text: """Extract, format and print information about Python stack traces."""
  
  import collections
  import itertools
  import linecache
  import sys
  
  __all__ = ['extract_stack', 'extract_tb', 'format_exception',
             'format_exception_only', 'format_list', 'format_stack',
             'format_tb', 'print_exc', 'format_exc', 'print_exception',
             'print_last', 'print_stack', 'print_tb', 'clear_frames',
             'FrameSummary', 'StackSummary', 'TracebackException',
             'walk_stack', 'walk_tb']
  
  #
  # Formatting and printing lists of traceback lines.
  #
  
  def print_list(extracted_list, file=None):
      """Print the list of tuples as returned by extract_tb() or
      extract_stack() as a formatted stack trace to the given file."""
      if file is None:
          file = sys.stderr
      for item in StackSummary.from_list(extracted_list).format():
          print >>file, item,; file.write("")
  
  def format_list(extracted_list):
      """Format a list of traceback entry tuples for printing.
  
      Given a list of tuples as returned by extract_tb() or
      extract_stack(), return a list of strings ready for printing.
      Each string in the resulting list corresponds to the item with the
      same index in the argument list.  Each string ends in a newline;
      the strings may contain internal newlines as well, for those items
      whose source text line is not None.
      """
      return StackSummary.from_list(extracted_list).format()
  
  #
  # Printing and Extracting Tracebacks.
  #
  
  def print_tb(tb, limit=None, file=None):
      """Print up to 'limit' stack trace entries from the traceback 'tb'.
  
      If 'limit' is omitted or None, all entries are printed.  If 'file'
      is omitted or None, the output goes to sys.stderr; otherwise
      'file' should be an open file or file-like object with a write()
      method.
      """
      print_list(extract_tb(tb, limit=limit), file=file)
  
  def format_tb(tb, limit=None):
      """A shorthand for 'format_list(extract_tb(tb, limit))'."""
      return extract_tb(tb, limit=limit).format()
  
  def extract_tb(tb, limit=None):
      """Return list of up to limit pre-processed entries from traceback.
  
      This is useful for alternate formatting of stack traces.  If
      'limit' is omitted or None, all entries are extracted.  A
      pre-processed stack trace entry is a quadruple (filename, line
      number, function name, text) representing the information that is
      usually printed for a stack trace.  The text is a string with
      leading and trailing whitespace stripped; if the source is not
      available it is None.
      """
      return StackSummary.extract(walk_tb(tb), limit=limit)
  
  #
  # Exception formatting and output.
  #
  
  _cause_message = (
      "\nThe above exception was the direct cause "
      "of the following exception:\n\n")
  
  _context_message = (
      "\nDuring handling of the above exception, "
      "another exception occurred:\n\n")
  
  
  def print_exception(etype, value, tb, limit=None, file=None, chain=True):
      """Print exception up to 'limit' stack trace entries from 'tb' to 'file'.
  
      This differs from print_tb() in the following ways: (1) if
      traceback is not None, it prints a header "Traceback (most recent
      call last):"; (2) it prints the exception type and value after the
      stack trace; (3) if type is SyntaxError and value has the
      appropriate format, it prints the line where the syntax error
      occurred with a caret on the next line indicating the approximate
      position of the error.
      """
      # format_exception has ignored etype for some time, and code such as cgitb
      # passes in bogus values as a result. For compatibility with such code we
      # ignore it here (rather than in the new TracebackException API).
      if file is None:
          file = sys.stderr
      for line in TracebackException(
              type(value), value, tb, limit=limit).format(chain=chain):
          print >>file, line,; file.write("")
  
  
  def format_exception(etype, value, tb, limit=None, chain=True):
      """Format a stack trace and the exception information.
  
      The arguments have the same meaning as the corresponding arguments
      to print_exception().  The return value is a list of strings, each
      ending in a newline and some containing internal newlines.  When
      these lines are concatenated and printed, exactly the same text is
      printed as does print_exception().
      """
      # format_exception has ignored etype for some time, and code such as cgitb
      # passes in bogus values as a result. For compatibility with such code we
      # ignore it here (rather than in the new TracebackException API).
      return list(TracebackException(
          type(value), value, tb, limit=limit).format(chain=chain))
  
  
  def format_exception_only(etype, value):
      """Format the exception part of a traceback.
  
