Server Processing

When a server recieves a request for a CGI resource:

Locate the script, eg. via normal filename resolution rules
Establish CGI environment
Call script, capture any output or errors
If successful: emit HTTP header, emit script output
If failed: emit HTTP error response

Example: Target Output

<html>
  <head>
    <title>Demo static HTML page</title>
  </head>
  <body>
    <h1>Hello COMP249!</h1>
  </body>
</html>

CGI Script in Python

#!/usr/local/bin/python
print "Content-type: text/html\n\n"
print "<html>"
print "<head>"
print "<title>Demo static HTML page</title>"
print "</head>"
print "<body>"
print "<h1>Hello COMP249!</h1>"
print "</body>"
print "</html>"

http://platypus.ics.mq.edu.au/~cassidy/cgi-bin/demo.py

The first print statement is the Content-type line that specifies the media type of the output that is generated. This line is actually part of the HTTP response header sent back to the client.

This line is immediately followed by a blank line which must not contain any spaces or tabs (the '\n\n' bit). Remember the HTTP protocol.

CGI programs often fail since the programmer forgot the blank line.

After the blank line comes the HTML encoded text which is displayed on the user's browser.

On Platypus

The OS (or web server) uses the hashbang line to find out how to run the program:
- Windows: #!C:/Python25/python
- Unix: #!/usr/local/bin/python
On Unix, the Python script needs to be made executable with chmod (or use WinSCP):
```
chmod 755 demo.py
# or equivalently
chmod a+rx demo.py
```
You can also test the script on the command line:
```
  > cd ~/html/cgi-bin
  cgi-bin> ./demo.py
	    
```

CGI Script Processing

Python Numbers

>>> 1+2

3



>>> #add comments
...

>>>7/2

3

>>>7./2

3.5

Python Strings

In Python, strings can be enclosed by single quotes or double quotes.

>>> 'a tring'
'a tring'
>>> "a string"
'a string'

>>> print 'This is', 'a test'
This is a test

Multi-line string can be written as

>>> str="line 1 \n\
line 2 \n\
line 3"
>>> print str
line 1
line 2
line 3
>>>

Alternatively, use triple-quotes """ or ''' to surround the string.

>>> str="""line 1
line 2
line 3"""
>>> print str
line 1
line 2
line 3
>>>

Python Lists

A Python list contains a set of values of compound data types.

>>> a=['hi', 10, 'hello', 200]
>>> a
['hi', 10, 'hello', 200]
>>> a[0]
'hi'
>>> a[1]
10
>>> a[:2]
['hi', 10]
>>> a[2:]
['hello', 200]
>>> a. append(300)
>>> a
['hi', 10, 'hello', 200, 300]
>>> a[4]=3
>>> a
['hi', 10, 'hello', 200, 3]
>>>a.remove(3)
>>> a
['hi', 10, 'hello', 200]
>>> 200 in a
True
>>> a.index(200)
3

Python Tuples

A Python tuple contains a set of values of compund data types.

>>> a='hi', 10, 'hello', 200
>>> a
('hi', 10, 'hello', 200)
>>> a[0]
'hi'
>>> a[1]
10
>>> a[:2]
('hi', 10)
>>> a[2:]
('hello', 200)
>>> len(a)
4
>>> 200 in a
True

>>> print "Hello %s! How do you like %s?" % ('COMP249 students', "today's topic") # %s is the placeholder

# what is the output?

Python Dictionaries

Dictionaries are indexed by keys, which can be any immutable type, e.g. string or numbers.
Keys are unique.
A dictionary is an unordered set of key:value pairs.

>>> dic={'hi': 101, 'hello': 200}
>>> dic
{'hi': 101, 'hello': 200}
>>> dic['hi']
101
>>> dic.keys()
['hi', 'hello']
>>> dic.has_key('hello')
Ture
>>> 'hello' in dic
True
>>> 200 in dic
False
>>> for k, v in dic.iteritems():
     print k, v
...
hi 101
hello 200


>>> for x in dic:
      print x, dic[x]
...

hi 101
hello 200

if statements

x = int(raw_input("Enter an integer: "))

if  x < 0:
  print "negative"
elif x == 0:
  print "zero"
else:
  print "positive"

for statements

l = ['COMP125', 'COMP249', 'COMP344']
for x in l:

  print x, l.index(x)



output:

COMP125 0

COMP249 1
COMP344 2

l = ['COMP125', 'COMP249', 'COMP344']
for i in range(len(l)):

  print i,  l[i]



output:
0 COMP125
1 COMP249
2 COMP344



>>>range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>range(2:5)
[2, 3, 4]

functions

def fib1(n):
  a, b = 0, 1
  while b < n:
    print b
    a, b = b, a+b


>>>fib1(10)
1 
1
2
3
5
8


or




def fib2(n):
  result = []
  a, b = 0, 1
  while b < n:
    result.append(b)
    a, b = b, a+b
  return result

