About Python

November 18, 2006 About Python <a href="http://25f6akwb3kqffkcmzlnrxm9u9u.hop.clickbank.net/" style="font-family:arial;font-size:14px;text-decoration:underline;color:#006699;" target="_top">Tru-Guru Home Biz Mastery - High Conversion.</a> Python is an interpreted, general-purpose high-level programming language whose design philosophy emphasizes code readability. Python aims to combine "remarkable power with very clear syntax", and its standard library is large and comprehensive. Its use of indentation for block delimiters is unusual among popular programming languages. Python supports multiple programming paradigms, primarily but not limited to object oriented, imperative and, to a lesser extent, functional programming styles. It features a fully dynamic type system and automatic memory management, similar to that of Scheme, Ruby, Perl, and Tcl. Like other dynamic languages, Python is often used as a scripting language, but is also used in a wide range of non-scripting contexts. The reference implementation of Python (CPython) is free and open source software and has a community-based development model, as do all or nearly all of its alternative implementations. CPython is managed by the non-profit Python Software Foundation. Python interpreters are available for many operating systems, and Python programs can be compiled into stand-alone executable code for those systems, using tools included with the interpreter installation package. <DIV class=content> This tutorial covers two web programming models that can be used with Python: the CGI standard and mod_python. As a general rule if you are in a shared host environment then your only option will be to run python scripts as CGI. Some specialized hosting providers will let you use and configure mod_python. Mod_python's performance and flexibility is much superior to CGI. There is no need to read the mod_python section if you want CGI and vice versa. They are not related to each other. To follow this tutorial it is required that you have a basic Python programming knowledge. If you need to learn how to program visit the <A href="http://programming-crash-course.com">Python Tutorial</A> first. Please post any doubt, suggestion or critique at the <A href="http://codepoint.net/index.php/board,7.0.html" target=pcc>Web Python Board</A> and help making this a better tutorial. November 18, 2006 CGI Some host providers only let you run CGI scripts in a certain directory, often named cgi-bin. In this case all you have to do to run the script is to call it like this: http://my_server.tld/cgi-bin/my_script.py The script will have to be made executable by "others". Give it a 755 permission or check the executable boxes if there is a graphical FTP interface. Some hosts let you run CGI scripts in any directory. In some of these hosts you don't have to do anything to configure the directories. In others you will have to add these lines to a file named .htaccess in the directory you want to run CGI scripts from:<PRE class=program>Options +ExecCGI AddHandler cgi-script .py</PRE> If the file does not exist create it. All directories below a directory with a .htaccess file will inherit the configurations. So if you want to be able to run CGI scripts from all directories create this file in the document root. If you are using your own server then probably you won't need to do anything to run a CGI script at the cgi-bin directory. Just make sure there is a line like the next in httpd.conf and that it is not commented. The trailing slashs are required.<PRE class=program>ScriptAlias /cgi-bin/ "/path/to/cgi-bin/directory/"</PRE> If you are using the line above and want html files to be handled correctly in the cgi-bin directory add the next to httpd.conf. No trailing slash.<PRE class=program><Directory /path/to/cgi-bin/directory> AddHandler default-handler .html .htm </Directory></PRE> To run a script saved at the root: http://my_server.tld/my_script.py If it was saved in some directory: http://my_server.tld/some_dir/some_subdir/my_script.py If your desktop is the server then execute it like this: http://localhost/cgi-bin/my_script.py In Windows, sometimes Apache will listen on port 8080. In this case the above address will be written with the port: http://localhost:8080/cgi-bin/my_script.py Make sure all text files you upload to the server are uploaded as text (not binary), specially if you are in Windows, otherwise you will have problems. November 18, 2006 Hello World This is the classical "Hello World" in python CGI fashion:<PRE class=program>#!