      The arguments are the exception type and value such as given by
      sys.last_type and sys.last_value. The return value is a list of
      strings, each ending in a newline.
  
      Normally, the list contains a single string; however, for
      SyntaxError exceptions, it contains several lines that (when
      printed) display detailed information about where the syntax
      error occurred.
  
      The message indicating which exception occurred is always the last
      string in the list.
  
      """
      return list(TracebackException(etype, value, None).format_exception_only())
  
  
  # -- not official API but folk probably use these two functions.
  
  def _format_final_exc_line(etype, value):
      valuestr = _some_str(value)
      if value is None or not valuestr:
          line = "%s\n" % etype
      else:
          line = "%s: %s\n" % (etype, valuestr)
      return line
  
  def _some_str(value):
      try:
          return str(value)
      except:
          return '<unprintable %s object>' % type(value).__name__
  
  # --
  
  def print_exc(limit=None, file=None, chain=True):
      """Shorthand for 'print_exception(*sys.exc_info(), limit, file)'."""
      print_exception(*sys.exc_info(), limit=limit, file=file, chain=chain)
  
  def format_exc(limit=None, chain=True):
      """Like print_exc() but return a string."""
      return "".join(format_exception(*sys.exc_info(), limit=limit, chain=chain))
  
  def print_last(limit=None, file=None, chain=True):
      """This is a shorthand for 'print_exception(sys.last_type,
      sys.last_value, sys.last_traceback, limit, file)'."""
      if not hasattr(sys, "last_type"):
          raise ValueError("no last exception")
      print_exception(sys.last_type, sys.last_value, sys.last_traceback,
                      limit, file, chain)
  
  #
  # Printing and Extracting Stacks.
  #
  
  def print_stack(f=None, limit=None, file=None):
      """Print a stack trace from its invocation point.
  
      The optional 'f' argument can be used to specify an alternate
      stack frame at which to start. The optional 'limit' and 'file'
      arguments have the same meaning as for print_exception().
      """
      if f is None:
          f = sys._getframe().f_back
      print_list(extract_stack(f, limit=limit), file=file)
  
  
  def format_stack(f=None, limit=None):
      """Shorthand for 'format_list(extract_stack(f, limit))'."""
      if f is None:
          f = sys._getframe().f_back
      return format_list(extract_stack(f, limit=limit))
  
  
  def extract_stack(f=None, limit=None):
      """Extract the raw traceback from the current stack frame.
  
      The return value has the same format as for extract_tb().  The
      optional 'f' and 'limit' arguments have the same meaning as for
      print_stack().  Each item in the list is a quadruple (filename,
      line number, function name, text), and the entries are in order
      from oldest to newest stack frame.
      """
      if f is None:
          f = sys._getframe().f_back
      stack = StackSummary.extract(walk_stack(f), limit=limit)
      stack.reverse()
      return stack
  
  
  def clear_frames(tb):
      "Clear all references to local variables in the frames of a traceback."
      while tb is not None:
          try:
              tb.tb_frame.clear()
          except RuntimeError:
              # Ignore the exception raised if the frame is still executing.
              pass
          tb = tb.tb_next
  
  
  class FrameSummary:
      """A single frame from a traceback.
  
      - :attr:`filename` The filename for the frame.
      - :attr:`lineno` The line within filename for the frame that was
        active when the frame was captured.
      - :attr:`name` The name of the function or method that was executing
        when the frame was captured.
      - :attr:`line` The text from the linecache module for the
        of code that was running when the frame was captured.
      - :attr:`locals` Either None if locals were not supplied, or a dict
        mapping the name to the repr() of the variable.
      """
  
      __slots__ = ('filename', 'lineno', 'name', '_line', 'locals')
  
      def __init__(self, filename, lineno, name, *, lookup_line=True,
              locals=None, line=None):
          """Construct a FrameSummary.
  
