286 lines
9.7 KiB
Python
286 lines
9.7 KiB
Python
"""A strict, dependency-free frontmatter reader for kandidat.md and sak.md.
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No YAML runtime is allowed here (risk H7), so this module hand-rolls a parser
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that sits directly on the hard-filter path: `lonn.gulv_nok` and
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`geografi.maks_reisetid_min` are read from here and compared numerically by
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scoring. A hand-rolled parser on that path is only safe if it is loud, so the
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one rule this module never breaks is that anything outside the grammar below
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raises :class:`FrontmatterError` carrying the offending line number. It never
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guesses, never applies a default, and never skips a line it did not understand.
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Grammar
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-------
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The whole accepted grammar, and nothing beyond it::
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document := [ "---" NL entries "---" NL ] body
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entry := comment | blank | mapping | block
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comment := "#" ... -- only when "#" is the FIRST column
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mapping := KEY ":" SP scalar
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block := KEY ":" NL ( child | item )+
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child := " " KEY ":" SP scalar -- one level of nesting, no more
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item := " - " scalar -- a flat list, scalars only
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KEY := [A-Za-z_][A-Za-z0-9_]*
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scalar := integer | '"' text '"' | "'" text "'" | unquoted-text
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Indentation is exactly zero or two spaces; a tab in the indentation is an
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error rather than a width the reader has to assume. A `#` anywhere but the
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first column is data -- `notat: rolle nr # 3` keeps its hash, because trimming
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it would silently rewrite what the operator wrote. Inside double quotes only
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``\\"`` and ``\\\\`` are escapes; single quotes take the text verbatim.
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A duplicate key is refused instead of resolving last-wins: two salary floors
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in one profile must stop the run, not pick one.
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A document with no opening fence has no frontmatter -- that is an empty
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mapping and an untouched body, not an error. Rejecting a kandidat.md that
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lacks frontmatter is the section 5.1 contract check's job, not the reader's.
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Preservation
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------------
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:func:`render` writes values back exactly as they came in: no Unicode
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normalisation, no escaping of non-ASCII, no reordering. `paths.slug`
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normalises deliberately; this module must not, because the file on disk is
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the operator's own text and a round trip through here has to return the same
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bytes.
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"""
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import re
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from . import paths
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#: Keys are conservative identifiers: the frontmatter is a machine contract,
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#: not free text, and a key with a space in it is a typo worth stopping for.
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KEY_RE = re.compile(r"^([A-Za-z_][A-Za-z0-9_]*):(.*)$")
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INTEGER_RE = re.compile(r"^-?\d+$")
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FENCE = "---"
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INDENT = " "
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class FrontmatterError(Exception):
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"""A line outside the grammar, reported with the line it was on.
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``line`` is 1-based and counts from the start of the document, so it
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matches what an editor shows.
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"""
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def __init__(self, line, message):
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self.line = line
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super().__init__("line %d: %s" % (line, message))
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def parse(text):
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"""Parse ``text`` into ``(metadata, body)``.
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``metadata`` preserves key order. ``body`` is everything after the closing
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fence, byte for byte. A document without an opening fence returns
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``({}, text)``.
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"""
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if not text.startswith(FENCE + "\n") and text.rstrip("\n") != FENCE:
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return {}, text
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lines = text.split("\n")
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closing = _find_closing_fence(lines)
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body = "\n".join(lines[closing + 1:])
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return _parse_entries(lines[1:closing], first_lineno=2), body
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def render(metadata, body=""):
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"""Render ``metadata`` and ``body`` back into a document.
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The inverse of :func:`parse` for any document written in the canonical
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form this function emits. Values are written as they are held: Norwegian
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characters go out as themselves, never escaped and never normalised.
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"""
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out = [FENCE]
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for key, value in metadata.items():
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if isinstance(value, dict):
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out.append("%s:" % key)
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out.extend("%s%s: %s" % (INDENT, k, _scalar_out(v)) for k, v in value.items())
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elif isinstance(value, list):
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out.append("%s:" % key)
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out.extend("%s- %s" % (INDENT, _scalar_out(item)) for item in value)
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else:
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out.append("%s: %s" % (key, _scalar_out(value)))
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out.append(FENCE)
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return "\n".join(out) + "\n" + body
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def read(root, *parts):
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"""Read and parse a file resolved under ``root`` via :func:`paths.safe_join`.
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Going through ``safe_join`` rather than ``os.path.join`` means a caller
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that hands in a traversing path gets a refusal here too, not only in the
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scripts that remembered to check.
