Python

1
Python

• Web Applications
• A KISS Introduction

2
Web Applications with Python

• Fetching, parsing, text processing
• Database client mySQL, etc., for building
  dynamic information on-the-fly
• Python CGI web pages or even web servers!

3
Fetching Pages

• Use urllib or urllib2 library
• Python is used frequently to parse data pages
  regular expressions are very important
• Nothing new here

gtgtgt from urllib2 import gtgtgt file
urlopen("http//www.mizzao.tk/mao") gtgtgt
file.read() 'lthtmlgt\nltheadgt\nlttitlegtA Simple
Pagelt/titlegt\nlt/headgt\nltbodygt\nNothing to
see...\nlt/bodygt\nlt/htmlgt gtgtgt file.read.split(\n
) (all the lines at once) gtgtgt file.readline() (on
e line at a time)
4
Parsing Data

• Say there is a webpage you want to pull certain
  information from every dayuse Python!
• You already know how to parse webpages though
  and that isnt really a web application
• Lots of complex parsers that are beyond the scope
  of this demo
• On to the interesting stuff!

5
Databases

• A database is a program that manages a set of
  data (gee, who would have guessed)
• Most databases are relational SQL databases
• SQL Structured Query Language, a standardized
  syntax for telling a database what to do
  although Microsofts SQL is different from
  everyone elses

6
Databases

• mySQL is the most widely used open-source
  database Python supports it!
• Uses library MySQLdb a mySQL API for Python
• Must install this yourself on Linux with a
  package manager or by tarball, on Windows with an
  archive

7
How SQL Databases Work

• Databases take some sort of connection as
  communication usually through TCP/IP
• Most databases are set up as a group of schemas,
  each containing a set of tables
• Each table contains a set of columns, each with a
  different data type
• Rows are the entries in a table

8
MySQL Queries

• The library is called _mysql, and it implements
  the standard Python Database API Specification
• Almost all database modules for python implement
  the API, which defines the set of functions
• connect() takes parameters to establish a
  connection, and select_db() chooses the schema to
  work with

gtgtgt from _mysql import gtgtgt db
connect(host"localhost",user"mao",passwd"") gtgtgt
db.select_db("test")
9
Some Python Database Commands
10
Some Simple Database Examples

• gtgtgt db.query("CREATE TABLE students (id INT NOT
  NULL AUTO_INCREMENT PRIMARY KEY, lname TEXT,
  fname TEXT)")
• gtgtgt db.query("INSERT INTO students (lname, fname)
  VALUES ('Mao', 'Andrew')")
• gtgtgt db.query("INSERT INTO students (lname, fname)
  VALUES ('Walker', 'James')")
• gtgtgt db.query("INSERT INTO students (lname, fname)
  VALUES ('Cohen', 'Ian')")
• gtgtgt db.query("SELECT FROM students")
• gtgtgt people db.store_result()
• gtgtgt people.fetch_row()
• (('1', 'Mao', 'Andrew'),)
• gtgtgt people.fetch_row()
• (('2', 'Walker', 'James'),)
• gtgtgt people.fetch_row()
• (('3', 'Cohen', 'Ian'),)
• gtgtgt people.fetch_row()
• ()
• gtgtgt db.close()

11
What Do We Do Now?

• So we have access to a database and we can get
  data from it but how do we get that across the
  web?
• Answer CGI!
• CGI stands for Common Gateway Interface Allows
  any script (perl, bash, python) to output to
  HTTP.
• Its how you submit your CSE 121 homework

12
Using Python through CGI

• First, you must set up the web server to use
  handle Python scripts with CGI
• With Apache, this is easy 2 lines in httpd.conf

AddHandler cgi-script .cgi .py ltDirectory
/gt Options ExecCGI lt/Directorygt
13
A Web Python Hello World!

• When we run a Python program as a CGI script, the
  output goes out directly and the file must be
  executable (x).
• The first line we have to print are the HTTP
  headers this tells the browser to read it as
  either text, or render it as HTML.
• Any line after that comprises the actual HTML or
  text itself

!/usr/bin/python Required header that tells
the browser how to render the text. print
"Content-Type text/plain\n\n" Print a simple
message to the display window. print "Hello,
World!\n"
14
Lo and Behold

• Notice that the first line went through as HTTP
  headers.

15
Tying It All Together

• We want to be able to receive some input to our
  python program, and then output something based
  on that input.
• The python cgi library can find all the HTTP POST
  and GET variables.
• We can then use these to query the database, etc.

16
A Final Simple Program

• cgitb is a library that dumps errors to the
  output you dont have to go searching for it in
  your log file.
• cgi.FieldStorage() is a pseudo-dictionary
  containing all the POST and GET variables and
  their values.

!/usr/bin/python import cgi,_mysql,cgitb
cgitb.enable() search cgi.FieldStorage()
returns a dictionary print "Content-Type
text/plain\n\n" if not (search.has_key("fname")
and search.has_key("lname")) print "Need a
first and last name!" exit fname,lname
search"fname".value,search"lname".value conn
_mysql.connect(host"localhost",user"mao") conn
.select_db("test") conn.query("SELECT FROM
students WHERE lname '"lname"' AND fname
'"fname"'") people conn.store_result() row
people.fetch_row() if row () print
fname,lname,"is not a student.\n" else print
fname,lname,"is a student.\n" conn.close()
17
HTML Form Interface
lthtmlgt ltheadgt lttitlegtStudent Searchlt/titlegt lt/head
gt ltbodygt ltform action "cgi-bin/search.py"
method 'GET'gt First Nameltinput type 'text'
name 'fname'gt ltbrgt Last Nameltinput type
'text' name 'lname'gt ltbrgt ltinput type
'submit' value 'Find this Person'gt lt/bodygt lt/htm
l
18
Questions?