/usr/bin/env python print "Content-Type: text/html" print print """\ <html> <body> <h2>Hello World!</h2> </body> </html> """</PRE> 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:<PRE class=program>#!/usr/bin/python #!/usr/bin/python2.3 #!/usr/bin/python2.4 #!c:\Python24\python.exe #!c:\Python25\python.exe</PRE> The first 3 lines above are Linux paths and the last 2 are Windows paths. 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.<PRE class=program>print "Content-Type: text/html" print</PRE> Many times the blank line will be written as \n:<PRE class=program>print "Content-Type: text/html\n"</PRE> 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. <H3>Client versus Server</H3> 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. November 18, 2006 Debugging To catch syntax error messages run the script in a local shell before uploading to the server. Header errors are hard to catch unless you have access to the server logs. In case you have, look for error_log and access_log in Linux and for error.log and access.log in Windows. 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:<PRE class=program>#!/usr/bin/env python print "Content-Type: text/html" print import cgitb; cgitb.enable() print 1/0</PRE> The handler() method can be used to handle only the catched exceptions:<PRE class=program>#!/usr/bin/env python print "Content-Type: text/html" print import cgitb try: f = open('non-existent-file.txt', 'r') except: cgitb.handler()</PRE> There is also the option for a crude approach making the header "text/plain" and setting the standard error to standard out:<PRE class=program>#!/usr/bin/env python print "Content-Type: text/plain" print import sys sys.stderr = sys.stdout f = open('non-existent-file.txt', 'r')</PRE> Will output this:<PRE>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'</PRE> Warning: These techniques expose information that can be used by an attacker. Use it only while developing/debugging. Once in production disable them. November 18, 2006 Forms The FieldStorage class of the cgi module has all that is needed to handle submited forms.<PRE class=program>import cgi form = cgi.FieldStorage() # instantiate only once!</PRE> It is transparent to the programmer if the data was submited by GET or by POST. The interface is exactly the same. November 18, 2006 Unique field names Suppose we have this HTML form which submits a field named name to a python CGI script named process_form.py: <PRE class="program"><html><body> <form method="get" action="process_form.py"> Name: <input type="text" name="name"> <input type="submit" value="Submit"> </form> </body></html></PRE> This is the process_form.py script: <PRE class="program">#!/usr/bin/env python import cgi form = cgi.FieldStorage() # instantiate only once! name = form.getfirst(‘name‘, ‘empty‘) # Avoid script injection escaping the user input name = cgi.escape(name) print """\ Content-Type: text/html\n <html><body> The submitted name was "%s" </body></html> """ % name</PRE> The getfirst() method returns the first value of the named field or a default or None if no field with that name was submited or if it is empty. If there is more than one field with the same name only the first will be returned. If the HTML form method is changed from get to post the process_form.py script will be the same. If the user inputed data is to be shown in a HTML document then it is necessary to escape it from HTML tags or else everything inside < > will be interpreted by the HTML parser including javascript code like <script type="text/javascript"> malicious code here </script> The cgi.escape() method will transform the above into safe HTML text: &lt;script type="text/javascript"&gt; malicious code here &lt;/script&gt; This is useful not only to prevent script injection but also to make it possible to display HTML source code as has just been done above. November 18, 2006 Multiple field names If there is more than one field with the same name like in HTML input check boxes then the method to be used is getlist(). It will return a list containing as many items (the values) as checked boxes. If no check box was checked the list will be empty. Sample HTML with check boxes:<PRE class=program><html><body> <form method="post" action="process_check.py"> Red<input type="checkbox" name="color" value="red"> Green<input type="checkbox" name="color" value="green"> <input type="submit" value="Submit"> </form> </body></html></PRE> And the corresponding process_check.py script:<PRE class=program>#!