Python Interview Questions and Answers

The classical "Hello World" in python CGI fashion:
#!/usr/bin/env python
print "Content-Type: text/html"
print
print """\
<html>
<body>
<h2>Hello World!
</body>
</html>
"""

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.

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