Hi all, 
I am using sage  on Debian (sid)
Sagemath 7.0 was working fine until an upgrade of python  to version 3 
Then it stop working with an syntax error due to python 3  
So I (re)lnked pyhton to python 2.7 but now it crashes for a reason I 
cannot determine 
here is the crash report 
Thanks in advance,

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***************************************************************************

IPython post-mortem report

{'commit_hash': u'1961f94',
 'commit_source': 'installation',
 'default_encoding': 'UTF-8',
 'ipython_path': 
'/home/laurent/sage-7.0/local/lib/python2.7/site-packages/IPython',
 'ipython_version': '4.0.0',
 'os_name': 'posix',
 'platform': 'Linux-4.8.0-2-amd64-x86_64-with-debian-stretch-sid',
 'sys_executable': '/home/laurent/sage-7.0/local/bin/python',
 'sys_platform': 'linux2',
 'sys_version': '2.7.13rc1 (default, Dec  4 2016, 14:12:39) \n[GCC 6.2.1 
20161124]'}

***************************************************************************



***************************************************************************

Crash traceback:

---------------------------------------------------------------------------
---------------------------------------------------------------------------
TypeError         Python 2.7.13rc1: /home/laurent/sage-7.0/local/bin/python
                                                   Wed Dec 14 13:08:03 2016
A problem occurred executing Python code.  Here is the sequence of function
calls leading up to the error, with the most recent (innermost) call last.
/home/laurent/sage-7.0/src/bin/sage-ipython in <module>()
      1 #!/usr/bin/env python
      2 # -*- coding: utf-8 -*-
      3 """
      4 Sage IPython startup script.
      5 """
      6 
      7 # Install extra readline commands before IPython initialization
      8 from sage.repl.readline_extra_commands import *
      9 
     10 from sage.repl.interpreter import SageTerminalApp
     11 
     12 app = SageTerminalApp.instance()
---> 13 app.initialize()
        global app.initialize = <bound method SageTerminalApp.initialize of 
<sage.repl.interpreter.SageTerminalApp object at 0x7ff82b5b40d0>>
     14 app.start()

<decorator-gen-111> in initialize(self=<sage.repl.interpreter.SageTerminalApp 
object>, argv=None)

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/traitlets/config/application.pyc
 in catch_config_error(method=<function initialize>, 
app=<sage.repl.interpreter.SageTerminalApp object>, *args=(None,), **kwargs={})
     60 
     61 
#-----------------------------------------------------------------------------
     62 # Application class
     63 
#-----------------------------------------------------------------------------
     64 
     65 @decorator
     66 def catch_config_error(method, app, *args, **kwargs):
     67     """Method decorator for catching invalid config 
(Trait/ArgumentErrors) during init.
     68 
     69     On a TraitError (generally caused by bad config), this will print 
the trait's
     70     message, and exit the app.
     71     
     72     For use on init methods, to prevent invoking excepthook on invalid 
input.
     73     """
     74     try:
---> 75         return method(app, *args, **kwargs)
        method = <function initialize at 0x7ff828527668>
        app = <sage.repl.interpreter.SageTerminalApp object at 0x7ff82b5b40d0>
        args = (None,)
        kwargs = {}
     76     except (TraitError, ArgumentError) as e:
     77         app.print_help()
     78         app.log.fatal("Bad config encountered during initialization:")
     79         app.log.fatal(str(e))
     80         app.log.debug("Config at the time: %s", app.config)
     81         app.exit(1)
     82 
     83 
     84 class ApplicationError(Exception):
     85     pass
     86 
     87 class LevelFormatter(logging.Formatter):
     88     """Formatter with additional `highlevel` record
     89     
     90     This field is empty if log level is less than highlevel_limit,

