Python Interview Questions And Answers Set - 7

The classical "Hello World" in python CGI fashion:
#!/usr/bin/env python
print "Content-Type: text/html"
print
print """\

Hello World!


"""

To test your setup save it with the .py extension, upload it to your server as text and make it executable before trying to run it.

The first line of a python CGI script sets the path where the python interpreter will be found in the server. Ask your provider what is the correct one. If it is wrong the script will fail. Some examples:

#!/usr/bin/python
#!/usr/bin/python2.3
#!/usr/bin/python2.4

It is necessary that the script outputs the HTTP header. The HTTP header consists of one or more messages followed by a blank line. If the output of the script is to be interpreted as HTML then the content type will be text/html. The blank line signals the end of the header and is required.

print "Content-Type: text/html"
print

If you change the content type to text/plain the browser will not interpret the script's output as HTML but as pure text and you will only see the HTML source. Try it now to never forget. A page refresh may be necessary for it to work.

Client versus Server

All python code will be executed at the server only. The client's agent (for example the browser) will never see a single line of python. Instead it will only get the script's output. This is something realy important to understand.

When programming for the Web you are in a client-server environment, that is, do not make things like trying to open a file in the client's computer as if the script were running there. It isn't.

How to Debugging in python?
Syntax and header errors are hard to catch unless you have access to the server logs. Syntax error messages can be seen if the script is run in a local shell before uploading to the server.

For a nice exceptions report there is the cgitb module. It will show a traceback inside a context. The default output is sent to standard output as HTML:

#!/usr/bin/env python
print "Content-Type: text/html"
print
import cgitb; cgitb.enable()
print 1/0

The handler() method can be used to handle only the catched exceptions:

#!/usr/bin/env python
print "Content-Type: text/html"
print
import cgitb
try:
f = open('non-existent-file.txt', 'r')
except:
cgitb.handler()

There is also the option for a crude approach making the header "text/plain" and setting the standard error to standard out:

#!/usr/bin/env python
print "Content-Type: text/plain"
print
import sys
sys.stderr = sys.stdout
f = open('non-existent-file.txt', 'r')

Will output this:

Traceback (most recent call last):
File "/var/www/html/teste/cgi-bin/text_error.py", line 6, in ?
f = open('non-existent-file.txt', 'r')
IOError: [Errno 2] No such file or directory: 'non-existent-file.txt'

Warning: These techniques expose information that can be used by an attacker. Use it only while developing/debugging. Once in production disable it.

How to use Cookies for Web python ?
HTTP is said to be a stateless protocol. What this means for web programmers is that every time a user loads a page it is the first time for the server. The server can't say whether this user has ever visited that site, if is he in the middle of a buying transaction, if he has already authenticated, etc.

A cookie is a tag that can be placed on the user's computer. Whenever the user loads a page from a site the site's script can send him a cookie. The cookie can contain anything the site needs to identify that user. Then within the next request the user does for a new page there goes back the cookie with all the pertinent information to be read by the script.

* Set the Cookie;

There are two basic cookie operations. The first is to set the cookie as an HTTP header to be sent to the client. The second is to read the cookie returned from the client also as an HTTP header.

This script will do the first one placing a cookie on the client's browser:

#!/usr/bin/env python
import time

# This is the message that contains the cookie
# and will be sent in the HTTP header to the client
print 'Set-Cookie: lastvisit=' + str(time.time());

# To save one line of code
# we replaced the print command with a '\n'
print 'Content-Type: text/html\n'
# End of HTTP header

print ''
print 'Server time is', time.asctime(time.localtime())
print ''

The Set-Cookie header contains the cookie. Save and run this code from your browser and take a look at the cookie saved there. Search for the cookie name, lastvisit, or for the domain name, or the server IP like 10.1.1.1 or 127.0.0.1.

The Cookie Object

The Cookie module can save us a lot of coding and errors and the next pages will use it in all cookie operations.

