save traceback for chained exception

IPython/core/ultratb.py

@@ -1,1505 +1,1506 b''
 # -*- coding: utf-8 -*-
 """
 Verbose and colourful traceback formatting.
 
 **ColorTB**
 
 I've always found it a bit hard to visually parse tracebacks in Python.  The
 ColorTB class is a solution to that problem.  It colors the different parts of a
 traceback in a manner similar to what you would expect from a syntax-highlighting
 text editor.
 
 Installation instructions for ColorTB::
 
     import sys,ultratb
     sys.excepthook = ultratb.ColorTB()
 
 **VerboseTB**
 
 I've also included a port of Ka-Ping Yee's "cgitb.py" that produces all kinds
 of useful info when a traceback occurs.  Ping originally had it spit out HTML
 and intended it for CGI programmers, but why should they have all the fun?  I
 altered it to spit out colored text to the terminal.  It's a bit overwhelming,
 but kind of neat, and maybe useful for long-running programs that you believe
 are bug-free.  If a crash *does* occur in that type of program you want details.
 Give it a shot--you'll love it or you'll hate it.
 
 .. note::
 
   The Verbose mode prints the variables currently visible where the exception
   happened (shortening their strings if too long). This can potentially be
   very slow, if you happen to have a huge data structure whose string
   representation is complex to compute. Your computer may appear to freeze for
   a while with cpu usage at 100%. If this occurs, you can cancel the traceback
   with Ctrl-C (maybe hitting it more than once).
 
   If you encounter this kind of situation often, you may want to use the
   Verbose_novars mode instead of the regular Verbose, which avoids formatting
   variables (but otherwise includes the information and context given by
   Verbose).
 
 .. note::
 
   The verbose mode print all variables in the stack, which means it can
   potentially leak sensitive information like access keys, or unencrypted
   password.
 
 Installation instructions for VerboseTB::
 
     import sys,ultratb
     sys.excepthook = ultratb.VerboseTB()
 
 Note:  Much of the code in this module was lifted verbatim from the standard
 library module 'traceback.py' and Ka-Ping Yee's 'cgitb.py'.
 
 Color schemes
 -------------
 
 The colors are defined in the class TBTools through the use of the
 ColorSchemeTable class. Currently the following exist:
 
   - NoColor: allows all of this module to be used in any terminal (the color
     escapes are just dummy blank strings).
 
   - Linux: is meant to look good in a terminal like the Linux console (black
     or very dark background).
 
   - LightBG: similar to Linux but swaps dark/light colors to be more readable
     in light background terminals.
 
   - Neutral: a neutral color scheme that should be readable on both light and
     dark background
 
 You can implement other color schemes easily, the syntax is fairly
 self-explanatory. Please send back new schemes you develop to the author for
 possible inclusion in future releases.
 
 Inheritance diagram:
 
 .. inheritance-diagram:: IPython.core.ultratb
    :parts: 3
 """
 
 #*****************************************************************************
 # Copyright (C) 2001 Nathaniel Gray <[e-mail]>
 # Copyright (C) 2001-2004 Fernando Perez <[e-mail]>
 #
 # Distributed under the terms of the BSD License.  The full license is in
 # the file COPYING, distributed as part of this software.
 #*****************************************************************************
 
 
 import dis
 import inspect
 import keyword
 import linecache
 import os
 import pydoc
 import re
 import sys
 import time
 import tokenize
 import traceback
 
 try:  # Python 2
     generate_tokens = tokenize.generate_tokens
 except AttributeError:  # Python 3
     generate_tokens = tokenize.tokenize
 
 # For purposes of monkeypatching inspect to fix a bug in it.
 from inspect import getsourcefile, getfile, getmodule, \
     ismodule, isclass, ismethod, isfunction, istraceback, isframe, iscode
 
 # IPython's own modules
 from IPython import get_ipython
 from IPython.core import debugger
 from IPython.core.display_trap import DisplayTrap
 from IPython.core.excolors import exception_colors
 from IPython.utils import PyColorize
 from IPython.utils import path as util_path
 from IPython.utils import py3compat
 from IPython.utils.data import uniq_stable
 from IPython.utils.terminal import get_terminal_size
 
 from logging import info, error, debug
 
 from importlib.util import source_from_cache
 
 import IPython.utils.colorable as colorable
 
 # Globals
 # amount of space to put line numbers before verbose tracebacks
 INDENT_SIZE = 8
 
 # Default color scheme.  This is used, for example, by the traceback
 # formatter.  When running in an actual IPython instance, the user's rc.colors
 # value is used, but having a module global makes this functionality available
 # to users of ultratb who are NOT running inside ipython.
 DEFAULT_SCHEME = 'NoColor'
 
 
 # Number of frame above which we are likely to have a recursion and will
 # **attempt** to detect it.  Made modifiable mostly to speedup test suite
 # as detecting recursion is one of our slowest test
 _FRAME_RECURSION_LIMIT = 500
 
 # ---------------------------------------------------------------------------
 # Code begins
 
 # Utility functions
 def inspect_error():
     """Print a message about internal inspect errors.
 
     These are unfortunately quite common."""
 
     error('Internal Python error in the inspect module.\n'
           'Below is the traceback from this internal error.\n')
 
 
 # This function is a monkeypatch we apply to the Python inspect module. We have
 # now found when it's needed (see discussion on issue gh-1456), and we have a
 # test case (IPython.core.tests.test_ultratb.ChangedPyFileTest) that fails if
 # the monkeypatch is not applied. TK, Aug 2012.
 def findsource(object):
     """Return the entire source file and starting line number for an object.
 
     The argument may be a module, class, method, function, traceback, frame,
     or code object.  The source code is returned as a list of all the lines
     in the file and the line number indexes a line in that list.  An IOError
     is raised if the source code cannot be retrieved.
 