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/IPython/terminal/ipapp.pyc
 in initialize(self=<sage.repl.interpreter.SageTerminalApp object>, argv=None)
    299 
    300         return super(TerminalIPythonApp, self).parse_command_line(argv)
    301     
    302     @catch_config_error
    303     def initialize(self, argv=None):
    304         """Do actions after construct, but before starting the app."""
    305         super(TerminalIPythonApp, self).initialize(argv)
    306         if self.subapp is not None:
    307             # don't bother initializing further, starting subapp
    308             return
    309         # print self.extra_args
    310         if self.extra_args and not self.something_to_run:
    311             self.file_to_run = self.extra_args[0]
    312         self.init_path()
    313         # create the shell
--> 314         self.init_shell()
        self.init_shell = <bound method SageTerminalApp.init_shell of 
<sage.repl.interpreter.SageTerminalApp object at 0x7ff82b5b40d0>>
    315         # and draw the banner
    316         self.init_banner()
    317         # Now a variety of things that happen after the banner is 
printed.
    318         self.init_gui_pylab()
    319         self.init_extensions()
    320         self.init_code()
    321 
    322     def init_shell(self):
    323         """initialize the InteractiveShell instance"""
    324         # Create an InteractiveShell instance.
    325         # shell.display_banner should always be False for the terminal
    326         # based app, because we call shell.show_banner() by hand below
    327         # so the banner shows *before* all extension loading stuff.
    328         self.shell = TerminalInteractiveShell.instance(parent=self,
    329                         display_banner=False, 
profile_dir=self.profile_dir,

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/repl/interpreter.pyc
 in init_shell(self=<sage.repl.interpreter.SageTerminalApp object>)
    793         self.shell.has_sage_extensions = SAGE_EXTENSION in 
self.extensions
    794 
    795         # Load the %lprun extension if available
    796         try:
    797             import line_profiler
    798         except ImportError:
    799             pass
    800         else:
    801             self.extensions.append('line_profiler')
    802 
    803         if self.shell.has_sage_extensions:
    804             self.extensions.remove(SAGE_EXTENSION)
    805 
    806             # load sage extension here to get a crash if
    807             # something is wrong with the sage library
--> 808             self.shell.extension_manager.load_extension(SAGE_EXTENSION)
        self.shell.extension_manager.load_extension = <bound method 
ExtensionManager.load_extension of <IPython.core.extensions.ExtensionManager 
object at 0x7ff826f68b10>>
        global SAGE_EXTENSION = 'sage'
    809 
    810 

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/IPython/core/extensions.pyc
 in load_extension(self=<IPython.core.extensions.ExtensionManager object>, 
module_str='sage')
     76 
     77         Returns the string "already loaded" if the extension is already 
loaded,
     78         "no load function" if the module doesn't have a 
load_ipython_extension
     79         function, or None if it succeeded.
     80         """
     81         if module_str in self.loaded:
     82             return "already loaded"
     83         
     84         from IPython.utils.syspathcontext import prepended_to_syspath
     85         
     86         with self.shell.builtin_trap:
     87             if module_str not in sys.modules:
     88                 with prepended_to_syspath(self.ipython_extension_dir):
     89                     __import__(module_str)
     90             mod = sys.modules[module_str]
---> 91             if self._call_load_ipython_extension(mod):
        self._call_load_ipython_extension = <bound method 
ExtensionManager._call_load_ipython_extension of 
<IPython.core.extensions.ExtensionManager object at 0x7ff826f68b10>>
        mod = <module 'sage' from 
'/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/__init__.pyc'>
     92                 self.loaded.add(module_str)
     93             else:
     94                 return "no load function"
     95 
     96     def unload_extension(self, module_str):
     97         """Unload an IPython extension by its module name.
     98 
     99         This function looks up the extension's name in ``sys.modules`` 
and
    100         simply calls ``mod.unload_ipython_extension(self)``.
    101         
    102         Returns the string "no unload function" if the extension 
doesn't define
    103         a function to unload itself, "not loaded" if the extension 
isn't loaded,
    104         otherwise None.
    105         """
    106         if module_str not in self.loaded:

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/IPython/core/extensions.pyc
 in _call_load_ipython_extension(self=<IPython.core.extensions.ExtensionManager 
object>, mod=<module 'sage' from 
'/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/__init__.pyc'>)
    123         """
    124         from IPython.utils.syspathcontext import prepended_to_syspath
    125 
    126         if (module_str in self.loaded) and (module_str in sys.modules):
    127             self.unload_extension(module_str)
    128             mod = sys.modules[module_str]
    129             with prepended_to_syspath(self.ipython_extension_dir):
    130                 reload(mod)
    131             if self._call_load_ipython_extension(mod):
    132                 self.loaded.add(module_str)
    133         else:
    134             self.load_extension(module_str)
    135 
    136     def _call_load_ipython_extension(self, mod):
    137         if hasattr(mod, 'load_ipython_extension'):
--> 138             mod.load_ipython_extension(self.shell)
        mod.load_ipython_extension = <function load_ipython_extension at 
0x7ff82b5ad0c8>
        self.shell = <sage.repl.interpreter.SageTerminalInteractiveShell object 
at 0x7ff828543750>
    139             return True
    140 
    141     def _call_unload_ipython_extension(self, mod):
    142         if hasattr(mod, 'unload_ipython_extension'):
    143             mod.unload_ipython_extension(self.shell)
    144             return True
    145 
    146     def install_extension(self, url, filename=None):
    147         """Download and install an IPython extension. 
    148 
    149         If filename is given, the file will be so named (inside the 
extension
    150         directory). Otherwise, the name from the URL will be used. The 
file must
    151         have a .py or .zip extension; otherwise, a ValueError will be 
raised.
    152 
    153         Returns the full path to the installed file.