>> l = fib2(10)
>>> l
[1, 1, 2, 3, 5, 8]

Generating a Table Dynamically

#!/usr/local/bin/python

print "Content-type: text/html\n\n"

print """
<html>  
        <head><title>Table Demo</title></head>  
        <body>    
        <h3>COMP249 Staff</h3>    
          <table> 
       <tr><th>Lecturer</th><th>Unit</th></tr>
"""
dict = {'Yan':'COMP249','Steve':'COMP249'}
for key, value in dict.items():
    print "    <tr><td>", key, "</td><td>", value, "</td></tr>"
print """    
    </table>  
  </body>
</html>
"""

The HTML Form Code

<html>
  <head><title>A simple HTML Form Page</title><head>
  <body>
     <h3> A simple HTML Form Page</h3>
     <hr>
     <form action="cgi-bin/process.py">
       <p><strong>Enter your name:</strong></p>
       <p><input type=text name=user></p>
       <p><input type=submit></p>
     </form>
  <body>
<html>

The Query String Format

The CGI script receives the form data via the query string.
In the previous example:
```
    user=Rona+Bates
```
Spaces are replaced with + or %20
Other special characters are scaped with % plus the ASCII code
Information from different form widgets is separated with &
```
    user=Rona+Bates&gender=female
```

How is the Query String transmitted?

If a GET method:
- The query string is encoded in the URL:
```
    http://.../cgi-bin/process.py?user=Rona+Bates
```
- The server places the information in the environment variable QUERY_STRING.
If a POST method:
- The query string is the HTTP request payload.
- The server sends the string to the CGI script on standard input
- The length of the query is passed through the environment variable CONTENT_LENGTH

GET vs. POST

GET encodes the query string in the URL

Easy to see what's being passed to the script
Can bookmark/link to a form submission, eg. a Google search
URLs can get long and ugly. Can hit length limits.
Form data shows up in server logs, browser history. (eg giggle)
But, remember GET is intended to be idempotent

GET vs. POST

POST encodes query as a message in the HTTP request.
- No information in the URL about the form data.
- Data is not hidden, but not stored in server logs.
- Allows larger payloads to be sent (eg. file upload).

A Simple CGI Script

#!/usr/local/bin/python

import cgi                            # imports cgi module
import cgitb; cgitb.enable()          # traceback manager, displays
                                      # errors in the Web browser

form = cgi.FieldStorage()             # retrieves form input
print "Content_type: text/html\n\n"   # output MIME type

html = """
<html>
  <head><title>Greetings</title></head>
  <body>
    <h3>Greetings</h3>
    <p>%s</p>    
  </body>
</html>
"""

A Simple CGI Script

if not form.has_key('user'):
    print html % "Who are you?" 
else:
    print html % ("Hello, %s." % form.getvalue('user'))

Examples: client_py.html process.py

The object form contains form input values, the getvalue method gets one of the values.
The FieldStorage handles either POST or GET data transparently
The string expressions (%) are used to insert the input text into a reply string, and the reply string into the triple-quoted HTML string block.

Environment Variables

Much of the additional information needed by CGI programs is available via HTTP environment variables.
- How is this information transmitted?
For example:
- SERVER_PORT
- SERVER_NAME
- SERVER_SOFTWARE
- QUERY_STRING
- REMOTE_ADDR
- REMOTE_HOST

Environment Variables in Python

In Python, HTTP environment variables are available (when set) via the os.environ dictionary.

#!/usr/local/bin/python

import os

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

if os.environ.has_key('SERVER_PORT'):
    server_port = os.environ['SERVER_PORT']
    print '<p> SERVER_PORT:', server_port, '</p>'
else:
    print '<p> SERVER_PORT: unknown </p>'

Query String

The QUERY_STRING variable always appears in the os.environ dictionary, even though its value is '' (the empty string).

#!/usr/local/bin/python

import os

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

if os.environ.has_key('QUERY_STRING') and  \               
           os.environ['QUERY_STRING'] != '':
     query_string = os.environ['QUERY_STRING']
     print '<h3> QUERY_STRING:', query_string, '</h3>'
else:
     print '<h3> QUERY_STRING: unknown </h3>'

Examples: name=Steve, name=Steve&age=21

COMP249: Week 5

COMP249: Week 5

Steve Cassidy and Yan Wang

Server Side Scripting

The Common Gateway Interface

CGI Programs

Location of CGI Programs

Server Processing

Example: Target Output

CGI Script in Python

On Platypus

CGI Script Processing

Python Numbers

Python Strings

Python Lists

Python Tuples

Python Dictionaries

if statements

for statements

functions

Generating a Table Dynamically

The HTML Form Code

The Query String Format

How is the Query String transmitted?

GET vs. POST

GET vs. POST

A Simple CGI Script

A Simple CGI Script

Environment Variables

Environment Variables in Python

Query String

Debugging a Script