     FIXED version with which we monkeypatch the stdlib to work around a bug."""
 
     file = getsourcefile(object) or getfile(object)
     # If the object is a frame, then trying to get the globals dict from its
     # module won't work. Instead, the frame object itself has the globals
     # dictionary.
     globals_dict = None
     if inspect.isframe(object):
         # XXX: can this ever be false?
         globals_dict = object.f_globals
     else:
         module = getmodule(object, file)
         if module:
             globals_dict = module.__dict__
     lines = linecache.getlines(file, globals_dict)
     if not lines:
         raise IOError('could not get source code')
 
     if ismodule(object):
         return lines, 0
 
     if isclass(object):
         name = object.__name__
         pat = re.compile(r'^(\s*)class\s*' + name + r'\b')
         # make some effort to find the best matching class definition:
         # use the one with the least indentation, which is the one
         # that's most probably not inside a function definition.
         candidates = []
         for i, line in enumerate(lines):
             match = pat.match(line)
             if match:
                 # if it's at toplevel, it's already the best one
                 if line[0] == 'c':
                     return lines, i
                 # else add whitespace to candidate list
                 candidates.append((match.group(1), i))
         if candidates:
             # this will sort by whitespace, and by line number,
             # less whitespace first
             candidates.sort()
             return lines, candidates[0][1]
         else:
             raise IOError('could not find class definition')
 
     if ismethod(object):
         object = object.__func__
     if isfunction(object):
         object = object.__code__
     if istraceback(object):
         object = object.tb_frame
     if isframe(object):
         object = object.f_code
     if iscode(object):
         if not hasattr(object, 'co_firstlineno'):
             raise IOError('could not find function definition')
         pat = re.compile(r'^(\s*def\s)|(.*(?<!\w)lambda(:|\s))|^(\s*@)')
         pmatch = pat.match
         # fperez - fix: sometimes, co_firstlineno can give a number larger than
         # the length of lines, which causes an error.  Safeguard against that.
         lnum = min(object.co_firstlineno, len(lines)) - 1
         while lnum > 0:
             if pmatch(lines[lnum]):
                 break
             lnum -= 1
 
         return lines, lnum
     raise IOError('could not find code object')
 
 
 # This is a patched version of inspect.getargs that applies the (unmerged)
 # patch for http://bugs.python.org/issue14611 by Stefano Taschini.  This fixes
 # https://github.com/ipython/ipython/issues/8205 and
 # https://github.com/ipython/ipython/issues/8293
 def getargs(co):
     """Get information about the arguments accepted by a code object.
 
     Three things are returned: (args, varargs, varkw), where 'args' is
     a list of argument names (possibly containing nested lists), and
     'varargs' and 'varkw' are the names of the * and ** arguments or None."""
     if not iscode(co):
         raise TypeError('{!r} is not a code object'.format(co))
 
     nargs = co.co_argcount
     names = co.co_varnames
     args = list(names[:nargs])
     step = 0
 
     # The following acrobatics are for anonymous (tuple) arguments.
     for i in range(nargs):
         if args[i][:1] in ('', '.'):
             stack, remain, count = [], [], []
             while step < len(co.co_code):
                 op = ord(co.co_code[step])
                 step = step + 1
                 if op >= dis.HAVE_ARGUMENT:
                     opname = dis.opname[op]
                     value = ord(co.co_code[step]) + ord(co.co_code[step+1])*256
                     step = step + 2
                     if opname in ('UNPACK_TUPLE', 'UNPACK_SEQUENCE'):
                         remain.append(value)
                         count.append(value)
                     elif opname in ('STORE_FAST', 'STORE_DEREF'):
                         if op in dis.haslocal:
                             stack.append(co.co_varnames[value])
                         elif op in dis.hasfree:
                             stack.append((co.co_cellvars + co.co_freevars)[value])
                         # Special case for sublists of length 1: def foo((bar))
                         # doesn't generate the UNPACK_TUPLE bytecode, so if
                         # `remain` is empty here, we have such a sublist.
                         if not remain:
                             stack[0] = [stack[0]]
                             break
                         else:
                             remain[-1] = remain[-1] - 1
                             while remain[-1] == 0:
                                 remain.pop()
                                 size = count.pop()
                                 stack[-size:] = [stack[-size:]]
                                 if not remain:
                                     break
                                 remain[-1] = remain[-1] - 1
                             if not remain:
                                 break
             args[i] = stack[0]
 
     varargs = None
     if co.co_flags & inspect.CO_VARARGS:
         varargs = co.co_varnames[nargs]
         nargs = nargs + 1
     varkw = None
     if co.co_flags & inspect.CO_VARKEYWORDS:
         varkw = co.co_varnames[nargs]
     return inspect.Arguments(args, varargs, varkw)
 
 
 # Monkeypatch inspect to apply our bugfix.
 def with_patch_inspect(f):
     """
     Deprecated since IPython 6.0
     decorator for monkeypatching inspect.findsource
     """
 
     def wrapped(*args, **kwargs):
         save_findsource = inspect.findsource
         save_getargs = inspect.getargs
         inspect.findsource = findsource
         inspect.getargs = getargs
         try:
             return f(*args, **kwargs)
         finally:
             inspect.findsource = save_findsource
             inspect.getargs = save_getargs
 
     return wrapped
 
 
 def fix_frame_records_filenames(records):
     """Try to fix the filenames in each record from inspect.getinnerframes().
 
     Particularly, modules loaded from within zip files have useless filenames
     attached to their code object, and inspect.getinnerframes() just uses it.
     """
     fixed_records = []
     for frame, filename, line_no, func_name, lines, index in records:
         # Look inside the frame's globals dictionary for __file__,
         # which should be better. However, keep Cython filenames since
         # we prefer the source filenames over the compiled .so file.
         if not filename.endswith(('.pyx', '.pxd', '.pxi')):
             better_fn = frame.f_globals.get('__file__', None)
             if isinstance(better_fn, str):
                 # Check the type just in case someone did something weird with
                 # __file__. It might also be None if the error occurred during
                 # import.
                 filename = better_fn
         fixed_records.append((frame, filename, line_no, func_name, lines, index))
     return fixed_records
 
 
 @with_patch_inspect
 def _fixed_getinnerframes(etb, context=1, tb_offset=0):
     LNUM_POS, LINES_POS, INDEX_POS = 2, 4, 5
 
     records = fix_frame_records_filenames(inspect.getinnerframes(etb, context))
     # If the error is at the console, don't build any context, since it would
     # otherwise produce 5 blank lines printed out (there is no file at the
     # console)
     rec_check = records[tb_offset:]
     try:
         rname = rec_check[0][1]
         if rname == '<ipython console>' or rname.endswith('<string>'):
             return rec_check
     except IndexError:
         pass
 
     aux = traceback.extract_tb(etb)
     assert len(records) == len(aux)
     for i, (file, lnum, _, _) in enumerate(aux):
         maybeStart = lnum - 1 - context // 2
         start = max(maybeStart, 0)
         end = start + context
         lines = linecache.getlines(file)[start:end]
         buf = list(records[i])
         buf[LNUM_POS] = lnum
         buf[INDEX_POS] = lnum - 1 - start
         buf[LINES_POS] = lines
         records[i] = tuple(buf)
     return records[tb_offset:]
 
 # Helper function -- largely belongs to VerboseTB, but we need the same
 # functionality to produce a pseudo verbose TB for SyntaxErrors, so that they
 # can be recognized properly by ipython.el's py-traceback-line-re
 # (SyntaxErrors have to be treated specially because they have no traceback)
 
 
 def _format_traceback_lines(lnum, index, lines, Colors, lvals, _line_format):
     """
     Format tracebacks lines with pointing arrow, leading numbers...
 