/usr/bin/env python import cgi form = cgi.FieldStorage() # getlist() returns a list containing the # values of the fields with the given name colors = form.getlist(‘color‘) print "Content-Type: text/html\n" print ‘<html><body>‘ print ‘The colors list:‘, colors for color in colors: print ‘‘, cgi.escape(color), ‘‘ print ‘</body></html>‘</PRE> November 18, 2006 File Upload The python scripts in this page and in the next one will try to save an uploaded file in a directory named files in the directory where it is running. If the directory where the script is running is /path/to/dir then the /path/to/dir/files directory must exist. If it does not it will fail. To upload a file the HTML form must have the enctype attribute set to multipart/form-data. The input tag with the file type will create a "Browse" button.<PRE class=program><html><body> <form enctype="multipart/form-data" action="save_file.py" method="post"> <p>File: <input type="file" name="file"></p> <p><input type="submit" value="Upload"></p> </form> </body></html></PRE> The getfirst() and getlist() methods will only return the file(s) content. To also get the filename it is necessary to access a nested FieldStorage instance by its index in the top FieldStorage instance.<PRE class=program>#!/usr/bin/env python import cgi, os import cgitb; cgitb.enable() try: # Windows needs stdio set for binary mode. import msvcrt msvcrt.setmode (0, os.O_BINARY) # stdin = 0 msvcrt.setmode (1, os.O_BINARY) # stdout = 1 except ImportError: pass form = cgi.FieldStorage() # A nested FieldStorage instance holds the file fileitem = form[‘file‘] # Test if the file was uploaded if fileitem.filename: # strip leading path from file name to avoid directory traversal attacks fn = os.path.basename(fileitem.filename) open(‘files/‘ + fn, ‘wb‘).write(fileitem.file.read()) message = ‘The file "‘ + fn + ‘" was uploaded successfully‘ else: message = ‘No file was uploaded‘ print """\ Content-Type: text/html\n <html><body> %s </body></html> """ % (message,)</PRE> A directory traversal attack is one where the attacker submits a file with a leading path like in ../../attacker_program. This way he can save a program wherever the Apache user has write permission. Or read a file if the target script reads files. November 18, 2006 Big File Upload To handle big files without using all the available memory a generator can be used. The generator will return the file in small chunks:<PRE class=program>#!/usr/bin/env python import cgi, os import cgitb; cgitb.enable() try: # Windows needs stdio set for binary mode. import msvcrt msvcrt.setmode (0, os.O_BINARY) # stdin = 0 msvcrt.setmode (1, os.O_BINARY) # stdout = 1 except ImportError: pass form = cgi.FieldStorage() # Generator to buffer file chunks def fbuffer(f, chunk_size=10000): while True: chunk = f.read(chunk_size) if not chunk: break yield chunk # A nested FieldStorage instance holds the file fileitem = form[‘file‘] # Test if the file was uploaded if fileitem.filename: # strip leading path from file name to avoid directory traversal attacks fn = os.path.basename(fileitem.filename) f = open(‘files/‘ + fn, ‘wb‘, 10000) # Read the file in chunks for chunk in fbuffer(fileitem.file): f.write(chunk) f.close() message = ‘The file "‘ + fn + ‘" was uploaded successfully‘ else: message = ‘No file was uploaded‘ print """\ Content-Type: text/html\n <html><body> %s </body></html> """ % (message,)</PRE> November 18, 2006 Shell Commands It is possible to execute shell commands through CGI. The subprocess.Popen class is what is necessary. This module is new in python 2.4. os.popen4 can also be used if a hosting provider does not offer 2.4. The script in this page is for educational purposes only. Read the <A href="#warning">warning</A>. If you need a CGI Shell use the one on the next page. This handy getshellcmd.py script gets anything in its query string and execute it as a shell command. It works for Unix with a bash shell only.<PRE class=fixed>#!