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"""
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target = paths.safe_join(root, *parts)
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with open(target, "r", encoding="utf-8") as handle:
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return parse(handle.read())
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def _find_closing_fence(lines):
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for index in range(1, len(lines)):
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if lines[index] == FENCE:
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return index
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raise FrontmatterError(
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1, "frontmatter fence opened here was never closed with %r" % FENCE
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)
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def _parse_entries(lines, first_lineno):
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metadata = {}
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open_key = None
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open_lineno = None
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block = None # None until the first child decides mapping or list
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for offset, line in enumerate(lines):
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lineno = first_lineno + offset
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if line.strip() == "":
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continue
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if line.startswith("#"):
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continue
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indent = _indent_width(line, lineno)
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content = line[indent:]
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if indent == 0:
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if open_key is not None and block is None:
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raise _empty_block(open_key, open_lineno)
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open_key, open_lineno, block = _top_level(metadata, content, lineno)
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else:
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if open_key is None:
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raise FrontmatterError(
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lineno, "indented line has no parent key above it"
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)
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block = _child(metadata, open_key, block, content, lineno)
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if open_key is not None and block is None:
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raise _empty_block(open_key, open_lineno)
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return metadata
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def _top_level(metadata, content, lineno):
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if content.startswith("- "):
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raise FrontmatterError(
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lineno, "list item at column 0 has no key to belong to"
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)
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match = KEY_RE.match(content)
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if match is None:
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raise FrontmatterError(
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lineno, "expected %r, got %r" % ("key: value", content)
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)
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key, rest = match.group(1), match.group(2)
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if key in metadata:
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raise FrontmatterError(lineno, "duplicate key %r" % key)
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if rest.strip() == "":
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return key, lineno, None
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metadata[key] = _scalar_in(rest.strip(), lineno)
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return None, None, None
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def _child(metadata, open_key, block, content, lineno):
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if content.startswith("- "):
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if block == "mapping":
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raise FrontmatterError(
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lineno, "list item under %r, which already holds a mapping" % open_key
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)
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metadata.setdefault(open_key, [])
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metadata[open_key].append(_scalar_in(content[2:].strip(), lineno))
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return "list"
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match = KEY_RE.match(content)
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if match is None:
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raise FrontmatterError(
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lineno, "expected %r or %r, got %r" % (" key: value", " - item", content)
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)
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if block == "list":
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raise FrontmatterError(
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lineno, "mapping entry under %r, which already holds a list" % open_key
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)
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key, rest = match.group(1), match.group(2)
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if rest.strip() == "":
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raise FrontmatterError(
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lineno, "%r nests deeper than one level, which the grammar does not allow" % key
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)
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nested = metadata.setdefault(open_key, {})
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if key in nested:
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raise FrontmatterError(lineno, "duplicate key %r under %r" % (key, open_key))
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nested[key] = _scalar_in(rest.strip(), lineno)
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return "mapping"
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def _indent_width(line, lineno):
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leading = line[: len(line) - len(line.lstrip(" \t"))]
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if "\t" in leading:
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raise FrontmatterError(
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lineno, "tab in the indentation; use exactly two spaces per level"
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)
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if len(leading) not in (0, 2):
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raise FrontmatterError(
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lineno,
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"indentation is %d spaces; the grammar allows 0 or 2" % len(leading),
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)
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return len(leading)
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def _empty_block(key, lineno):
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return FrontmatterError(
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lineno, "%r has no value and no indented block beneath it" % key
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)
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def _scalar_in(raw, lineno):
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if raw.startswith('"'):
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if len(raw) < 2 or not raw.endswith('"'):
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raise FrontmatterError(lineno, "unterminated double-quoted value")
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return _unescape(raw[1:-1])
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if raw.startswith("'"):
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if len(raw) < 2 or not raw.endswith("'"):
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raise FrontmatterError(lineno, "unterminated single-quoted value")
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return raw[1:-1]
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if INTEGER_RE.match(raw):
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return int(raw)
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return raw
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def _unescape(text):
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out = []
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index = 0
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while index < len(text):
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char = text[index]
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if char == "\\" and index + 1 < len(text) and text[index + 1] in '"\\':
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out.append(text[index + 1])
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index += 2
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continue
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out.append(char)
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index += 1
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return "".join(out)
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def _scalar_out(value):
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if isinstance(value, bool):
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# Booleans are not in the grammar; letting one through here would
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# write a value the reader cannot read back.
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raise ValueError("the grammar has no boolean; write the word you mean")
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if isinstance(value, int):
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return str(value)
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text = str(value)
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if _needs_quoting(text):
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return '"%s"' % text.replace("\\", "\\\\").replace('"', '\\"')
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return text
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def _needs_quoting(text):
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if text == "" or text != text.strip():
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return True
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if text[0] in "#\"'":
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return True
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# An unquoted digit string would come back as an int and stop being the
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# string it was written as.
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return bool(INTEGER_RE.match(text))
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