     Parameters
     ==========
 
     lnum: int
     index: int
     lines: list[string]
     Colors:
         ColorScheme used.
     lvals: bytes
         Values of local variables, already colored, to inject just after the error line.
     _line_format: f (str) -> (str, bool)
         return (colorized version of str, failure to do so)
     """
     numbers_width = INDENT_SIZE - 1
     res = []
 
     for i,line in enumerate(lines, lnum-index):
         line = py3compat.cast_unicode(line)
 
         new_line, err = _line_format(line, 'str')
         if not err:
             line = new_line
 
         if i == lnum:
             # This is the line with the error
             pad = numbers_width - len(str(i))
             num = '%s%s' % (debugger.make_arrow(pad), str(lnum))
             line = '%s%s%s %s%s' % (Colors.linenoEm, num,
                                     Colors.line, line, Colors.Normal)
         else:
             num = '%*s' % (numbers_width, i)
             line = '%s%s%s %s' % (Colors.lineno, num,
                                   Colors.Normal, line)
 
         res.append(line)
         if lvals and i == lnum:
             res.append(lvals + '\n')
     return res
 
 def is_recursion_error(etype, value, records):
     try:
         # RecursionError is new in Python 3.5
         recursion_error_type = RecursionError
     except NameError:
         recursion_error_type = RuntimeError
 
     # The default recursion limit is 1000, but some of that will be taken up
     # by stack frames in IPython itself. >500 frames probably indicates
     # a recursion error.
     return (etype is recursion_error_type) \
            and "recursion" in str(value).lower() \
            and len(records) > _FRAME_RECURSION_LIMIT
 
 def find_recursion(etype, value, records):
     """Identify the repeating stack frames from a RecursionError traceback
 
     'records' is a list as returned by VerboseTB.get_records()
 
     Returns (last_unique, repeat_length)
     """
     # This involves a bit of guesswork - we want to show enough of the traceback
     # to indicate where the recursion is occurring. We guess that the innermost
     # quarter of the traceback (250 frames by default) is repeats, and find the
     # first frame (from in to out) that looks different.
     if not is_recursion_error(etype, value, records):
         return len(records), 0
 
     # Select filename, lineno, func_name to track frames with
     records = [r[1:4] for r in records]
     inner_frames = records[-(len(records)//4):]
     frames_repeated = set(inner_frames)
 
     last_seen_at = {}
     longest_repeat = 0
     i = len(records)
     for frame in reversed(records):
         i -= 1
         if frame not in frames_repeated:
             last_unique = i
             break
 
         if frame in last_seen_at:
             distance = last_seen_at[frame] - i
             longest_repeat = max(longest_repeat, distance)
 
         last_seen_at[frame] = i
     else:
         last_unique = 0 # The whole traceback was recursion
 
     return last_unique, longest_repeat
 
 #---------------------------------------------------------------------------
 # Module classes
 class TBTools(colorable.Colorable):
     """Basic tools used by all traceback printer classes."""
 
     # Number of frames to skip when reporting tracebacks
     tb_offset = 0
 
     def __init__(self, color_scheme='NoColor', call_pdb=False, ostream=None, parent=None, config=None):
         # Whether to call the interactive pdb debugger after printing
         # tracebacks or not
         super(TBTools, self).__init__(parent=parent, config=config)
         self.call_pdb = call_pdb
 
         # Output stream to write to.  Note that we store the original value in
         # a private attribute and then make the public ostream a property, so
         # that we can delay accessing sys.stdout until runtime.  The way
         # things are written now, the sys.stdout object is dynamically managed
         # so a reference to it should NEVER be stored statically.  This
         # property approach confines this detail to a single location, and all
         # subclasses can simply access self.ostream for writing.
         self._ostream = ostream
 
         # Create color table
         self.color_scheme_table = exception_colors()
 
         self.set_colors(color_scheme)
         self.old_scheme = color_scheme  # save initial value for toggles
 
         if call_pdb:
             self.pdb = debugger.Pdb()
         else:
             self.pdb = None
 
     def _get_ostream(self):
         """Output stream that exceptions are written to.
 
         Valid values are:
 
         - None: the default, which means that IPython will dynamically resolve
           to sys.stdout.  This ensures compatibility with most tools, including
           Windows (where plain stdout doesn't recognize ANSI escapes).
 
         - Any object with 'write' and 'flush' attributes.
         """
         return sys.stdout if self._ostream is None else self._ostream
 
     def _set_ostream(self, val):
         assert val is None or (hasattr(val, 'write') and hasattr(val, 'flush'))
         self._ostream = val
 
     ostream = property(_get_ostream, _set_ostream)
 
     def get_parts_of_chained_exception(self, evalue):
         def get_chained_exception(exception_value):
             cause = getattr(exception_value, '__cause__', None)
             if cause:
                 return cause
             if getattr(exception_value, '__suppress_context__', False):
                 return None
             return getattr(exception_value, '__context__', None)
 
         chained_evalue = get_chained_exception(evalue)
 
         if chained_evalue:
             return chained_evalue.__class__, chained_evalue, chained_evalue.__traceback__
 
     def prepare_chained_exception_message(self, cause):
         direct_cause = "\nThe above exception was the direct cause of the following exception:\n"
         exception_during_handling = "\nDuring handling of the above exception, another exception occurred:\n"
 
         if cause:
             message = [[direct_cause]]
         else:
             message = [[exception_during_handling]]
         return message
 
     def set_colors(self, *args, **kw):
         """Shorthand access to the color table scheme selector method."""
 
         # Set own color table
         self.color_scheme_table.set_active_scheme(*args, **kw)
         # for convenience, set Colors to the active scheme
         self.Colors = self.color_scheme_table.active_colors
         # Also set colors of debugger
         if hasattr(self, 'pdb') and self.pdb is not None:
             self.pdb.set_colors(*args, **kw)
 
     def color_toggle(self):
         """Toggle between the currently active color scheme and NoColor."""
 
         if self.color_scheme_table.active_scheme_name == 'NoColor':
             self.color_scheme_table.set_active_scheme(self.old_scheme)
             self.Colors = self.color_scheme_table.active_colors
         else:
             self.old_scheme = self.color_scheme_table.active_scheme_name
             self.color_scheme_table.set_active_scheme('NoColor')
             self.Colors = self.color_scheme_table.active_colors
 
     def stb2text(self, stb):
         """Convert a structured traceback (a list) to a string."""
         return '\n'.join(stb)
 
     def text(self, etype, value, tb, tb_offset=None, context=5):
         """Return formatted traceback.
 
