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Python
Another CGI
Strings
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Single and double quotes interchangeable
Triple quotes (""") allow multi-line literal quoting
Escaping of \n, %, etc by default
Prefix r can be used for regexps, r'[a-z].*'
• Any variable can be converted to a string with
str(v). Can go back with int(s), float(s)
• Strings operations:
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Concatenation with +, repeating with *
Length with len()
Substrings with []. Interesting index options.
Special boolean operator in
String Methods
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count
find
isdigit
upper, lower
rjust, ljust
strip
replace
Tuples and Lists
• Tuples store a set of elements. Syntax:
– foo = ("Korn", "Jeff")
• Lists also store elements but unlike tuples
can be modified (thus cheaper). Syntax:
– foo = ["Korn", "Jeff"]
– foo[1] = "Jeffrey"
• List operations:
– append, extend, +, *, len, index, in, pop, sort,
reverse, join, etc.
• Be careful about references!
Dictionary
• Dictionaries are associative arrays (arrays
indexed by string or number). Values can
be any object.
• Syntax:
– student = {'name' : 'Guido', 'score': 95 }
• Operations:
– has_key, keys, values, len, get, copy, update
• Example:
print student['name']
Statements
• Standard if, then, else
if person == 'Korn':
status = 'teacher'
elif person == 'Cano'
status = 'yankee'
else:
status = 'unknown'
print person, status
• While loops
while n <= 20:
s.append(' ')
n += 2
if magic:
break
a, b = 0, 1
while b < 100:
print b
a, b = b, a + b
for loops
• Similar to shell:
for val in ["foo", "bar", "baz"]:
print "Hello %s" % val
• To do c-style loops, use special function
range:
for val in range(5, 20, 2):
print val
# prints 5, 7, 9 ... 19
• Also: xrange
Exceptions
• Run-time errors are possible with Python,
but they can be caught with try-except:
try:
n = float(inputstring)
m = n * 2
except ValueError, msg:
print msg
# will print: invalid literal for float(): foo
• You can also use: raise, finally
Functions
• Defined with def:
def my_func(foo, bar):
print foo + bar
• Types not needed
• Arguments can have defaults:
def my_func(foo, bar=12):
print foo + bar
my_func(3)
15
my_func(3, bar=3)
• Variables in functions are local
– global changes this
• return used to return values
Modules
• Python modules are libraries of reusable
code with specific functionalities
• Standard modules are distributed with
Python to do a variety of things.
• Modules names are unique, but functions in
modules don't conflict with other modules.
• Modules can have sub-modules.
Using Modules
• Include modules in your program with use, e.g.
import math incorporates the math module
import math
print math.log(4906, 2)
12.0
print math.sqrt(100)
10
Important modules
• sys
– sys.argv, sys.path, sys.platform, sys.exit,
sys.stdin, sys.stderr, sys.stdout
• os
– os.getcwd, os.environ, os.chdir, os.listdir,
os.mkdir, os.rmdir, os.remove, os.system,
os.popen, os.getpid
• os.path
– os.path.abspath, os.path.dirname,
os.path.basename, os.path.join, os.path.split,
os.isfile, os.isdir
HTTP
• Protocol for the Web
• Client-server model
– Client (user agent) is typically web browser
(e.g. Firefox)
– Server is web server (e.g. Apache)
• Client request a resource identified by
URLs
– e.g. http://cs.nyu.edu/csweb/index.html
Apache HTTP Server
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Open source HTTP (Web) server
Most popular web server since 1996
The “A” in LAMP
Part of Apache Software Foundation
– Other projects: Ant, Hadoop, Tomcat,
SpamAssassin, …
HTTP Transactions
• HTTP request to web server
GET /v40images/nyu.gif HTTP/1.1
Host: www.nyu.edu
• HTTP response to web client
HTTP/1.1 200 OK
Content-type: image/gif
Content-length: 3210
HTML Example
<html>
<head>
<title>Some Document</title>
</head>
<body>
<h2>Some Topics</h2>
This is an HTML document
<p>
This is another paragraph
</body>
</html>
HTML
• File format that describes a webpage
• Can be written by hand, or generated by a
program
• A good way to generate a HTML file is by
writing a shell or Perl script
Gateways
• Interface between resource and a web server
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
HTTP
Web Server
Gateway
resource
CGI
• Common Gateway Interface - a standard
interface for running helper applications to
generate dynamic contents
– Specify the encoding of data passed to programs
• Allow HTML documents to be created on the fly
• Transparent to clients
– Client sends regular HTTP request
– Web server receives HTTP request, runs CGI program,
and sends contents back in HTTP responses
• CGI programs can be written in any language
