some metadata about string at load time, eg if there is "|" take note of it for when it gets dumped. On a new element, you should be able to add that metadata.

[perlpunk]

@schollii sure, that would be possible, and ruamel.yaml does it.
But PyYAML has other issues right now that are more urgent IMHO, like YAML 1.2 support