Replace the exception dance with light input validation.

django_tables2/tables.py

@@ -35,22 +35,15 @@ class TableData(object):
             self.queryset = data
         # otherwise it must be convertable to a list
         else:
-            try:
+            # do some light validation
+            if hasattr(data, '__iter__') or (hasattr(data, '__len__') and hasattr(data, '__getitem__')):
                 self.list = list(data)
-            except Exception as ex:
-                if six.PY3:
-                    raise ValueError(
-                        'data must be QuerySet-like (have count and '
-                        'order_by) or support list(data) -- %s has '
-                        'neither' % type(data).__name__
-                    )
-                else:
-                    # really horrible, but this syntax is not supported on PY3 and would not work otherwise
-                    exec("""raise ValueError, (
-                        'data must be QuerySet-like (have count and '
-                        'order_by) or support list(data) -- %s has '
-                        'neither. Original exception: %s' % (type(data).__name__, ex)
-                    ), sys.exc_info()[2]""")
+            else:
+                raise ValueError(
+                    'data must be QuerySet-like (have count and '
+                    'order_by) or support list(data) -- %s has '
+                    'neither' % type(data).__name__
+                )
 
     def __len__(self):
         if not hasattr(self, "_length"):

tests/core.py

@@ -165,7 +165,30 @@ def should_support_haystack_data_source():
 
     table = PersonTable(SearchQuerySet().all())
     table.as_html()
+
+
+@core.test
+def data_validation():
+    with raises(ValueError):
+        table = OrderedTable(None)
+    
+    class Bad:
+        def __len__(self):
+            pass
+      
+    with raises(ValueError):
+        table = OrderedTable(Bad())
 
+    class Ok:
+        def __len__(self):
+            return 1
+        def __getitem__(self, pos):
+            if pos != 0:
+                raise IndexError()
+            return {'a': 1}
+    
+    table = OrderedTable(Ok())
+    assert len(table.rows) == 1
 
 @core.test
 def ordering():