          :param lookup_line: If True, `linecache` is consulted for the source
              code line. Otherwise, the line will be looked up when first needed.
          :param locals: If supplied the frame locals, which will be captured as
              object representations.
          :param line: If provided, use this instead of looking up the line in
              the linecache.
          """
          self.filename = filename
          self.lineno = lineno
          self.name = name
          self._line = line
          if lookup_line:
              self.line
          self.locals = \
              dict((k, repr(v)) for k, v in locals.items()) if locals else None
  
      def __eq__(self, other):
          if isinstance(other, FrameSummary):
              return (self.filename == other.filename and
                      self.lineno == other.lineno and
                      self.name == other.name and
                      self.locals == other.locals)
          if isinstance(other, tuple):
              return (self.filename, self.lineno, self.name, self.line) == other
          return NotImplemented
  
      def __getitem__(self, pos):
          return (self.filename, self.lineno, self.name, self.line)[pos]
  
      def __iter__(self):
          return iter([self.filename, self.lineno, self.name, self.line])
  
      def __repr__(self):
          return "<FrameSummary file {filename}, line {lineno} in {name}>".format(
              filename=self.filename, lineno=self.lineno, name=self.name)
  
      @property
      def line(self):
          if self._line is None:
              self._line = linecache.getline(self.filename, self.lineno).strip()
          return self._line
  
  
  def walk_stack(f):
      """Walk a stack yielding the frame and line number for each frame.
  
      This will follow f.f_back from the given frame. If no frame is given, the
      current stack is used. Usually used with StackSummary.extract.
      """
      if f is None:
          f = sys._getframe().f_back.f_back
      while f is not None:
          yield f, f.f_lineno
          f = f.f_back
  
  
  def walk_tb(tb):
      """Walk a traceback yielding the frame and line number for each frame.
  
      This will follow tb.tb_next (and thus is in the opposite order to
      walk_stack). Usually used with StackSummary.extract.
      """
      while tb is not None:
          yield tb.tb_frame, tb.tb_lineno
          tb = tb.tb_next
  
  
  class StackSummary(list):
      """A stack of frames."""
  
      @classmethod
      def extract(klass, frame_gen, *, limit=None, lookup_lines=True,
              capture_locals=False):
          """Create a StackSummary from a traceback or stack object.
  
          :param frame_gen: A generator that yields (frame, lineno) tuples to
              include in the stack.
          :param limit: None to include all frames or the number of frames to
              include.
          :param lookup_lines: If True, lookup lines for each frame immediately,
              otherwise lookup is deferred until the frame is rendered.
          :param capture_locals: If True, the local variables from each frame will
              be captured as object representations into the FrameSummary.
          """
          if limit is None:
              limit = getattr(sys, 'tracebacklimit', None)
              if limit is not None and limit < 0:
                  limit = 0
          if limit is not None:
              if limit >= 0:
                  frame_gen = itertools.islice(frame_gen, limit)
              else:
                  frame_gen = collections.deque(frame_gen, maxlen=-limit)
  
          result = klass()
          fnames = set()
          for f, lineno in frame_gen:
              co = f.f_code
              filename = co.co_filename
              name = co.co_name
  
              fnames.add(filename)
              linecache.lazycache(filename, f.f_globals)
              # Must defer line lookups until we have called checkcache.
              if capture_locals:
                  f_locals = f.f_locals
              else:
                  f_locals = None
              result.append(FrameSummary(
                  filename, lineno, name, lookup_line=False, locals=f_locals))
          for filename in fnames:
              linecache.checkcache(filename)
          # If immediate lookup was desired, trigger lookups now.
          if lookup_lines:
              for f in result:
                  f.line
          return result
  
      @classmethod
      def from_list(klass, a_list):
          """Create a StackSummary from a simple list of tuples.
  
