Preserve front matter ordering

It’s a tricky thing to do with PyYAML, but using http://stackoverflow.com/a/21048064/5483904 it’s possible. Once you fix the loader up, the things that stand in the way are:

• parse takes in defaults as **kwargs, which is automatically a dict. Additionally, you can’t specify a custom loader, so you have to set it globally.
• Once you fix that, Post.__init__ takes the metadata as **kwargs yet again, which destroys the ordered dict received by the prior function

I suspect to really do this, one would have to break backwards compat. Here’s my hacky way of accomplishing this in case anyone else wants this Right Now:

#  Patch yaml first

import collections
import yaml

_mapping_tag = yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG

def dict_representer(dumper, data):
    return dumper.represent_dict(data.items())

def dict_constructor(loader, node):
    loader.flatten_mapping(node)
    return collections.OrderedDict(loader.construct_pairs(node))

try:
    yaml.CSafeDumper.add_representer(collections.OrderedDict, dict_representer)
except ImportError:
    pass

yaml.SafeDumper.add_representer(collections.OrderedDict, dict_representer)
yaml.SafeLoader.add_constructor(_mapping_tag, dict_constructor)

# patch frontmatter second
import frontmatter

def _parse(text, **defaults):
    text = frontmatter.u(text)

    # metadata starts with defaults
    metadata = defaults.copy()

    try:
        _, fm, content = frontmatter.FM_BOUNDARY.split(text, 2)
    except ValueError:
        return metadata, text

    fm = yaml.safe_load(fm)

    if isinstance(fm, dict):
        fm.update(metadata)
        metadata = fm

    return metadata, content.strip()

def _loads(text, **defaults):
    metadata, content = _parse(text, **defaults)
    post = frontmatter.Post(content)
    post.metadata = metadata
    return post

frontmatter.parse = _parse
frontmatter.loads = _loads

[dirkcuys]

I’ve managed to get order preserving frontmatter! The quick version is:

1. upgrade to python >= 3.7
2. Add sort_keys=False to frontmatter.dump ie frontmatter.dump(post, f, sort_keys=False)

Since python 3.7 insertion order for dicts are maintained (no OrderedDict needed) and since then PyYaml have added the option to not sort keys when dumping. kwargs is passed to handler.export, which is passed to yaml.dump and things just work!

[eyeseast]

I’m happy if anyone has a PR that can solve this. For anything in the core library, updates should be added at the handler level.