"""
Specialised psycopg2 cursor with shortcut functions useful for testing.
"""
import psycopg2.extras

class CursorForTesting(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)