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/__init__.pyc in 
load_ipython_extension(*args=(<sage.repl.interpreter.SageTerminalInteractiveShell
 object>,))
      1 __all__ = ['all']
      2 
      3 # IPython calls this when starting up
      4 def load_ipython_extension(*args):
      5     import sage.repl.ipython_extension
----> 6     sage.repl.ipython_extension.load_ipython_extension(*args)
        sage.repl.ipython_extension.load_ipython_extension = <function 
load_ipython_extension at 0x7ff826f17488>
        args = (<sage.repl.interpreter.SageTerminalInteractiveShell object at 
0x7ff828543750>,)

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/repl/ipython_extension.pyc
 in wrapper(*args=(<sage.repl.interpreter.SageTerminalInteractiveShell 
object>,), **kwargs={})
    448         ....:     if work:
    449         ....:         return 'foo worked'
    450         ....:     raise RuntimeError("foo didn't work")
    451         sage: foo(False)
    452         Traceback (most recent call last):
    453         ...
    454         RuntimeError: foo didn't work
    455         sage: foo(True)
    456         'foo worked'
    457         sage: foo(False)
    458         sage: foo(True)
    459     """
    460     @wraps(func)
    461     def wrapper(*args, **kwargs):
    462         if not wrapper.has_run:
--> 463             result = func(*args, **kwargs)
        result = undefined
        global func = undefined
        args = (<sage.repl.interpreter.SageTerminalInteractiveShell object at 
0x7ff828543750>,)
        kwargs = {}
    464             wrapper.has_run = True
    465             return result
    466     wrapper.has_run = False
    467     return wrapper
    468 
    469 
    470 @run_once
    471 def load_ipython_extension(ip):
    472     """
    473     Load the extension in IPython.
    474     """
    475     # this modifies ip
    476     SageCustomizations(shell=ip)

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/repl/ipython_extension.pyc
 in 
load_ipython_extension(ip=<sage.repl.interpreter.SageTerminalInteractiveShell 
object>)
    461     def wrapper(*args, **kwargs):
    462         if not wrapper.has_run:
    463             result = func(*args, **kwargs)
    464             wrapper.has_run = True
    465             return result
    466     wrapper.has_run = False
    467     return wrapper
    468 
    469 
    470 @run_once
    471 def load_ipython_extension(ip):
    472     """
    473     Load the extension in IPython.
    474     """
    475     # this modifies ip
--> 476     SageCustomizations(shell=ip)
        global SageCustomizations = <class 
'sage.repl.ipython_extension.SageCustomizations'>
        global shell = undefined
        ip = <sage.repl.interpreter.SageTerminalInteractiveShell object at 
0x7ff828543750>

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/repl/ipython_extension.pyc
 in __init__(self=<sage.repl.ipython_extension.SageCustomizations object>, 
shell=<sage.repl.interpreter.SageTerminalInteractiveShell object>)
    330         """
    331         self.shell = shell
    332 
    333         self.auto_magics = SageMagics(shell)
    334         self.shell.register_magics(self.auto_magics)
    335 
    336         import sage.misc.edit_module as edit_module
    337         self.shell.set_hook('editor', edit_module.edit_devel)
    338 
    339         self.init_inspector()
    340         self.init_line_transforms()
    341 
    342         import inputhook
    343         inputhook.install()
    344 
--> 345         import sage.all # until sage's import hell is fixed
        sage.all = undefined
    346 
    347         self.shell.verbose_quit = True
    348         self.set_quit_hook()
    349 
    350         self.register_interface_magics()
    351 
    352         if SAGE_IMPORTALL == 'yes':
    353             self.init_environment()
    354 
    355 
    356     def register_interface_magics(self):
    357         """
    358         Register magics for each of the Sage interfaces
    359         """
    360         from sage.misc.superseded import deprecation

