PyYAML dump format

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.

In my case, I want " if value contains a { or a }, otherwise nothing. For example:

 en:
   key1: value is 1
   key2: 'value is {1}'

To perform that, copy function represent_str() from file representer.py in module PyYaml and use another style if string contains { or a }:

def represent_str(self, data):
    tag = None
    style = None
    # Add these two lines:
    if '{' in data or '}' in data:
        style = '"'
    try:
        data = unicode(data, 'ascii')
        tag = u'tag:yaml.org,2002:str'
    except UnicodeDecodeError:
        try:
            data = unicode(data, 'utf-8')
            tag = u'tag:yaml.org,2002:str'
        except UnicodeDecodeError:
            data = data.encode('base64')
            tag = u'tag:yaml.org,2002:binary'
            style = '|'
    return self.represent_scalar(tag, data, style=style)

To use it in your code:

import yaml

def represent_str(self, data):
  ...

yaml.add_representer(str, represent_str)

In this case, no diffences between keys and values and that's enough for me. If you want a different style for keys and values, perform the same thing with function represent_mapping