prize18-10  Exercise 3: Locating the Faulty Stage
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Plan: build four correct round-trip stages (file, JSON, database, export). For each experiment replace exactly one stage by a faulty version and call
first_bad_stage, which feeds the text through the stages in order and returns the first one whose output differs from the original.

ex3.py:

  import json, sqlite3, tempfile, pathlib

  original = "Zo\u00eb\u2019s caf\u00e9 \u20ac5"
  tmp = pathlib.Path(tempfile.mkdtemp())

  def lenient_decode(raw):
      out = []
      for b in raw:
          try:
              out.append(bytes([b]).decode("cp1252"))
          except UnicodeDecodeError:
              out.append(chr(b))
      return "".join(out)

  def file_ok(s):
      p = tmp / "a.txt"
      p.write_text(s, encoding="utf-8", newline="\n")
      return p.read_text(encoding="utf-8")

  def json_ok(s):
      return json.loads(json.dumps({"v": s}))["v"]

  def db_ok(s):
      con = sqlite3.connect(":memory:")
      con.execute("CREATE TABLE t (v TEXT)")
      con.execute("INSERT INTO t VALUES (?)", (s,))
      return con.execute("SELECT v FROM t").fetchone()[0]

  def wrong_read(s):
      return lenient_decode(s.encode("utf-8"))

  def to_ascii(s):
      return s.encode("ascii", errors="replace").decode("ascii")

  def first_bad_stage(text, stages):
      current = text
      for name, fn in stages:
          current = fn(current)
          if current != text:
              return name, current
      return None, current

  names = ["file", "json", "database", "export"]
  good = {"file": file_ok, "json": json_ok, "database": db_ok, "export": json_ok}
  for fault, bad in [(None, None), ("file", wrong_read), ("database", wrong_read), ("export", to_ascii)]:
      stages = [(n, bad if n == fault else good[n]) for n in names]
      name, result = first_bad_stage(original, stages)
      print(f"fault in {fault!s:9} -> first bad stage: {name!s:9} {result}")

Real output (Python 3.14):

  fault in None      -> first bad stage: None      Zoë’s café €5
  fault in file      -> first bad stage: file      Zoë’s café €5
  fault in database  -> first bad stage: database  Zoë’s café €5
  fault in export    -> first bad stage: export    Zo??s caf? ?5

Each fault is reported at the stage where it was injected, and the fault-free run reports none. The mojibake outputs could be repaired with peel (they are
UTF-8 bytes read as Windows-1252); the last output has question marks where characters could not be encoded, so the information is gone: the repair
must be made by fixing the export stage and exporting again from the original data.