First attempt at Python port, expand tests.

packages/python/package.json

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+{
+  "name": "@terrace/python",
+  "version": "0.0.1",
+  "license": "MIT",
+  "scripts": {
+    "test": "python3 ./test/index.py"
+  }
+}

packages/python/parser.py

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+from typing import TypedDict
+
+class LineData(TypedDict):
+  line: str
+  indent: str
+  level: int
+  offsetHead: int
+  offsetTail: int
+
+def createLineData(line: str = '', indent: str = ' ') -> LineData:
+  return { "line": line, "indent": indent, "level": 0, "offsetHead": 0, "offsetTail": 0 }
+
+def parseLine(lineData: LineData) -> LineData:
+  # if ((typeof lineData !== 'object' || !lineData) || typeof lineData.level !== 'number') throw new Error(`'lineData' must be an object with string line and numeric level properties`)
+  # if (typeof lineData.indent !== 'string' || lineData.indent.length === 0 || lineData.indent.length > 1) throw new Error(`'lineData.indent' must be a single-character string`)
+  # if (typeof lineData.line !== 'string') throw new Error(`'lineData.line' must be a string`)
+
+  level = 0
+
+  # Repeat previous level for blank lines.
+  if len(lineData['line']) == 0:
+    lineData['level'] = lineData['level']
+    lineData['offsetHead'] = 0
+    lineData['offsetTail'] = 0
+  else:
+    while level < len(lineData['line']) and lineData['line'][level] == lineData['indent'] and level <= lineData['level'] + 1:
+      level += 1
+    lineData['level'] = level
+    lineData['offsetHead'] = level
+    lineData['offsetTail'] = level
+
+    while lineData['offsetTail'] < len(lineData['line']) and lineData['line'][lineData['offsetTail']] != ' ':
+      lineData['offsetTail'] += 1
+
+  return lineData

packages/python/test/index.py

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+import sys
+import os
+
+sys.path.insert(1, os.path.join(sys.path[0], '..'))
+
+from parser import createLineData, parseLine
+
+def next():
+  return sys.stdin.readline().rstrip('\n')
+
+def linedata_basic ():
+  lineData = createLineData('')
+
+  while l := next():
+    lineData['line'] = l
+    parseLine(lineData)
+    print("| level {level} | indent {indent} | offsetHead {offsetHead} | offsetTail {offsetTail} | line {line} |".format(
+      level = lineData['level'], indent = lineData['indent'], offsetHead = lineData['offsetHead'], offsetTail = lineData['offsetTail'], line = lineData['line']
+    ))
+
+def linedata_tabs ():
+  lineData = createLineData('', '\t')
+
+  while l := next():
+    lineData['line'] = l
+    parseLine(lineData)
+    print("| level {level} | indent {indent} | offsetHead {offsetHead} | offsetTail {offsetTail} | line {line} |".format(
+      level = lineData['level'], indent = lineData['indent'], offsetHead = lineData['offsetHead'], offsetTail = lineData['offsetTail'], line = lineData['line']
+    ))
+
+def linedata_head_tail ():
+  lineData = createLineData('')
+
+  while l := next():
+    lineData['line'] = l
+    parseLine(lineData)
+    head = lineData['line'][lineData['offsetHead']:lineData['offsetTail']] if len(lineData['line']) > lineData['offsetTail'] else ''
+    tail = lineData['line'][lineData['offsetTail'] + 1:] if len(lineData['line']) > lineData['offsetTail'] + 1 else ''
+
+    print("| head {head} | tail {tail} |".format(
+      head = head, tail = tail
+    ))
+
+
+tests = {
+  'linedata:basic': linedata_basic,
+  'linedata:tabs': linedata_tabs,
+  'linedata:head-tail': linedata_head_tail
+}
+
+
+def main():
+  testName = sys.argv[1]
+  test = tests[testName]
+  test()
+
+if __name__ == "__main__":
+  main()