          This method supports the older Python API. Each tuple should be a
          4-tuple with (filename, lineno, name, line) elements.
          """
          # While doing a fast-path check for isinstance(a_list, StackSummary) is
          # appealing, idlelib.run.cleanup_traceback and other similar code may
          # break this by making arbitrary frames plain tuples, so we need to
          # check on a frame by frame basis.
          result = StackSummary()
          for frame in a_list:
              if isinstance(frame, FrameSummary):
                  result.append(frame)
              else:
                  filename, lineno, name, line = frame
                  result.append(FrameSummary(filename, lineno, name, line=line))
          return result
  
      def format(self):
          """Format the stack ready for printing.
  
          Returns a list of strings ready for printing.  Each string in the
          resulting list corresponds to a single frame from the stack.
          Each string ends in a newline; the strings may contain internal
          newlines as well, for those items with source text lines.
  
          For long sequences of the same frame and line, the first few
          repetitions are shown, followed by a summary line stating the exact
          number of further repetitions.
          """
          result = []
          last_file = None
          last_line = None
          last_name = None
          count = 0
          for frame in self:
              if (last_file is not None and last_file == frame.filename and
                  last_line is not None and last_line == frame.lineno and
                  last_name is not None and last_name == frame.name):
                  count += 1
              else:
                  if count > 3:
                      result.append(f'  [Previous line repeated {count-3} more times]\n')
                  last_file = frame.filename
                  last_line = frame.lineno
                  last_name = frame.name
                  count = 0
              if count >= 3:
                  continue
              row = []
              row.append('  File "{}", line {}, in {}\n'.format(
                  frame.filename, frame.lineno, frame.name))
              if frame.line:
                  row.append('    {}\n'.format(frame.line.strip()))
              if frame.locals:
                  for name, value in sorted(frame.locals.items()):
                      row.append('    {name} = {value}\n'.format(name=name, value=value))
              result.append(''.join(row))
          if count > 3:
              result.append(f'  [Previous line repeated {count-3} more times]\n')
          return result
  
  
  class TracebackException:
      """An exception ready for rendering.
  
      The traceback module captures enough attributes from the original exception
      to this intermediary form to ensure that no references are held, while
      still being able to fully print or format it.
  
      Use `from_exception` to create TracebackException instances from exception
      objects, or the constructor to create TracebackException instances from
      individual components.
  
      - :attr:`__cause__` A TracebackException of the original *__cause__*.
      - :attr:`__context__` A TracebackException of the original *__context__*.
      - :attr:`__suppress_context__` The *__suppress_context__* value from the
        original exception.
      - :attr:`stack` A `StackSummary` representing the traceback.
      - :attr:`exc_type` The class of the original traceback.
      - :attr:`filename` For syntax errors - the filename where the error
        occurred.
      - :attr:`lineno` For syntax errors - the linenumber where the error
        occurred.
      - :attr:`text` For syntax errors - the text where the error
        occurred.
      - :attr:`offset` For syntax errors - the offset into the text where the
        error occurred.
      - :attr:`msg` For syntax errors - the compiler error message.
      """
  
