fix: Allow heading introspection on base tier classes

The heading property now returns None for base tier classes (Lookup,
Manual, Imported, Computed, Part) instead of raising an error. This
allows Python's help() and inspect modules to work correctly.

User-defined table classes still get the helpful error message when
trying to access heading on a non-activated schema.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/table.py

@@ -120,15 +120,23 @@ def table_name(self):
     def class_name(self):
         return self.__class__.__name__
 
+    # Base tier class names that should not raise errors when heading is None
+    _base_tier_classes = frozenset({"Table", "UserTable", "Lookup", "Manual", "Imported", "Computed", "Part"})
+
     @property
     def heading(self):
         """
         Return the table's heading, or raise a helpful error if not configured.
 
         Overrides QueryExpression.heading to provide a clear error message
         when the table is not properly associated with an activated schema.
+        For base tier classes (Lookup, Manual, etc.), returns None to support
+        introspection (e.g., help()).
         """
         if self._heading is None:
+            # Don't raise error for base tier classes - they're used for introspection
+            if self.__class__.__name__ in self._base_tier_classes:
+                return None
             raise DataJointError(
                 f"Table `{self.__class__.__name__}` is not properly configured. "
                 "Ensure the schema is activated before using the table. "