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/all.py in 
<module>()
     69 
     70 from sage.env import SAGE_ROOT, SAGE_DOC, SAGE_LOCAL, DOT_SAGE, SAGE_ENV
     71 
     72 if sys.version_info[:2] < (2, 5):
     73     print >>sys.stderr, "Sage requires Python 2.5 or newer"
     74     sys.exit(1)
     75 
     76 ###################################################################
     77 
     78 # This import also setups the interrupt handler
     79 from sage.ext.interrupt import AlarmInterrupt, SignalError, 
sig_on_reset as sig_on_count
     80 
     81 from time                import sleep
     82 
     83 import sage.misc.lazy_import
---> 84 from sage.misc.all       import *         # takes a while
        global sage.misc.all = undefined
     85 from sage.typeset.all    import *
     86 from sage.repl.all       import *
     87 
     88 from sage.misc.sh import sh
     89 
     90 from sage.libs.all       import *
     91 from sage.data_structures.all import *
     92 from sage.doctest.all    import *
     93 try:
     94     from sage.dev.all    import *
     95 except ImportError:
     96     pass   # dev scripts are disabled
     97 
     98 from sage.structure.all  import *
     99 from sage.rings.all      import *

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/misc/all.py in 
<module>()
     73                         series_precision, set_series_precision)
     74 
     75 from sage_eval import sage_eval, sageobj
     76 
     77 from sage_input import sage_input
     78 
     79 from cython import cython_lambda, cython_create_local_so
     80 from cython_c import cython_compile as cython
     81 lazy_import("sage.misc.cython_c", "cython_compile", "pyrex", 
deprecation=9552)
     82 lazy_import("sage.misc.cython_c", "cython_compile", "sagex", 
deprecation=9552)
     83 
     84 from persist import save, load, dumps, loads, db, db_save
     85 
     86 from func_persist import func_persist
     87 
---> 88 from functional import (additive_order,
        global functional = undefined
        global additive_order = undefined
        global base_ring = undefined
        global base_field = undefined
        global basis = undefined
        global category = undefined
        global charpoly = undefined
        global characteristic_polynomial = undefined
        global coerce = undefined
        global cyclotomic_polynomial = undefined
        global decomposition = undefined
        global denominator = undefined
        global det = undefined
        global dimension = undefined
        global dim = undefined
        global discriminant = undefined
        global disc = undefined
        global eta = undefined
        global fcp = undefined
        global gen = undefined
        global gens = undefined
        global hecke_operator = undefined
        global image = undefined
        global integral = undefined
        global integrate = undefined
        global integral_closure = undefined
        global interval = undefined
        global xinterval = undefined
        global is_commutative = undefined
        global is_even = undefined
        global is_integrally_closed = undefined
        global is_field = undefined
        global is_odd = undefined
        global kernel = undefined
        global krull_dimension = undefined
        global lift = undefined
        global log = undefined
        global log_b = undefined
        global minimal_polynomial = undefined
        global minpoly = undefined
        global multiplicative_order = undefined
        global ngens = undefined
        global norm = undefined
        global numerator = undefined
        global numerical_approx = undefined
        global n = undefined
        global N = undefined
        global objgens = undefined
        global objgen = undefined
        global one = undefined
        global order = undefined
        global rank = undefined
        global regulator = undefined
        global round = undefined
        global quotient = undefined
        global quo = undefined
        global isqrt = undefined
        global squarefree_part = undefined
        global symbolic_sum = undefined
        global sum = undefined
        global transpose = undefined
        global zero = undefined
     89                         base_ring,
     90                         base_field,
     91                         basis,
     92                         category,
     93                         charpoly,
     94                         characteristic_polynomial,
     95                         coerce,
     96                         cyclotomic_polynomial,
     97                         decomposition,
     98                         denominator,
     99                         det,
    100                         dimension,
    101                         dim,
    102                         discriminant,
    103                         disc,

