python - PyYAML dump format

45
votes

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.

3
pythonpython-3.xyamlquotespyyaml
yaml.dump has a default_style argument. Using default_style='"' will keep your string values in double quotes, but your keys and any other value types will also be wrapped in double quotes. - dirn
Thanks!, I'll keep it in mind, it would have been really useful if it wasn't for the keys :( - nicosantangelo
You'll probably have a hard time ordering the keys, too. yaml.load gives you a dict; its keys are unordered. yaml.dump probably outputs in whatever order the iteration goes. - dirn
The new file represents exactly the same information (in YAML) as the origin file; there is no reason to keep the same format. - user707650
@Evert that's true, but I wanted to keep the format because it's useful given the context of the sublime package I have created github.com/NicoSantangelo/sublime-text-i18n-rails - nicosantangelo

3 Answers

83
votes

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.

4
votes

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

2
votes

First

To represent dictionary data is used following code:

mapping = list(mapping.items())
    try:
        mapping = sorted(mapping)
    except TypeError:
        pass

It is why ordering is changed

Second

Information about how scalar type was presented (with double quote or not) is lost when reading (this is principal approach of library)

Summary

You can create own class based on 'Dumper' and to overload method 'represent_mapping' for changing behaviour how dictionary will be presented

For saving information about double quotes for scalar you must also create own class based on 'Loader', but i am afraid that it will affect and other classes and will doing it difficult