#!/usr/bin/env python
import time, Cookie

# Instantiate a SimpleCookie object
cookie = Cookie.SimpleCookie()

# The SimpleCookie instance is a mapping
cookie['lastvisit'] = str(time.time())

# Output the HTTP message containing the cookie
print cookie
print 'Content-Type: text/html\n'

print ''
print 'Server time is', time.asctime(time.localtime())
print ''

It does not seem as much for this extremely simple code, but wait until it gets complex and the Cookie module will be your friend.

* Retrieve the Cookie;

The returned cookie will be available as a string in the os.environ dictionary with the key 'HTTP_COOKIE':

cookie_string = os.environ.get('HTTP_COOKIE')

The load() method of the SimpleCookie object will parse that string rebuilding the object's mapping:

cookie.load(cookie_string)

Complete code:

#!/usr/bin/env python
import Cookie, os, time

cookie = Cookie.SimpleCookie()
cookie['lastvisit'] = str(time.time())

print cookie
print 'Content-Type: text/html\n'

print ''
print '

Server time is', time.asctime(time.localtime()), '

'

# The returned cookie is available in the os.environ dictionary
cookie_string = os.environ.get('HTTP_COOKIE')

# The first time the page is run there will be no cookies
if not cookie_string:
print '

First visit or cookies disabled

'

else: # Run the page twice to retrieve the cookie
print '

The returned cookie string was "' + cookie_string + '"

'

# load() parses the cookie string
cookie.load(cookie_string)
# Use the value attribute of the cookie to get it
lastvisit = float(cookie['lastvisit'].value)

print '

Your last visit was at',
print time.asctime(time.localtime(lastvisit)), '

'

print ''

When the client first loads the page there will be no cookie in the client's computer to be returned. The second time the page is requested then the cookie saved in the last run will be sent to the server.

* Morsels

In the previous cookie retrieve program the lastvisit cookie value was retrieved through its value attribute:

lastvisit = float(cookie['lastvisit'].value)

When a new key is set for a SimpleCookie object a Morsel instance is created:

>>> import Cookie
>>> import time
>>>
>>> cookie = Cookie.SimpleCookie()
>>> cookie

>>>
>>> cookie['lastvisit'] = str(time.time())
>>> cookie['lastvisit']

>>>
>>> cookie['lastvisit'].value
'1159535133.33'

Each cookie, a Morsel instance, can only have a predefined set of keys: expires, path, commnent, domain, max-age, secure and version. Any other key will raise an exception.

#!/usr/bin/env python
import Cookie, time

cookie = Cookie.SimpleCookie()

# name/value pair
cookie['lastvisit'] = str(time.time())

# expires in x seconds after the cookie is output.
# the default is to expire when the browser is closed
cookie['lastvisit']['expires'] = 30 * 24 * 60 * 60


# path in which the cookie is valid.
# if set to '/' it will valid in the whole domain.
# the default is the script's path.
cookie['lastvisit']['path'] = '/cgi-bin'

# the purpose of the cookie to be inspected by the user
cookie['lastvisit']['comment'] = 'holds the last user\'s visit date'

# domain in which the cookie is valid. always stars with a dot.
# to make it available in all subdomains
# specify only the domain like .my_site.com
cookie['lastvisit']['domain'] = '.www.my_site.com'

# discard in x seconds after the cookie is output
# not supported in most browsers
cookie['lastvisit']['max-age'] = 30 * 24 * 60 * 60

# secure has no value. If set directs the user agent to use
# only (unspecified) secure means to contact the origin
# server whenever it sends back this cookie
cookie['lastvisit']['secure'] = ''

# a decimal integer, identifies to which version of
# the state management specification the cookie conforms.
cookie['lastvisit']['version'] = 1

print 'Content-Type: text/html\n'

print '

', cookie, '

'
for morsel in cookie:
print '

', morsel, '=', cookie[morsel].value
print '

'
for key in cookie[morsel]:
print key, '=', cookie[morsel][key], '
'
print '