Support for Pandoc flavored YAML front matter

[zengxs]

Wanted to see about adding support for Pandoc flavored YAML front matter. That flavor allows use 3 dots (...) to ending a YAML front matter.

---

From YAML specs:

• Three dashes --- is used to separate directives from document content. This also serves to signal the start of a document if no directives are present.
• Three dots ... indicate the end of a document without starting a new one, for use in communication channels

[NGenetzky]

@zengxs , in theory, this should be possible by using the YAMLHandler to instantiate a handler and then use it. Unfortunately, this doesn't work. Since this issue is the answer to the universe 😺 I might consider creating a PR for it if you had a need for it.

import re
from frontmatter import YAMLHandler
yaml_pandoc_re = re.compile(r'^(?:---)|(?:\.\.\.)$', re.MULTILINE)
handler = YAMLHandler(
    fm_boundary=yaml_pandoc_re, start_delimiter='---', end_delimiter='...',
)

[eyeseast]

Here's a solution, for anyone still watching this:

class PandocHandler(frontmatter.YAMLHandler):
    START_DELIMITER = "---"
    END_DELIMITER = "..."

    def split(self, text):
        "It's going to take a few passes to split here"
        _, start, text = text.partition(self.START_DELIMITER)
        fm, end, content = text.partition(self.END_DELIMITER)
        return fm, content

post = frontmatter.loads(text, handler=PandocHandler())

Pandoc format uses different start and end delimiters, which trips up parsing a bit. To make it work, we need a custom handler that deals with each part separately.

There are probably other ways to solve this, but this will work.