         Subclasses may override this if they add extra arguments.
         """
         tb_list = self.structured_traceback(etype, value, tb,
                                             tb_offset, context)
         return self.stb2text(tb_list)
 
     def structured_traceback(self, etype, evalue, tb, tb_offset=None,
                              context=5, mode=None):
         """Return a list of traceback frames.
 
         Must be implemented by each class.
         """
         raise NotImplementedError()
 
 
 #---------------------------------------------------------------------------
 class ListTB(TBTools):
     """Print traceback information from a traceback list, with optional color.
 
     Calling requires 3 arguments: (etype, evalue, elist)
     as would be obtained by::
     
       etype, evalue, tb = sys.exc_info()
       if tb:
         elist = traceback.extract_tb(tb)
       else:
         elist = None
 
     It can thus be used by programs which need to process the traceback before
     printing (such as console replacements based on the code module from the
     standard library).
 
     Because they are meant to be called without a full traceback (only a
     list), instances of this class can't call the interactive pdb debugger."""
 
     def __init__(self, color_scheme='NoColor', call_pdb=False, ostream=None, parent=None, config=None):
         TBTools.__init__(self, color_scheme=color_scheme, call_pdb=call_pdb,
                          ostream=ostream, parent=parent,config=config)
 
     def __call__(self, etype, value, elist):
         self.ostream.flush()
         self.ostream.write(self.text(etype, value, elist))
         self.ostream.write('\n')
 
     def _extract_tb(self, tb):
         if tb:
             return traceback.extract_tb(tb)
         else:
             return None
 
     def structured_traceback(self, etype, evalue, etb=None, tb_offset=None,
                              context=5):
         """Return a color formatted string with the traceback info.
 
         Parameters
         ----------
         etype : exception type
           Type of the exception raised.
 
         evalue : object
           Data stored in the exception
 
         etb : object
           If list: List of frames, see class docstring for details.
           If Traceback: Traceback of the exception.
 
         tb_offset : int, optional
           Number of frames in the traceback to skip.  If not given, the
           instance evalue is used (set in constructor).
 
         context : int, optional
           Number of lines of context information to print.
 
         Returns
         -------
         String with formatted exception.
         """
         # This is a workaround to get chained_exc_ids in recursive calls
         # etb should not be a tuple if structured_traceback is not recursive
         if isinstance(etb, tuple):
             etb, chained_exc_ids = etb
         else:
             chained_exc_ids = set()
 
         if isinstance(etb, list):
             elist = etb
         elif etb is not None:
             elist = self._extract_tb(etb)
         else:
             elist = []
         tb_offset = self.tb_offset if tb_offset is None else tb_offset
         Colors = self.Colors
         out_list = []
         if elist:
 
             if tb_offset and len(elist) > tb_offset:
                 elist = elist[tb_offset:]
 
             out_list.append('Traceback %s(most recent call last)%s:' %
                             (Colors.normalEm, Colors.Normal) + '\n')
             out_list.extend(self._format_list(elist))
         # The exception info should be a single entry in the list.
         lines = ''.join(self._format_exception_only(etype, evalue))
         out_list.append(lines)
 
         exception = self.get_parts_of_chained_exception(evalue)
 
         if exception and not id(exception[1]) in chained_exc_ids:
             chained_exception_message = self.prepare_chained_exception_message(
                 evalue.__cause__)[0]
             etype, evalue, etb = exception
             # Trace exception to avoid infinite 'cause' loop
             chained_exc_ids.add(id(exception[1]))
             chained_exceptions_tb_offset = 0
             out_list = (
                 self.structured_traceback(
                     etype, evalue, (etb, chained_exc_ids),
                     chained_exceptions_tb_offset, context)
                 + chained_exception_message
                 + out_list)
 
         return out_list
 
     def _format_list(self, 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.
 
         Lifted almost verbatim from traceback.py
         """
 
         Colors = self.Colors
         list = []
         for filename, lineno, name, line in extracted_list[:-1]:
             item = '  File %s"%s"%s, line %s%d%s, in %s%s%s\n' % \
                    (Colors.filename, filename, Colors.Normal,
                     Colors.lineno, lineno, Colors.Normal,
                     Colors.name, name, Colors.Normal)
             if line:
                 item += '    %s\n' % line.strip()
             list.append(item)
         # Emphasize the last entry
         filename, lineno, name, line = extracted_list[-1]
         item = '%s  File %s"%s"%s, line %s%d%s, in %s%s%s%s\n' % \
                (Colors.normalEm,
                 Colors.filenameEm, filename, Colors.normalEm,
                 Colors.linenoEm, lineno, Colors.normalEm,
                 Colors.nameEm, name, Colors.normalEm,
                 Colors.Normal)
         if line:
             item += '%s    %s%s\n' % (Colors.line, line.strip(),
                                       Colors.Normal)
         list.append(item)
         return list
 
     def _format_exception_only(self, etype, value):
         """Format the exception part of a traceback.
 
         The arguments are the exception type and value such as given by
         sys.exc_info()[:2]. 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 the
         always last string in the list.
 
         Also lifted nearly verbatim from traceback.py
         """
         have_filedata = False
         Colors = self.Colors
         list = []
         stype = py3compat.cast_unicode(Colors.excName + etype.__name__ + Colors.Normal)
         if value is None:
             # Not sure if this can still happen in Python 2.6 and above
             list.append(stype + '\n')
         else:
             if issubclass(etype, SyntaxError):
                 have_filedata = True
                 if not value.filename: value.filename = "<string>"
                 if value.lineno:
                     lineno = value.lineno
                     textline = linecache.getline(value.filename, value.lineno)
                 else:
                     lineno = 'unknown'
                     textline = ''
                 list.append('%s  File %s"%s"%s, line %s%s%s\n' % \
                             (Colors.normalEm,
                              Colors.filenameEm, py3compat.cast_unicode(value.filename), Colors.normalEm,
                              Colors.linenoEm, lineno, Colors.Normal  ))
                 if textline == '':
                     textline = py3compat.cast_unicode(value.text, "utf-8")
 
                 if textline is not None:
                     i = 0
                     while i < len(textline) and textline[i].isspace():
                         i += 1
                     list.append('%s    %s%s\n' % (Colors.line,
                                                   textline.strip(),
                                                   Colors.Normal))
                     if value.offset is not None:
                         s = '    '
                         for c in textline[i:value.offset - 1]:
                             if c.isspace():
                                 s += c
                             else:
                                 s += ' '
                         list.append('%s%s^%s\n' % (Colors.caret, s,
                                                    Colors.Normal))
 
             try:
                 s = value.msg
             except Exception:
                 s = self._some_str(value)
             if s:
                 list.append('%s%s:%s %s\n' % (stype, Colors.excName,
                                               Colors.Normal, s))
             else:
                 list.append('%s\n' % stype)
 