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/misc/functional.py
 in <module>()
     11 
     12 - David Joyner (2005-12-20): More Examples
     13 """
     14 
     15 
#*****************************************************************************
     16 #       Copyright (C) 2004 William Stein <wst...@gmail.com>
     17 #
     18 # This program is free software: you can redistribute it and/or modify
     19 # it under the terms of the GNU General Public License as published by
     20 # the Free Software Foundation, either version 2 of the License, or
     21 # (at your option) any later version.
     22 #                  http://www.gnu.org/licenses/
     23 
#*****************************************************************************
     24 
     25 import sage.misc.latex
---> 26 import sage.interfaces.expect
        global sage.interfaces.expect = undefined
     27 import sage.interfaces.mathematica
     28 
     29 
     30 from sage.rings.complex_double import CDF
     31 from sage.rings.real_double import RDF, RealDoubleElement
     32 
     33 import sage.rings.real_mpfr
     34 import sage.rings.complex_field
     35 import sage.rings.integer
     36 
     37 import __builtin__
     38 
     39 LOG_TEN_TWO_PLUS_EPSILON = 3.321928094887363 # a small overestimate of 
log(10,2)
     40 
     41 
##############################################################################

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/interfaces/expect.py
 in <module>()
     40 
#*****************************************************************************
     41 
     42 import os
     43 import sys
     44 import weakref
     45 import time
     46 import gc
     47 import quit
     48 import cleaner
     49 import six
     50 from random import randrange
     51 
     52 import pexpect
     53 from pexpect import ExceptionPexpect
     54 from sage.interfaces.sagespawn import SageSpawn
---> 55 from sage.interfaces.interface import (Interface, InterfaceElement,
        global sage.interfaces.interface = undefined
        global Interface = undefined
        global InterfaceElement = undefined
        global InterfaceFunction = undefined
        global InterfaceFunctionElement = undefined
        global AsciiArtString = undefined
     56         InterfaceFunction, InterfaceFunctionElement, AsciiArtString)
     57 
     58 from sage.structure.element import RingElement
     59 
     60 from sage.misc.misc import SAGE_TMP_INTERFACE
     61 from sage.env import SAGE_EXTCODE, LOCAL_IDENTIFIER
     62 from sage.misc.object_multiplexer import Multiplex
     63 
     64 from six import reraise as raise_
     65 
     66 BAD_SESSION = -2
     67 
     68 # The subprocess is a shared resource.  In a multi-threaded
     69 # environment, there would have to be a lock to control access to the
     70 # subprocess.  Fortunately, Sage does not use Python threads.

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/interfaces/interface.py
 in <module>()
     30 #
     31 #    This code is distributed in the hope that it will be useful,
     32 #    but WITHOUT ANY WARRANTY; without even the implied warranty of
     33 #    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     34 #    General Public License for more details.
     35 #
     36 #  The full text of the GPL is available at:
     37 #
     38 #                  http://www.gnu.org/licenses/
     39 
#*****************************************************************************
     40 
     41 import operator
     42 import six
     43 
     44 from sage.structure.sage_object import SageObject
---> 45 from sage.structure.parent_base import ParentWithBase
        global sage.structure.parent_base = undefined
        global ParentWithBase = undefined
     46 from sage.structure.element import RingElement, parent
     47 
     48 import sage.misc.sage_eval
     49 
     50 
     51 class AsciiArtString(str):
     52     def __repr__(self):
     53         return str(self)
     54 
     55 
     56 class Interface(ParentWithBase):
     57     """
     58     Interface interface object.
     59     """
     60     def __init__(self, name):

sage/structure/parent_base.pyx in init sage.structure.parent_base 
(/home/laurent/sage-7.0/src/build/cythonized/sage/structure/parent_base.c:3476)()

sage/structure/parent_old.pyx in init sage.structure.parent_old 
(/home/laurent/sage-7.0/src/build/cythonized/sage/structure/parent_old.c:9608)()

sage/structure/parent.pyx in init sage.structure.parent 
(/home/laurent/sage-7.0/src/build/cythonized/sage/structure/parent.c:31329)()

sage/structure/category_object.pyx in init sage.structure.category_object 
(/home/laurent/sage-7.0/src/build/cythonized/sage/structure/category_object.c:13329)()

