refactor: improve error messages and use capsule source in UDTF example

Author: kosiew

examples/python-udtf-table-capsule-regression.py

@@ -10,19 +10,23 @@ def main() -> None:
     """Demonstrate current failure modes around table provider capsules."""
 
     ctx = SessionContext()
+    capsule_source = Table(ctx.sql("SELECT 1 AS value"))
 
     @udtf("table_from_sql")
     def table_from_sql_udtf() -> Table:
         """Return a DataFusion Table constructed from a SQL query."""
 
-        return Table(ctx.sql("SELECT 1 AS value"))
+        return capsule_source
 
     ctx.register_udtf(table_from_sql_udtf)
 
     try:
-        ctx.sql("SELECT * FROM table(table_from_sql())").collect()
+        ctx.sql("SELECT * FROM table_from_sql()").collect()
     except NotImplementedError as err:
-        print("Collecting from table_from_sql() failed:", err)
+        print(
+            "Collecting from table_from_sql() failed because the table provider capsule is missing:",
+            err,
+        )
 
     ctx.register_table("numbers", Table(ctx.sql("SELECT 1 AS value")))
 
@@ -31,7 +35,10 @@ def table_from_sql_udtf() -> Table:
     try:
         getattr(numbers, "__datafusion_table_provider__")
     except AttributeError as err:
-        print("Accessing __datafusion_table_provider__ on catalog table failed:", err)
+        print(
+            "Accessing __datafusion_table_provider__ on catalog table failed because the capsule attribute is missing:",
+            err,
+        )
 
 
 if __name__ == "__main__":