Question

How are you supposed to handle exceptions when there are no rows for this id. How should the calling code handle this

  def function(id)
    mysql_conn = MySQLdb.connect(..)    
    cursor = mysql_conn.cursor()
    cursor.execute("""select val from fooo where id = %s""", (id))
    row = cursor.fetchone()
    return row[0]

Answer 1

Whenever I find myself fetching just one row I always like to use the following (although not the best performance wise) construct.

cursor.execute("SELECT * FROM foo;")
for row in cursor:
    return row
# No row handling behind the loop.
return "No rows found"

This effectively returns when the first iteration of the loop runs. Which can only happen if there is at least 1 row in the result set. And otherwise will run the code behind the for loop.

You can raise a custom exception, or return a special value such as None that lets the calling function know nicely that there were no rows.