<pre> 
#!/usr/bin/python3
###################################################################################
# Enter a login
# Redirect to another page if login is entered
###################################################################################
import os
import cgi, cgitb

cgitb.enable()

elements = cgi.FieldStorage()
username = elements.getvalue('user') or 'Guest'   #get html form fields
password = elements.getvalue('pswd') or ''        
    
msg = ''

#-------------------------------------------------------------------------------------
# Function to validate entry fields 
#-------------------------------------------------------------------------------------
def validate():
    global msg                             #needed to modify global variable
                                           #no need if access to global is read only
    if username=='':
        msg = 'Please enter a login username (any value is OK)'
    if password=='':
        msg = 'Please enter a password (any value is OK)'

#-------------------------------------------------------------------------------------
# Function to display the form 
#-------------------------------------------------------------------------------------
def display():
                                           
    print('Content-Type: text/html')            #reponse header             
    print('\n')                                 #end all response headers

    print(F'''
        &lt;!DOCTYPE html>
        &lt;html>
        &lt;body bgcolor="lightyellow">
        &lt;h1>&lt;center>Enter a Login&lt;/center>&lt;/h1>
        &lt;form name="form1" method="POST" >             &lt;!--the name of the current Python script-->
        &lt;fieldset style='width:580px; height:100px; border-color:gold'>
        &lt;legend>&lt;i>Enter login below&lt;/i>&lt;/legend>    
        &lt;table bgcolor=lightgray width=570px>
            &lt;tr>&lt;td>&lt;b>Username
                &lt;td>&lt;input type="text" name="user" value="{username}">
            &lt;tr>&lt;td>&lt;b>Password
                &lt;td>&lt;input type="text" name="pswd" value="{password}">
                &lt;i>(Any value is OK)&lt;/i>
            &lt;tr>&lt;td>&lt;td>&lt;input type="submit" value="  Login " />&lt;/td>
        &lt;/table>
        &lt;br>&lt;p style='color:red'> {msg} &lt;/p>
        &lt;/body>
        &lt;/html>
    ''')                                       
    
#==========================================================================
# main
#==========================================================================
if not elements:                                    #if first time 
    display()                                       #display the page

if elements:                                        #if subsequent time
    validate()                                      #validate                            

    if msg:                                         #if errors
        display()                                   #redisplay the same page
    else:
        print('Set-Cookie: login=' + username)      #set a cookie     
        print('Location: page2.py')                 #REDIRECT to another page     
        print('\n')                                 #end the response headers


#=== link to see the python code =================================================
import os, sys
sys.path.insert(0,'/home/sultans/web/python/demo')
import zCode                          #import func to display the Python code
filename = os.path.abspath(__file__)  #get absolute file name 
zCode.display(filename)               #call it
#=================================================================================</pre>