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/categories/category.py
 in <module>()
     90 #  Copyright (C) 2005      David Kohel <ko...@maths.usyd.edu> and
     91 #                          William Stein <wst...@math.ucsd.edu>
     92 #                2008-2014 Nicolas M. Thiery <nthiery at users.sf.net>
     93 #
     94 #  Distributed under the terms of the GNU General Public License (GPL)
     95 #                  http://www.gnu.org/licenses/
     96 
#*****************************************************************************
     97 
     98 import inspect
     99 from warnings import warn
    100 from sage.misc.abstract_method import abstract_method, 
abstract_methods_of_class
    101 from sage.misc.cachefunc import cached_method, cached_function
    102 from sage.misc.c3_controlled import _cmp_key, _cmp_key_named, 
C3_sorted_merge
    103 from sage.misc.lazy_attribute import lazy_attribute
    104 from sage.misc.lazy_import import lazy_import
--> 105 from sage.misc.unknown import Unknown
        global sage.misc.unknown = undefined
        global Unknown = undefined
    106 from sage.misc.weak_dict import WeakValueDictionary
    107 
    108 from sage.structure.sage_object import SageObject
    109 from sage.structure.unique_representation import UniqueRepresentation
    110 from sage.structure.dynamic_class import DynamicMetaclass, dynamic_class
    111 
    112 from sage.categories.category_cy_helper import category_sort_key, 
_sort_uniq, _flatten_categories, join_as_tuple
    113 
    114 _join_cache = WeakValueDictionary()
    115 
    116 class Category(UniqueRepresentation, SageObject):
    117     r"""
    118     The base class for modeling mathematical categories, like for 
example:
    119 
    120     - ``Groups()``: the category of groups

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/misc/unknown.py 
in <module>()
    144             sage: for a in l: print ([a <= b for b in l])
    145             [True, True, True]
    146             [False, True, True]
    147             [False, False, True]
    148         """
    149         if other is self:
    150             return 0
    151         if isinstance(other, bool):
    152             if other:
    153                 return -1
    154             else:
    155                 return +1
    156         else:
    157             raise ValueError("Unable to compare {} with 
{}".format(self, other))
    158 
--> 159 Unknown = UnknownClass()
        global Unknown = undefined
        global UnknownClass = None

sage/misc/classcall_metaclass.pyx in 
sage.misc.classcall_metaclass.ClasscallMetaclass.__call__ 
(/home/laurent/sage-7.0/src/build/cythonized/sage/misc/classcall_metaclass.c:1239)()

sage/misc/cachefunc.pyx in sage.misc.cachefunc.CachedFunction.__call__ 
(/home/laurent/sage-7.0/src/build/cythonized/sage/misc/cachefunc.c:5371)()

/home/laurent/sage-7.0/local/lib/python2.7/site-packages/sage/structure/unique_representation.pyc
 in __classcall__(cls=<class 'sage.misc.unknown.UnknownClass'>, *args=(), 
**options={})
   1006     @weak_cached_function # automatically a staticmethod
   1007     def __classcall__(cls, *args, **options):
   1008         """
   1009         Construct a new object of this class or reuse an existing one.
   1010 
   1011         See also :class:`CachedRepresentation` and
   1012         :class:`UniqueRepresentation` for a discussion.
   1013 
   1014         EXAMPLES::
   1015 
   1016             sage: x = UniqueRepresentation()
   1017             sage: y = UniqueRepresentation()
   1018             sage: x is y   # indirect doctest
   1019             True
   1020         """
-> 1021         instance = typecall(cls, *args, **options)
        instance = undefined
        global typecall = <built-in function typecall>
        cls = <class 'sage.misc.unknown.UnknownClass'>
        args = ()
        options = {}
   1022         assert isinstance( instance, cls )
   1023         if instance.__class__.__reduce__ == 
CachedRepresentation.__reduce__:
   1024             instance._reduction = (cls, args, options)
   1025         return instance
   1026 
   1027     @classmethod
   1028     def _clear_cache_(cls):
   1029         """
   1030         Remove all instances of this class from the cache.
   1031 
   1032         EXAMPLES:
   1033 
   1034         If ``cls`` overloads 
:meth:`~sage.structure.unique_representation.CachedRepresentation.__classcall__`,
   1035         clearing the cache still works, because ``cls.mro()``
   1036         is searched until a ``__classcall__`` with an attribute

sage/misc/classcall_metaclass.pyx in sage.misc.classcall_metaclass.typecall 
(/home/laurent/sage-7.0/src/build/cythonized/sage/misc/classcall_metaclass.c:1665)()

TypeError: sage.misc.fast_methods.WithEqualityById.__new__(UnknownClass) is not 
safe, use object.__new__()

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