PyYAML dump format

Question

I know there are a few questions about this on SO, but I couldn't find what I was looking for.

I'm using pyyaml to read (.load()) a .yml file, modify or add a key, and then write it (.dump()) again. The problem is that I want to keep the file format post-dump, but it changes.

For example, I edit the key en.test.index.few to say "Bye" instead of "Hello"

Python:

with open(path, 'r', encoding = "utf-8") as yaml_file:
    self.dict = pyyaml.load(yaml_file)

Then, afther changing the key:

with open(path, 'w', encoding = "utf-8") as yaml_file:
    dump = pyyaml.dump(self.dict, default_flow_style = False, allow_unicode = True, encoding = None)
    yaml_file.write( dump )

Yaml:

Before:

en:
  test:
    new: "Bye"
    index:
      few: "Hello"
  anothertest: "Something"

After:

en:
  anothertest: Something
  test:
    index:
      few: Hello
    new: Bye

Is there a way to keep the same format?, for example the qoutes and order. Am I using the wrong tool for this?

I know maybe the original file it's not entirely correct, but I have no control over it (it's a Ruby on Rails i18n file).

Thank you very much.

Answer 1

Below, ruamel.yaml is used instead.

ruamel.yaml is actively maintained. Unlike PyYAML, ruamel.yaml supports:

• YAML <= 1.2. PyYAML only supports YAML <= 1.1. This is vital, as YAML 1.2 intentionally breaks backward compatibility with YAML 1.1 in several edge cases. This would usually be a bad thing. In this case, this renders YAML 1.2 a strict superset of JSON. Since YAML 1.1 is not a strict superset of JSON, this is a good thing.
• Roundtrip preservation. When calling yaml.dump() to dump a dictionary loaded by a prior call to yaml.load():
  • PyYAML naively ignores all input formatting – including comments, ordering, quoting, and whitespace. Discarded like so much digital refuse into the nearest available bit bucket.
  • ruamel.yaml cleverly respects all input formatting. Everything. The whole stylistic enchilada. The entire literary shebang. All.

Library Migration

Switching from PyYAML to ruamel.yaml in existing applications is typically as simple as changing the library import to:

from ruamel import yaml

This works because ruamel.yaml is a PyYAML fork that conforms to the PyYAML API.

No other changes should be needed. The yaml.load() and yaml.dump() functions should continue to behave as expected.

Roundtrip Preservation and What It Can Do for You

For backward compatibility with PyYaml, the yaml.load() and yaml.dump() functions do not perform roundtrip preservation by default. To do so, explicitly pass:

• The optional Loader=ruamel.yaml.RoundTripLoader keyword parameter to yaml.load().
• The optional Dumper=ruamel.yaml.RoundTripDumper keyword parameter to yaml.dump().

An example kindly "borrowed" from ruamel.yaml documentation:

import ruamel.yaml

inp = """\
# example
name:
  # Yet another Great Duke of Hell. He's not so bad, really.
  family: TheMighty
  given: Ashtaroth
"""

code = ruamel.yaml.load(inp, Loader=ruamel.yaml.RoundTripLoader)
code['name']['given'] = 'Astarte'  # Oh no you didn't.

print(ruamel.yaml.dump(code, Dumper=ruamel.yaml.RoundTripDumper), end='')

It is done. Comments, ordering, quoting, and whitespace will now be preserved intact.