         # sync with user hooks
         if have_filedata:
             ipinst = get_ipython()
             if ipinst is not None:
                 ipinst.hooks.synchronize_with_editor(value.filename, value.lineno, 0)
 
         return list
 
     def get_exception_only(self, etype, value):
         """Only print the exception type and message, without a traceback.
 
         Parameters
         ----------
         etype : exception type
         value : exception value
         """
         return ListTB.structured_traceback(self, etype, value)
 
     def show_exception_only(self, etype, evalue):
         """Only print the exception type and message, without a traceback.
 
         Parameters
         ----------
         etype : exception type
         value : exception value
         """
         # This method needs to use __call__ from *this* class, not the one from
         # a subclass whose signature or behavior may be different
         ostream = self.ostream
         ostream.flush()
         ostream.write('\n'.join(self.get_exception_only(etype, evalue)))
         ostream.flush()
 
     def _some_str(self, value):
         # Lifted from traceback.py
         try:
             return py3compat.cast_unicode(str(value))
         except:
             return u'<unprintable %s object>' % type(value).__name__
 
 
 #----------------------------------------------------------------------------
 class VerboseTB(TBTools):
     """A port of Ka-Ping Yee's cgitb.py module that outputs color text instead
     of HTML.  Requires inspect and pydoc.  Crazy, man.
 
     Modified version which optionally strips the topmost entries from the
     traceback, to be used with alternate interpreters (because their own code
     would appear in the traceback)."""
 
     def __init__(self, color_scheme='Linux', call_pdb=False, ostream=None,
                  tb_offset=0, long_header=False, include_vars=True,
                  check_cache=None, debugger_cls = None,
                  parent=None, config=None):
         """Specify traceback offset, headers and color scheme.
 
         Define how many frames to drop from the tracebacks. Calling it with
         tb_offset=1 allows use of this handler in interpreters which will have
         their own code at the top of the traceback (VerboseTB will first
         remove that frame before printing the traceback info)."""
         TBTools.__init__(self, color_scheme=color_scheme, call_pdb=call_pdb,
                          ostream=ostream, parent=parent, config=config)
         self.tb_offset = tb_offset
         self.long_header = long_header
         self.include_vars = include_vars
         # By default we use linecache.checkcache, but the user can provide a
         # different check_cache implementation.  This is used by the IPython
         # kernel to provide tracebacks for interactive code that is cached,
         # by a compiler instance that flushes the linecache but preserves its
         # own code cache.
         if check_cache is None:
             check_cache = linecache.checkcache
         self.check_cache = check_cache
 
         self.debugger_cls = debugger_cls or debugger.Pdb
 
     def format_records(self, records, last_unique, recursion_repeat):
         """Format the stack frames of the traceback"""
         frames = []
         for r in records[:last_unique+recursion_repeat+1]:
             #print '*** record:',file,lnum,func,lines,index  # dbg
             frames.append(self.format_record(*r))
 
         if recursion_repeat:
             frames.append('... last %d frames repeated, from the frame below ...\n' % recursion_repeat)
             frames.append(self.format_record(*records[last_unique+recursion_repeat+1]))
 
         return frames
 
     def format_record(self, frame, file, lnum, func, lines, index):
         """Format a single stack frame"""
         Colors = self.Colors  # just a shorthand + quicker name lookup
         ColorsNormal = Colors.Normal  # used a lot
         col_scheme = self.color_scheme_table.active_scheme_name
         indent = ' ' * INDENT_SIZE
         em_normal = '%s\n%s%s' % (Colors.valEm, indent, ColorsNormal)
         undefined = '%sundefined%s' % (Colors.em, ColorsNormal)
         tpl_link = '%s%%s%s' % (Colors.filenameEm, ColorsNormal)
         tpl_call = 'in %s%%s%s%%s%s' % (Colors.vName, Colors.valEm,
                                         ColorsNormal)
         tpl_call_fail = 'in %s%%s%s(***failed resolving arguments***)%s' % \
                         (Colors.vName, Colors.valEm, ColorsNormal)
         tpl_local_var = '%s%%s%s' % (Colors.vName, ColorsNormal)
         tpl_global_var = '%sglobal%s %s%%s%s' % (Colors.em, ColorsNormal,
                                                  Colors.vName, ColorsNormal)
         tpl_name_val = '%%s %s= %%s%s' % (Colors.valEm, ColorsNormal)
 
         if not file:
             file = '?'
         elif file.startswith(str("<")) and file.endswith(str(">")):
             # Not a real filename, no problem...
             pass
         elif not os.path.isabs(file):
             # Try to make the filename absolute by trying all
             # sys.path entries (which is also what linecache does)
             for dirname in sys.path:
                 try:
                     fullname = os.path.join(dirname, file)
                     if os.path.isfile(fullname):
                         file = os.path.abspath(fullname)
                         break
                 except Exception:
                     # Just in case that sys.path contains very
                     # strange entries...
                     pass
 
         file = py3compat.cast_unicode(file, util_path.fs_encoding)
         link = tpl_link % util_path.compress_user(file)
         args, varargs, varkw, locals_ = inspect.getargvalues(frame)
 
         if func == '?':
             call = ''
         elif func == '<module>':
             call = tpl_call % (func, '')
         else:
             # Decide whether to include variable details or not
             var_repr = eqrepr if self.include_vars else nullrepr
             try:
                 call = tpl_call % (func, inspect.formatargvalues(args,
                                                                  varargs, varkw,
                                                                  locals_, formatvalue=var_repr))
             except KeyError:
                 # This happens in situations like errors inside generator
                 # expressions, where local variables are listed in the
                 # line, but can't be extracted from the frame.  I'm not
                 # 100% sure this isn't actually a bug in inspect itself,
                 # but since there's no info for us to compute with, the
                 # best we can do is report the failure and move on.  Here
                 # we must *not* call any traceback construction again,
                 # because that would mess up use of %debug later on.  So we
                 # simply report the failure and move on.  The only
                 # limitation will be that this frame won't have locals
                 # listed in the call signature.  Quite subtle problem...
                 # I can't think of a good way to validate this in a unit
                 # test, but running a script consisting of:
                 #  dict( (k,v.strip()) for (k,v) in range(10) )
                 # will illustrate the error, if this exception catch is
                 # disabled.
                 call = tpl_call_fail % func
 
