Dynamic Web content and forms processing in Python

First we process the inputs; see Section 7.19, “class Inputs: All the script's input”. We'll also open the database file. The 'c' mode argument causes it to be created if necessary; the 's' mode suffix requests that operations against be synchronized to the disk immediately. (If multiple copies of reader.cgi are running, gdbm will take care of locking the file.)

# - - -   m a i n

def main():
    '''Main program.

      [ if cgi.FieldStorage() requests cookie deletion ->
          file DB_NAME  :=  that file with no entry for the
              user's cookie, and with all expired entries deleted
          sys.stdout  +:=  (header to delete the cookie) +
              (HTML header) + (blank line) + (deletion-successful page)
        else if (the user has no cookie) or
        (the user's cookie is not a key in file DB_NAME) ->
          file DB_NAME  :=  that file  with a new entry whose key is a
              random string and whose value is chapter 1 and an
              expiration time as specified by cgi.FieldStorage(),
              and with expired entries deleted
          sys.stdout  +:=  (header to set that random string as
              the cookie) + (HTML header) + (blank line) +
             (page showing the chapter for that db entry)
        else ->
          file DB_NAME  :=  that file with the entry for the
              user's cookie, updated to the chapter number
              specified by cgi.FieldStorage(), and with expired
              entries deleted
          sys.stdout  +:=  (HTML header) + (blank line) +
              (page showing the chapter for that db entry) ]    
    '''

First we'll transform all the inputs into an instance of the Inputs class, and open our database file.

    #-- 1 --
    # [ inputs  :=  an Inputs instance representing all form
    #       name-value pairs and relevant environmental variables
    #       sent to this script
    inputs   =  Inputs()

For the database file name, see Section 7.6.13, “DB_NAME”; see also Section 7.21, “class UserDatabase”.

    #-- 2 --
    # [ if DB_NAME can be opened as a gdbm file ->
    #     db  :=  a UserDatabase instance representing that file
    #   else ->
    #     file DB_NAME  :=  a new, empty gdbm file
    #     db  :=  a UserDatabase instance representing that file ]
    db  =  UserDatabase()

If the user wants the cookie deleted, that's a special case with a different output page; see Section 7.8, “deleteCase(): Delete the cookie”. Otherwise we generate the usual page.

    #-- 3 --
    # [ if inputs.forget ->
    #     db  :=  db with the entry for key inputs.userId deleted
    #     sys.stdout  +:=  (header to delete the cookie) +
    #         (HTML header) + (blank line) +
    #         (deletion-successful page)
    #   else if (inputs.userId is None) or (inputs.userId is not
    #   a key in db)->
    #     db  :=  db with a new entry whose key is a random
    #         string and whose value is chapter 1 and an expiration time
    #         as specified by inputs
    #     sys.stdout  +:=  (header to set that random string as
    #         the cookie) + (HTML header) + (blank line) +
    #         (page showing the chapter for that db entry)
    #   else ->
    #     db  :=  db with the entry for inputs.userId updated
    #         to the page number specified by inputs
    #     sys.stdout  +:=  (HTML header) + (blank line) +
    #         (page showing the chapter for that db entry) ]
    if inputs.forget:
        deleteCase(inputs, db)
    else:
        chapterCase(inputs, db)

Finally, to prevent the database from growing without limit, we'll delete expired entries; see Section 7.26, “UserDatabase.cleanup()”.

    #-- 4 --
    # [ db  :=  db with all expired entries deleted ]
    db.cleanup()
    db.close()