improve error message when disconnected

[dimitri-yatsenko]

trafficstars

When the connection to server goes away, error messages are misleading:

>>> Session()

---------------------------------------------------------------------------
DataJointError                            Traceback (most recent call last)
/usr/local/lib/python3.5/dist-packages/IPython/core/formatters.py in __call__(self, obj)
    309             method = get_real_method(obj, self.print_method)
    310             if method is not None:
--> 311                 return method()
    312             return None
    313         else:

/work/datajoint-python/datajoint/relational_operand.py in _repr_html_(self)
    364                         **dict(zip_longest(self.heading.blobs, [], fillvalue="'=BLOB='")))  # replace blobs with =BLOB=
    365         info = self.heading.table_info
--> 366         count = len(rel)
    367 
    368         css = """

/work/datajoint-python/datajoint/relational_operand.py in __len__(self)
    458         number of tuples in the relation.
    459         """
--> 460         return U().aggr(self, n='count(*)').fetch1['n']
    461 
    462     def __bool__(self):

/work/datajoint-python/datajoint/fetch.py in __getitem__(self, item)
    273         result = self._relation.proj(*attributes).fetch(**behavior)
    274         if len(result) != 1:
--> 275             raise DataJointError('fetch1 should only return one tuple. %d tuples were found' % len(result))
    276         return_values = tuple(
    277             next(to_dicts(result[self._relation.primary_key]))

DataJointError: fetch1 should only return one tuple. 0 tuples were found

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/usr/local/lib/python3.5/dist-packages/IPython/core/formatters.py in __call__(self, obj)
    670                 type_pprinters=self.type_printers,
    671                 deferred_pprinters=self.deferred_printers)
--> 672             printer.pretty(obj)
    673             printer.flush()
    674             return stream.getvalue()

/usr/local/lib/python3.5/dist-packages/IPython/lib/pretty.py in pretty(self, obj)
    381                             if callable(meth):
    382                                 return meth(obj, self, cycle)
--> 383             return _default_pprint(obj, self, cycle)
    384         finally:
    385             self.end_group()

/usr/local/lib/python3.5/dist-packages/IPython/lib/pretty.py in _default_pprint(obj, p, cycle)
    501     if _safe_getattr(klass, '__repr__', None) not in _baseclass_reprs:
    502         # A user-provided repr. Find newlines and replace them with p.break_()
--> 503         _repr_pprint(obj, p, cycle)
    504         return
    505     p.begin_group(1, '<')

/usr/local/lib/python3.5/dist-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    699     """A pprint that just redirects to the normal repr function."""
    700     # Find newlines and replace them with p.break_()
--> 701     output = repr(obj)
    702     for idx,output_line in enumerate(output.splitlines()):
    703         if idx:

/work/datajoint-python/datajoint/relational_operand.py in __repr__(self)
    337 
    338     def __repr__(self):
--> 339         return super().__repr__() if config['loglevel'].lower() == 'debug' else self.preview()
    340 
    341     def preview(self, limit=None, width=None):

/work/datajoint-python/datajoint/relational_operand.py in preview(self, limit, width)
    349         if width is None:
    350             width = config['display.width']
--> 351         tuples = rel.fetch(limit=limit)
    352         columns = rel.heading.names
    353         widths = {f: min(max([len(f)] + [len(str(e)) for e in tuples[f]]) + 4, width) for f in columns}

/work/datajoint-python/datajoint/fetch.py in __call__(self, **kwargs)
    156         else:
    157             ret = list(cur.fetchall())
--> 158             ret = np.array(ret, dtype=heading.as_dtype)
    159             for blob_name in heading.blobs:
    160                 ret[blob_name] = list(map(unpack_, ret[blob_name]))

ValueError: size of tuple must match number of fields.

Make the message more informative and suggest a solution: re-connect.