         # Don't attempt to tokenize binary files.
         if file.endswith(('.so', '.pyd', '.dll')):
             return '%s %s\n' % (link, call)
 
         elif file.endswith(('.pyc', '.pyo')):
             # Look up the corresponding source file.
             try:
                 file = source_from_cache(file)
             except ValueError:
                 # Failed to get the source file for some reason
                 # E.g. https://github.com/ipython/ipython/issues/9486
                 return '%s %s\n' % (link, call)
 
         def linereader(file=file, lnum=[lnum], getline=linecache.getline):
             line = getline(file, lnum[0])
             lnum[0] += 1
             return line
 
         # Build the list of names on this line of code where the exception
         # occurred.
         try:
             names = []
             name_cont = False
 
             for token_type, token, start, end, line in generate_tokens(linereader):
                 # build composite names
                 if token_type == tokenize.NAME and token not in keyword.kwlist:
                     if name_cont:
                         # Continuation of a dotted name
                         try:
                             names[-1].append(token)
                         except IndexError:
                             names.append([token])
                         name_cont = False
                     else:
                         # Regular new names.  We append everything, the caller
                         # will be responsible for pruning the list later.  It's
                         # very tricky to try to prune as we go, b/c composite
                         # names can fool us.  The pruning at the end is easy
                         # to do (or the caller can print a list with repeated
                         # names if so desired.
                         names.append([token])
                 elif token == '.':
                     name_cont = True
                 elif token_type == tokenize.NEWLINE:
                     break
 
         except (IndexError, UnicodeDecodeError, SyntaxError):
             # signals exit of tokenizer
             # SyntaxError can occur if the file is not actually Python
             #  - see gh-6300
             pass
         except tokenize.TokenError as msg:
             # Tokenizing may fail for various reasons, many of which are
             # harmless. (A good example is when the line in question is the
             # close of a triple-quoted string, cf gh-6864). We don't want to
             # show this to users, but want make it available for debugging
             # purposes.
             _m = ("An unexpected error occurred while tokenizing input\n"
                   "The following traceback may be corrupted or invalid\n"
                   "The error message is: %s\n" % msg)
             debug(_m)
 
         # Join composite names (e.g. "dict.fromkeys")
         names = ['.'.join(n) for n in names]
         # prune names list of duplicates, but keep the right order
         unique_names = uniq_stable(names)
 
         # Start loop over vars
         lvals = ''
         lvals_list = []
         if self.include_vars:
             for name_full in unique_names:
                 name_base = name_full.split('.', 1)[0]
                 if name_base in frame.f_code.co_varnames:
                     if name_base in locals_:
                         try:
                             value = repr(eval(name_full, locals_))
                         except:
                             value = undefined
                     else:
                         value = undefined
                     name = tpl_local_var % name_full
                 else:
                     if name_base in frame.f_globals:
                         try:
                             value = repr(eval(name_full, frame.f_globals))
                         except:
                             value = undefined
                     else:
                         value = undefined
                     name = tpl_global_var % name_full
                 lvals_list.append(tpl_name_val % (name, value))
         if lvals_list:
             lvals = '%s%s' % (indent, em_normal.join(lvals_list))
 
         level = '%s %s\n' % (link, call)
 
         if index is None:
             return level
         else:
             _line_format = PyColorize.Parser(style=col_scheme, parent=self).format2
             return '%s%s' % (level, ''.join(
                 _format_traceback_lines(lnum, index, lines, Colors, lvals,
                                          _line_format)))
 
     def prepare_header(self, etype, long_version=False):
         colors = self.Colors  # just a shorthand + quicker name lookup
         colorsnormal = colors.Normal  # used a lot
         exc = '%s%s%s' % (colors.excName, etype, colorsnormal)
         width = min(75, get_terminal_size()[0])
         if long_version:
             # Header with the exception type, python version, and date
             pyver = 'Python ' + sys.version.split()[0] + ': ' + sys.executable
             date = time.ctime(time.time())
 
             head = '%s%s%s\n%s%s%s\n%s' % (colors.topline, '-' * width, colorsnormal,
                                            exc, ' ' * (width - len(str(etype)) - len(pyver)),
                                            pyver, date.rjust(width) )
             head += "\nA problem occurred executing Python code.  Here is the sequence of function" \
                     "\ncalls leading up to the error, with the most recent (innermost) call last."
         else:
             # Simplified header
             head = '%s%s' % (exc, 'Traceback (most recent call last)'. \
                              rjust(width - len(str(etype))) )
 
         return head
 
     def format_exception(self, etype, evalue):
         colors = self.Colors  # just a shorthand + quicker name lookup
         colorsnormal = colors.Normal  # used a lot
         # Get (safely) a string form of the exception info
         try:
             etype_str, evalue_str = map(str, (etype, evalue))
         except:
             # User exception is improperly defined.
             etype, evalue = str, sys.exc_info()[:2]
             etype_str, evalue_str = map(str, (etype, evalue))
         # ... and format it
         return ['%s%s%s: %s' % (colors.excName, etype_str,
                                 colorsnormal, py3compat.cast_unicode(evalue_str))]
 
     def format_exception_as_a_whole(self, etype, evalue, etb, number_of_lines_of_context, tb_offset):
         """Formats the header, traceback and exception message for a single exception.
 