      def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
              lookup_lines=True, capture_locals=False, _seen=None):
          # NB: we need to accept exc_traceback, exc_value, exc_traceback to
          # permit backwards compat with the existing API, otherwise we
          # need stub thunk objects just to glue it together.
          # Handle loops in __cause__ or __context__.
          if _seen is None:
              _seen = set()
          _seen.add(id(exc_value))
          # Gracefully handle (the way Python 2.4 and earlier did) the case of
          # being called with no type or value (None, None, None).
          if (exc_value and exc_value.__cause__ is not None
              and id(exc_value.__cause__) not in _seen):
              cause = TracebackException(
                  type(exc_value.__cause__),
                  exc_value.__cause__,
                  exc_value.__cause__.__traceback__,
                  limit=limit,
                  lookup_lines=False,
                  capture_locals=capture_locals,
                  _seen=_seen)
          else:
              cause = None
          if (exc_value and exc_value.__context__ is not None
              and id(exc_value.__context__) not in _seen):
              context = TracebackException(
                  type(exc_value.__context__),
                  exc_value.__context__,
                  exc_value.__context__.__traceback__,
                  limit=limit,
                  lookup_lines=False,
                  capture_locals=capture_locals,
                  _seen=_seen)
          else:
              context = None
          self.exc_traceback = exc_traceback
          self.__cause__ = cause
          self.__context__ = context
          self.__suppress_context__ = \
              exc_value.__suppress_context__ if exc_value else False
          # TODO: locals.
          self.stack = StackSummary.extract(
              walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines,
              capture_locals=capture_locals)
          self.exc_type = exc_type
          # Capture now to permit freeing resources: only complication is in the
          # unofficial API _format_final_exc_line
          self._str = _some_str(exc_value)
          if exc_type and issubclass(exc_type, SyntaxError):
              # Handle SyntaxError's specially
              self.filename = exc_value.filename
              self.lineno = str(exc_value.lineno)
              self.text = exc_value.text
              self.offset = exc_value.offset
              self.msg = exc_value.msg
          if lookup_lines:
              self._load_lines()
  
      @classmethod
      def from_exception(cls, exc, *args, **kwargs):
          """Create a TracebackException from an exception."""
          return cls(type(exc), exc, exc.__traceback__, *args, **kwargs)
  
      def _load_lines(self):
          """Private API. force all lines in the stack to be loaded."""
          for frame in self.stack:
              frame.line
          if self.__context__:
              self.__context__._load_lines()
          if self.__cause__:
              self.__cause__._load_lines()
  
      def __eq__(self, other):
          return self.__dict__ == other.__dict__
  
      def __str__(self):
          return self._str
  
      def format_exception_only(self):
          """Format the exception part of the traceback.
  
          The return value is a generator of strings, each ending in a newline.
  
          Normally, the generator emits a single string; however, for
          SyntaxError exceptions, it emites several lines that (when
          printed) display detailed information about where the syntax
          error occurred.
  
          The message indicating which exception occurred is always the last
          string in the output.
          """
          if self.exc_type is None:
              yield _format_final_exc_line(None, self._str)
              return
  
          stype = self.exc_type.__qualname__
          smod = self.exc_type.__module__
          if smod not in ("__main__", "builtins"):
              stype = smod + '.' + stype
  
          if not issubclass(self.exc_type, SyntaxError):
              yield _format_final_exc_line(stype, self._str)
              return
  
          # It was a syntax error; show exactly where the problem was found.
          filename = self.filename or "<string>"
          lineno = str(self.lineno) or '?'
          yield '  File "{}", line {}\n'.format(filename, lineno)
  
          badline = self.text
          offset = self.offset
          if badline is not None:
              yield '    {}\n'.format(badline.strip())
              if offset is not None:
                  caretspace = badline.rstrip('\n')
                  offset = min(len(caretspace), offset) - 1
                  caretspace = caretspace[:offset].lstrip()
                  # non-space whitespace (likes tabs) must be kept for alignment
                  caretspace = ((c.isspace() and c or ' ') for c in caretspace)
                  yield '    {}^\n'.format(''.join(caretspace))
          msg = self.msg or "<no detail available>"
          yield "{}: {}\n".format(stype, msg)
  
      def format(self, *, chain=True):
          """Format the exception.
  
          If chain is not *True*, *__cause__* and *__context__* will not be formatted.
  
          The return value is a generator of strings, each ending in a newline and
          some containing internal newlines. `print_exception` is a wrapper around
          this method which just prints the lines to a file.
  
          The message indicating which exception occurred is always the last
          string in the output.
          """
          if chain:
              if self.__cause__ is not None:
                  yield from self.__cause__.format(chain=chain)
                  yield _cause_message
              elif (self.__context__ is not None and
                  not self.__suppress_context__):
                  yield from self.__context__.format(chain=chain)
                  yield _context_message
          if self.exc_traceback is not None:
              yield 'Traceback (most recent call last):\n'
          yield from self.stack.format()
          yield from self.format_exception_only()
  