How CGI Works
HTTP request
Web Server
HTTP response
spawn process
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Document
Script
Forms
• HTML forms are used to collect user input
• Data sent via HTTP request
• Server launches CGI script to process data
<form method=POST
action=“http://cs.nyu.edu/~unixtool/cgi-bin/search.cgi”>
Enter your query: <input type=text name=Search>
<input type=submit>
</form>
Input Types
• Text Field
<input type=text name=zipcode>
• Radio Buttons
<input type=radio name=size value=“S”> Small
<input type=radio name=size value=“M”> Medium
<input type=radio name=size value=“L”> Large
• Checkboxes
<input type=checkbox name=extras value=“lettuce”> Lettuce
<input type=checkbox name=extras value=“tomato”> Tomato
• Text Area
<textarea name=address cols=50 rows=4>
…
</textarea>
Submit Button
• Submits the form for processing by the CGI
script specified in the form tag
<input type=submit value=“Submit Order”>
HTTP Methods
• Determine how form data are sent to web
server
• Two methods:
– GET
• Form variables stored in URL
– POST
• Form variables sent as content of HTTP request
Encoding Form Values
• Browser sends form variable as name-value pairs
– name1=value1&name2=value2&name3=value3
• Names are defined in form elements
– <input type=text name=ssn maxlength=9>
• Special characters are replaced with %## (2-digit
hex number), spaces replaced with +
– e.g. “10/20 Wed” is encoded as “10%2F20+Wed”
GET/POST examples
GET:
GET /cgi-bin/myscript.pl?name=Bill%20Gates&
company=Microsoft HTTP/1.1
HOST: www.cs.nyu.edu
POST:
POST /cgi-bin/myscript.pl HTTP/1.1
HOST: www.cs.nyu.edu
…other headers…
name=Bill%20Gates&company=Microsoft
GET or POST?
• GET method for
– Retrieving information, e.g. from a database
– A “safe” method - no action taken other than retrieval
– Embedding data in URL without form element
• POST method for
– Forms with many fields or long fields
– Sending data for updating database, posting to bulletin
board, etc.
• GET requests may be cached by clients browsers
or proxies, but not POST requests
Parsing Form Input
• Method stored in HTTP_METHOD
• GET: Data encoded into
QUERY_STRING
• POST: Data in standard input (from body
of request)
• Most scripts parse input into an associative
array
– Parse it yourself, or
– Use available libraries (e.g. Perl CGI module)
CGI Environment Variables
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DOCUMENT_ROOT
HTTP_HOST
HTTP_REFERER
HTTP_USER_AGENT
HTTP_COOKIE
REMOTE_ADDR
REMOTE_HOST
REMOTE_USER
REQUEST_METHOD
SERVER_NAME
SERVER_PORT
Example: Comment Form
Part 1: HTML Form
<html>
<center>
<H1>Anonymous Comment Submission</H1>
</center>
Please enter your comment below which will
be sent anonymously to <tt>[email protected]</tt>.
If you want to be extra cautious, access this
page through <a
href="http://www.anonymizer.com">Anonymizer</a>.
<p>
<form action=cgi-bin/comment.cgi method=post>
<textarea name=comment rows=20 cols=80>
</textarea>
<input type=submit value="Submit Comment">
</form>
</html>
Part 2: CGI Script (ksh)
#!/home/unixtool/bin/ksh
. cgi-lib.ksh
ReadParse
PrintHeader
# Read special functions to help parse
print -r -- "${Cgi.comment}" | /bin/mailx -s "COMMENT" kornj
print
print
print
print
"<H2>You submitted the comment</H2>"
"<pre>"
-r -- "${Cgi.comment}"
"</pre>"
Example: Find words in Dictionary
<form action=dict.cgi>
Regular expression: <input type=“text”
name=re value=".*">
<input type=submit>
</form>
Example: CGI Script (ksh)
#!/home/unixtool/bin/ksh
PATH=$PATH:.
. cgi-lib.ksh
ReadParse
PrintHeader
print "<H1> Words matching <tt>${Cgi.re}</tt> in the dictionary
</H1>\n";
print "<OL>"
grep "${Cgi.re}" /usr/dict/words | while read word
do
print "<LI> $word"
done
print "</OL>"
Debugging
• Debugging can be tricky, since error
messages don't always print well as HTML
• One method: run interactively
$ QUERY_STRING='birthday=10/15/03'
$ ./birthday.cgi
Content-type: text/html
<html>
Your birthday is <tt>10/15/02</tt>.
</html>
A Python CGI Script
#!/usr/bin/python
import cgi
import cgitb
cgitb.enable()
form = cgi.FieldStorage()
bday = form['birthday'].value
# Print header
print 'Content-Type: text/html'
print
# Your HTML body
print "Your birthday is %s.\n" % bday
Debugging Python CGI Scripts
• Debugging CGI script is tricky - error messages
don’t always come up on your browser
• Run script with test data
$ python cgiScript prod=“MacBook” price=“1800”
Content-Type: text/html
<html>
…
</html>
CGI Benefits
• Simple
• Language independent
• UNIX tools are good for this because
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Work well with text
Integrate programs well
Easy to prototype
No compilation (CGI scripts)