         This may be called multiple times by Python 3 exception chaining
         (PEP 3134).
         """
         # some locals
         orig_etype = etype
         try:
             etype = etype.__name__
         except AttributeError:
             pass
 
         tb_offset = self.tb_offset if tb_offset is None else tb_offset
         head = self.prepare_header(etype, self.long_header)
         records = self.get_records(etb, number_of_lines_of_context, tb_offset)
 
         if records is None:
             return ""
 
         last_unique, recursion_repeat = find_recursion(orig_etype, evalue, records)
 
         frames = self.format_records(records, last_unique, recursion_repeat)
 
         formatted_exception = self.format_exception(etype, evalue)
         if records:
             filepath, lnum = records[-1][1:3]
             filepath = os.path.abspath(filepath)
             ipinst = get_ipython()
             if ipinst is not None:
                 ipinst.hooks.synchronize_with_editor(filepath, lnum, 0)
 
         return [[head] + frames + [''.join(formatted_exception[0])]]
 
     def get_records(self, etb, number_of_lines_of_context, tb_offset):
         try:
             # Try the default getinnerframes and Alex's: Alex's fixes some
             # problems, but it generates empty tracebacks for console errors
             # (5 blanks lines) where none should be returned.
             return _fixed_getinnerframes(etb, number_of_lines_of_context, tb_offset)
         except UnicodeDecodeError:
             # This can occur if a file's encoding magic comment is wrong.
             # I can't see a way to recover without duplicating a bunch of code
             # from the stdlib traceback module. --TK
             error('\nUnicodeDecodeError while processing traceback.\n')
             return None
         except:
             # FIXME: I've been getting many crash reports from python 2.3
             # users, traceable to inspect.py.  If I can find a small test-case
             # to reproduce this, I should either write a better workaround or
             # file a bug report against inspect (if that's the real problem).
             # So far, I haven't been able to find an isolated example to
             # reproduce the problem.
             inspect_error()
             traceback.print_exc(file=self.ostream)
             info('\nUnfortunately, your original traceback can not be constructed.\n')
             return None
 
     def structured_traceback(self, etype, evalue, etb, tb_offset=None,
                              number_of_lines_of_context=5):
         """Return a nice text document describing the traceback."""
 
         formatted_exception = self.format_exception_as_a_whole(etype, evalue, etb, number_of_lines_of_context,
                                                                tb_offset)
 
         colors = self.Colors  # just a shorthand + quicker name lookup
         colorsnormal = colors.Normal  # used a lot
         head = '%s%s%s' % (colors.topline, '-' * min(75, get_terminal_size()[0]), colorsnormal)
         structured_traceback_parts = [head]
         chained_exceptions_tb_offset = 0
         lines_of_context = 3
         formatted_exceptions = formatted_exception
         exception = self.get_parts_of_chained_exception(evalue)
         if exception:
             formatted_exceptions += self.prepare_chained_exception_message(evalue.__cause__)
             etype, evalue, etb = exception
         else:
             evalue = None
         chained_exc_ids = set()
         while evalue:
             formatted_exceptions += self.format_exception_as_a_whole(etype, evalue, etb, lines_of_context,
                                                                      chained_exceptions_tb_offset)
             exception = self.get_parts_of_chained_exception(evalue)
 
             if exception and not id(exception[1]) in chained_exc_ids:
                 chained_exc_ids.add(id(exception[1])) # trace exception to avoid infinite 'cause' loop
                 formatted_exceptions += self.prepare_chained_exception_message(evalue.__cause__)
                 etype, evalue, etb = exception
             else:
                 evalue = None
 
         # we want to see exceptions in a reversed order:
         # the first exception should be on top
         for formatted_exception in reversed(formatted_exceptions):
             structured_traceback_parts += formatted_exception
 
         return structured_traceback_parts
 
     def debugger(self, force=False):
         """Call up the pdb debugger if desired, always clean up the tb
         reference.
 
         Keywords:
 
           - force(False): by default, this routine checks the instance call_pdb
             flag and does not actually invoke the debugger if the flag is false.
             The 'force' option forces the debugger to activate even if the flag
             is false.
 
         If the call_pdb flag is set, the pdb interactive debugger is
         invoked. In all cases, the self.tb reference to the current traceback
         is deleted to prevent lingering references which hamper memory
         management.
 
         Note that each call to pdb() does an 'import readline', so if your app
         requires a special setup for the readline completers, you'll have to
         fix that by hand after invoking the exception handler."""
 
         if force or self.call_pdb:
             if self.pdb is None:
                 self.pdb = self.debugger_cls()
             # the system displayhook may have changed, restore the original
             # for pdb
             display_trap = DisplayTrap(hook=sys.__displayhook__)
             with display_trap:
                 self.pdb.reset()
                 # Find the right frame so we don't pop up inside ipython itself
                 if hasattr(self, 'tb') and self.tb is not None:
                     etb = self.tb
                 else:
                     etb = self.tb = sys.last_traceback
                 while self.tb is not None and self.tb.tb_next is not None:
                     self.tb = self.tb.tb_next
                 if etb and etb.tb_next:
                     etb = etb.tb_next
                 self.pdb.botframe = etb.tb_frame
                 self.pdb.interaction(None, etb)
 
         if hasattr(self, 'tb'):
             del self.tb
 
     def handler(self, info=None):
         (etype, evalue, etb) = info or sys.exc_info()
         self.tb = etb
         ostream = self.ostream
         ostream.flush()
         ostream.write(self.text(etype, evalue, etb))
         ostream.write('\n')
         ostream.flush()
 
     # Changed so an instance can just be called as VerboseTB_inst() and print
     # out the right info on its own.
     def __call__(self, etype=None, evalue=None, etb=None):
         """This hook can replace sys.excepthook (for Python 2.1 or higher)."""
         if etb is None:
             self.handler()
         else:
             self.handler((etype, evalue, etb))
         try:
             self.debugger()
         except KeyboardInterrupt:
             print("\nKeyboardInterrupt")
 
 
 #----------------------------------------------------------------------------
 class FormattedTB(VerboseTB, ListTB):
     """Subclass ListTB but allow calling with a traceback.
 
     It can thus be used as a sys.excepthook for Python > 2.1.
 
     Also adds 'Context' and 'Verbose' modes, not available in ListTB.
 
     Allows a tb_offset to be specified. This is useful for situations where
     one needs to remove a number of topmost frames from the traceback (such as
     occurs with python programs that themselves execute other python code,
     like Python shells).  """
 
     def __init__(self, mode='Plain', color_scheme='Linux', call_pdb=False,
                  ostream=None,
                  tb_offset=0, long_header=False, include_vars=False,
                  check_cache=None, debugger_cls=None,
                  parent=None, config=None):
 
         # NEVER change the order of this list. Put new modes at the end:
         self.valid_modes = ['Plain', 'Context', 'Verbose', 'Minimal']
         self.verbose_modes = self.valid_modes[1:3]
 
         VerboseTB.__init__(self, color_scheme=color_scheme, call_pdb=call_pdb,
                            ostream=ostream, tb_offset=tb_offset,
                            long_header=long_header, include_vars=include_vars,
                            check_cache=check_cache, debugger_cls=debugger_cls,
                            parent=parent, config=config)
 
