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Transcript
Python
Web Applications
A KISS Introduction
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!
Fetching Pages
Use urllib or urllib2 library
Python is used frequently to parse data pages – regular
expressions are very important
Nothing new here
>>> from urllib2 import *
>>> file = urlopen("http://www.mizzao.tk/~mao")
>>> file.read()
'<html>\n<head>\n<title>A Simple
Page</title>\n</head>\n<body>\nNothing to
see...\n</body>\n</html>‘
>>> file.read.split(‘\n’) (all the lines at once)
>>> file.readline()
(one line at a time)
Parsing Data
Say there is a webpage you want to pull certain
information from every day…use Python!
You already know how to parse webpages though
– and that isn’t really a web application
Lots of complex parsers that are beyond the
scope of this demo
On to the interesting stuff!
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 Microsoft’s SQL is
different from everyone else’s
Databases
mySQL is the most widely used opensource 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
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
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
>>> from _mysql import *
>>> db = connect(host="localhost",user="mao",passwd="")
>>> db.select_db("test")
Some Python Database Commands
Command
What it does
_mysql.connect(params)
Returns a new object that represents the database
conn.select_db(db)
Selects the schema (database) to use after connecting
conn.query(string)
Queries the database with a SQL syntactic string
conn.store_result()
Returns all the results of the last query at once
conn.use_result()
Returns an iterator over the results of the last query
conn.fetch_row(maxrows)
Returns up to maxrows of the query
conn.info()
Returns some information about the last query
conn.close()
Closes the database connection
Some Simple Database Examples
>>> db.query("CREATE TABLE students (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, lname
TEXT, fname TEXT)")
>>> db.query("INSERT INTO students (lname, fname) VALUES ('Mao', 'Andrew')")
>>> db.query("INSERT INTO students (lname, fname) VALUES ('Walker', 'James')")
>>> db.query("INSERT INTO students (lname, fname) VALUES ('Cohen', 'Ian')")
>>> db.query("SELECT * FROM students")
>>> people = db.store_result()
>>> people.fetch_row()
(('1', 'Mao', 'Andrew'),)
>>> people.fetch_row()
(('2', 'Walker', 'James'),)
>>> people.fetch_row()
(('3', 'Cohen', 'Ian'),)
>>> people.fetch_row()
()
>>> db.close()
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.
It’s how you submit your CSE 121 homework
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
…
<Directory />
Options ExecCGI
</Directory>
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"
Lo and Behold…
Notice that
the first line
went
through as
HTTP
headers.
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.
A Final Simple Program
#!/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()
cgitb is a library that dumps
errors to the output – you
don’t 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.
HTML Form Interface
<html>
<head>
<title>Student Search</title>
</head>
<body>
<form action = "cgi-bin/search.py" method = 'GET'>
First Name:<input type = 'text' name = 'fname'>
<br>
Last Name:<input type = 'text' name = 'lname'>
<br>
<input type = 'submit' value = 'Find this Person'>
</body>
</html
Questions?
