+"""
+Specialised psycopg2 cursor with shortcut functions useful for testing.
+"""
+import psycopg2.extras
+
+class TestingCursor(psycopg2.extras.DictCursor):
+ """ Extension to the DictCursor class that provides execution
+ short-cuts that simplify writing assertions.
+ """
+
+ def scalar(self, sql, params=None):
+ """ Execute a query with a single return value and return this value.
+ Raises an assertion when not exactly one row is returned.
+ """
+ self.execute(sql, params)
+ assert self.rowcount == 1
+ return self.fetchone()[0]
+
+
+ def row_set(self, sql, params=None):
+ """ Execute a query and return the result as a set of tuples.
+ Fails when the SQL command returns duplicate rows.
+ """
+ self.execute(sql, params)
+
+ result = set((tuple(row) for row in self))
+ assert len(result) == self.rowcount
+
+ return result
+
+
+ def table_exists(self, table):
+ """ Check that a table with the given name exists in the database.
+ """
+ num = self.scalar("""SELECT count(*) FROM pg_tables
+ WHERE tablename = %s""", (table, ))
+ return num == 1
+
+
+ def table_rows(self, table, where=None):
+ """ Return the number of rows in the given table.
+ """
+ if where is None:
+ return self.scalar('SELECT count(*) FROM ' + table)
+
+ return self.scalar('SELECT count(*) FROM {} WHERE {}'.format(table, where))
+
+
+ def execute_values(self, *args, **kwargs):
+ """ Execute the execute_values() function on the cursor.
+ """
+ psycopg2.extras.execute_values(self, *args, **kwargs)