         # Different types of tracebacks are joined with different separators to
         # form a single string.  They are taken from this dict
         self._join_chars = dict(Plain='', Context='\n', Verbose='\n',
                                 Minimal='')
         # set_mode also sets the tb_join_char attribute
         self.set_mode(mode)
 
     def structured_traceback(self, etype, value, tb, tb_offset=None, number_of_lines_of_context=5):
         tb_offset = self.tb_offset if tb_offset is None else tb_offset
         mode = self.mode
         if mode in self.verbose_modes:
             # Verbose modes need a full traceback
             return VerboseTB.structured_traceback(
                 self, etype, value, tb, tb_offset, number_of_lines_of_context
             )
         elif mode == 'Minimal':
             return ListTB.get_exception_only(self, etype, value)
         else:
             # We must check the source cache because otherwise we can print
             # out-of-date source code.
             self.check_cache()
             # Now we can extract and format the exception
             return ListTB.structured_traceback(
                 self, etype, value, tb, tb_offset, number_of_lines_of_context
             )
 
     def stb2text(self, stb):
         """Convert a structured traceback (a list) to a string."""
         return self.tb_join_char.join(stb)
 
 
     def set_mode(self, mode=None):
         """Switch to the desired mode.
 
         If mode is not specified, cycles through the available modes."""
 
         if not mode:
             new_idx = (self.valid_modes.index(self.mode) + 1 ) % \
                       len(self.valid_modes)
             self.mode = self.valid_modes[new_idx]
         elif mode not in self.valid_modes:
             raise ValueError('Unrecognized mode in FormattedTB: <' + mode + '>\n'
                                                                             'Valid modes: ' + str(self.valid_modes))
         else:
             self.mode = mode
         # include variable details only in 'Verbose' mode
         self.include_vars = (self.mode == self.valid_modes[2])
         # Set the join character for generating text tracebacks
         self.tb_join_char = self._join_chars[self.mode]
 
     # some convenient shortcuts
     def plain(self):
         self.set_mode(self.valid_modes[0])
 
     def context(self):
         self.set_mode(self.valid_modes[1])
 
     def verbose(self):
         self.set_mode(self.valid_modes[2])
 
     def minimal(self):
         self.set_mode(self.valid_modes[3])
 
 
 #----------------------------------------------------------------------------
 class AutoFormattedTB(FormattedTB):
     """A traceback printer which can be called on the fly.
 
     It will find out about exceptions by itself.
 
     A brief example::
 
         AutoTB = AutoFormattedTB(mode = 'Verbose',color_scheme='Linux')
         try:
           ...
         except:
           AutoTB()  # or AutoTB(out=logfile) where logfile is an open file object
     """
 
     def __call__(self, etype=None, evalue=None, etb=None,
                  out=None, tb_offset=None):
         """Print out a formatted exception traceback.
 
         Optional arguments:
           - out: an open file-like object to direct output to.
 
           - tb_offset: the number of frames to skip over in the stack, on a
           per-call basis (this overrides temporarily the instance's tb_offset
           given at initialization time.  """
 
         if out is None:
             out = self.ostream
         out.flush()
         out.write(self.text(etype, evalue, etb, tb_offset))
         out.write('\n')
         out.flush()
         # FIXME: we should remove the auto pdb behavior from here and leave
         # that to the clients.
         try:
             self.debugger()
         except KeyboardInterrupt:
             print("\nKeyboardInterrupt")
 
     def structured_traceback(self, etype=None, value=None, tb=None,
                              tb_offset=None, number_of_lines_of_context=5):
         if etype is None:
             etype, value, tb = sys.exc_info()
-        if not isinstance(tb, tuple):
+        if isinstance(tb, tuple):
             # tb is a tuple if this is a chained exception.
-            # We keep first traceback.
+            self.tb = tb[0]
+        else:
             self.tb = tb
         return FormattedTB.structured_traceback(
             self, etype, value, tb, tb_offset, number_of_lines_of_context)
 
 
 #---------------------------------------------------------------------------
 
 # A simple class to preserve Nathan's original functionality.
 class ColorTB(FormattedTB):
     """Shorthand to initialize a FormattedTB in Linux colors mode."""
 
     def __init__(self, color_scheme='Linux', call_pdb=0, **kwargs):
         FormattedTB.__init__(self, color_scheme=color_scheme,
                              call_pdb=call_pdb, **kwargs)
 
 
 class SyntaxTB(ListTB):
     """Extension which holds some state: the last exception value"""
 
     def __init__(self, color_scheme='NoColor', parent=None, config=None):
         ListTB.__init__(self, color_scheme, parent=parent, config=config)
         self.last_syntax_error = None
 
     def __call__(self, etype, value, elist):
         self.last_syntax_error = value
 
         ListTB.__call__(self, etype, value, elist)
 
     def structured_traceback(self, etype, value, elist, tb_offset=None,
                              context=5):
         # If the source file has been edited, the line in the syntax error can
         # be wrong (retrieved from an outdated cache). This replaces it with
         # the current value.
         if isinstance(value, SyntaxError) \
                 and isinstance(value.filename, str) \
                 and isinstance(value.lineno, int):
             linecache.checkcache(value.filename)
             newtext = linecache.getline(value.filename, value.lineno)
             if newtext:
                 value.text = newtext
         self.last_syntax_error = value
         return super(SyntaxTB, self).structured_traceback(etype, value, elist,
                                                           tb_offset=tb_offset, context=context)
 
     def clear_err_state(self):
         """Return the current error state and clear it"""
         e = self.last_syntax_error
         self.last_syntax_error = None
         return e
 
     def stb2text(self, stb):
         """Convert a structured traceback (a list) to a string."""
         return ''.join(stb)
 
 
 # some internal-use functions
 def text_repr(value):
     """Hopefully pretty robust repr equivalent."""
     # this is pretty horrible but should always return *something*
     try:
         return pydoc.text.repr(value)
     except KeyboardInterrupt:
         raise
     except:
         try:
             return repr(value)
         except KeyboardInterrupt:
             raise
         except:
             try:
                 # all still in an except block so we catch
                 # getattr raising
                 name = getattr(value, '__name__', None)
                 if name:
                     # ick, recursion
                     return text_repr(name)
                 klass = getattr(value, '__class__', None)
                 if klass:
                     return '%s instance' % text_repr(klass)
             except KeyboardInterrupt:
                 raise
             except:
                 return 'UNRECOVERABLE REPR FAILURE'
 
 
 def eqrepr(value, repr=text_repr):
     return '=%s' % repr(value)
 
 
 def nullrepr(value, repr=text_repr):
     return ''