/usr/bin/python2.4 import cgitb; cgitb.enable() # The subprocess module is new in 2.4 import os, urllib, subprocess as sub # Retrieve the command from the query string # and unencode the escaped %xx chars str_command = urllib.unquote(os.environ[‘QUERY_STRING‘]) p = sub.Popen([‘/bin/bash‘, ‘-c‘, str_command], stdout=sub.PIPE, stderr=sub.STDOUT) output = urllib.unquote(p.stdout.read()) print """\ Content-Type: text/html\n <html><body> <pre> $ %s %s </pre> </body></html> """ % (str_command, output)</PRE> Say you want to install Django in your site. Without this script you would have to download it to your local host, decompress it, and upload the uncompressed files by FTP. With CGI you download it using curl or wget directly to a directory in your site‘s hierarchy like a tmp directory: http://my_site.tld/getshellcmd.py?curl -o tmp/Django-0.95.tar.gz http://media.djangoproject.com/releases/0.95/Django-0.95.tar.gz The above is one only line. And the output in the browser:<PRE style="FONT-SIZE: 10px">$ curl -o tmp/Django-0.95.tar.gz http://media.djangoproject.com/releases/0.95/Django-0.95.tar.gz % Total % Received % Xferd Average Speed Time Curr. Dload Upload Total Current Left Speed 0 1257k 0 2479 0 0 7042 0 0:03:02 0:00:00 0:03:02 7042 4 1257k 4 62727 0 0 98k 0 0:00:12 0:00:00 0:00:12 217k 32 1257k 32 404k 0 0 241k 0 0:00:05 0:00:01 0:00:03 303k 49 1257k 49 623k 0 0 235k 0 0:00:05 0:00:02 0:00:02 270k 78 1257k 78 983k 0 0 271k 0 0:00:04 0:00:03 0:00:01 299k 100 1257k 100 1257k 0 0 309k 0 0:00:04 0:00:04 0:00:00 338k</PRE> Four seconds to download 1,257k in my host provider. Now to untar it issue the tar command as the query string: http://my_site.tld/getshellcmd.py?tar -xzvf tmp/Django-0.95.tar.gz Depending on the host absolute directory paths should be declared. Warning: If you ever use this sample code save it with another name and chmod it to 600 immediately after its use. Otherwise any one in the whole world will be able to execute whatever he wants in your host. November 18, 2006 Safer CGI Shell This is a CGI Shell for Unix servers with a bash shell. It has authentication and state management. It needs the <A href="http://webpython.codepoint.net/a_session_class">session module</A>. The latest version can be <A href="http://cgpy-shell.googlecode.com/svn/trunk/">downloaded</A> from the <A href="http://code.google.com/p/cgpy-shell/">project‘s site</A>. Get help in <A href="http://codepoint.net/index.php/topic,33.0.html">this topic</A>.<PRE class=program>#!/usr/bin/python2.4 import cgitb; cgitb.enable() import sys, os sys.path.append(os.path.dirname(os.environ[‘SCRIPT_FILENAME‘])) import session, time, cgi, urllib, subprocess as sub def login(form, sess): # You absolutely need to change these !!! passwords = {‘me‘:‘mine‘, ‘friend‘:‘his‘} username = form.getfirst(‘username‘, ‘‘) password = form.getfirst(‘password‘, ‘‘) if username and passwords.get(username) == password: sess.set_expires(365 * 24 * 60 * 60) sess.data[‘logged‘] = True sess.data[‘logged_until‘] = time.time() + 10 * 60 sess.data[‘empty_cmd‘] = False return True if not username or not password: message = ‘Enter user name and password to log into CGI Shell‘ elif username not in passwords or passwords.get(username) != password: message = ‘Wrong username or password‘ onload = """var el = document.getElementById(‘un‘); el.focus(); el.select()""" header(sess, onload) print """\ <div style="border:3px solid gray;width:350px;height:130px; margin:auto;padding:20px;" class="shell"> %s <form method="post" action="./cgi-shell.py"> Username: <input type="text" name="username" id="un" value="%s" class="shell" style="border:1px lime solid;padding:2px 4px;" /> Password: <input type="password" name="password" class="shell" style="border:1px lime solid;padding:2px 4px;" /> <p style="text-align:center;"><input type="submit" value="Login" class="button" /> </form> </div> """ % (message, username) return False def end(sess): print ‘</body>\n</html>‘ sess.close() sys.exit(0) def shell(form, sess): sess.data[‘logged_until‘] = time.time() + 10 * 60 sess.data.setdefault(‘output‘, []) cmd = form.getfirst(‘cmd‘, ‘‘) cur_dir = sess.data.setdefault(‘pwd‘, os.environ[‘DOCUMENT_ROOT‘]) if cmd or cmd == ‘‘ and sess.data[‘empty_cmd‘]: sess.data[‘cmd‘] = cmd cmd = urllib.unquote(cmd) p = sub.Popen([‘/bin/bash‘, ‘-c‘, cmd + ‘\necho $PWD‘], stdout=sub.PIPE, stderr=sub.STDOUT, cwd=cur_dir) prompt = ‘[@‘ + os.environ[‘SERVER_NAME‘] + ‘ ‘ prompt += os.path.basename(cur_dir) + ‘]$ ‘ output = [prompt + cmd + ‘\n‘] + p.stdout.readlines() sess.data[‘pwd‘] = (output[-1:])[0].rstrip() sess.data[‘output‘] = (sess.data[‘output‘] + output)[-501:-1] else: cmd = urllib.unquote(sess.data.setdefault(‘cmd‘, ‘‘)) sess.data[‘empty_cmd‘] = True cur_dir = sess.data[‘pwd‘] output = cgi.escape(‘‘.join(sess.data[‘output‘]).rstrip(), True) onload = """window.location.href = ‘#last‘; var el = document.getElementById(‘cmd‘);el.focus();el.select();""" prompt = ‘[&#064;‘ + os.environ[‘SERVER_NAME‘] + ‘ ‘ prompt += os.path.basename(cur_dir) + ‘]$ ‘ header(sess, onload) print """\ <div style="width:98%%;margin:0;"> <pre style="border-bottom:0;margin:0;padding:6px;width:100%%; overflow:auto;height:430px;border:3px solid gray;border-bottom:0;" class="shell" id="output">%s<span id="last"></pre> <form method="post" action="#last"> <p style="margin:0;padding:6px 6px 8px;width:100%%;border:3px solid gray; border-top:0;" class="shell">%s <input type="text" name="cmd" value="%s" class="shell" id="cmd" style="width:55%%;border:1px lime solid;padding:2px 4px;" /> <input type="submit" value="Submit" name="submit" class="button" /> <input type="submit" value="Logout" name="submit" class="button" /> </form> </div> """ % (output, prompt, cgi.escape(cmd, True)) end(sess) def header(sess, onload=‘‘): print """\ %s Content-Type: text/html\n <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html>\n<head><title>CGI Shell</title> <style type="text/css"> .shell {font-size:12px;font-family:monospace;background-color:black;color:lime;} </style>\n</head> <body onload="%s" style="background-color:lightgray;font-family:monospace;"> <h2 style="text-align:center;margin:8px auto">CGI Shell</h2> """ % (sess.cookie, onload) def main(): sess = session.Session() form = cgi.FieldStorage() if form.getfirst(‘submit‘) == ‘Logout‘ or \ sess.data.get(‘logged_until‘, 0) < time.time(): sess.data[‘logged‘] = False if not sess.data.get(‘logged‘) and not login(form, sess): end(sess) shell(form, sess) main()</PRE> November 18, 2006 Cookies HTTP is said to be a stateless protocol. What this means for web programmers is that every time a user loads a page it is the first time for the server. The server can‘t say whether this user has ever visited that site, if is he in the middle of a buying transaction, if he has already authenticated, etc. A cookie is a tag that can be placed on the user‘s computer. Whenever the user loads a page from a site the site‘s script can send him a cookie. The cookie can contain anything the site needs to identify that user. Then within the next request the user does for a new page there goes back the cookie with all the pertinent information to be read by the script. November 18, 2006 Set the Cookie There are two basic cookie operations. The first is to set the cookie as an HTTP header to be sent to the client. The second is to read the cookie returned from the client also as an HTTP header. This script will do the first one placing a cookie on the client‘s browser:<PRE class=program>#!/usr/bin/env python import time # This is the message that contains the cookie # and will be sent in the HTTP header to the client print ‘Set-Cookie: lastvisit=‘ + str(time.time()); # To save one line of code # we replaced the print command with a ‘\n‘ print ‘Content-Type: text/html\n‘ # End of HTTP header print ‘<html><body>‘ print ‘Server time is‘, time.asctime(time.localtime()) print ‘</body></html>‘</PRE> The Set-Cookie header contains the cookie. Save and run this code from your browser and take a look at the cookie saved there. Search for the cookie name, lastvisit, or for the domain name, or the server IP like 10.1.1.1 or 127.0.0.1. <H3>The Cookie Object</H3> The Cookie module can save us a lot of coding and errors and the next pages will use it in all cookie operations.<PRE class=program>#!/usr/bin/env python import time, Cookie # Instantiate a SimpleCookie object cookie = Cookie.SimpleCookie() # The SimpleCookie instance is a mapping cookie[‘lastvisit‘] = str(time.time()) # Output the HTTP message containing the cookie print cookie print ‘Content-Type: text/html\n‘ print ‘<html><body>‘ print ‘Server time is‘, time.asctime(time.localtime()) print ‘</body></html>‘</PRE> It does not seem as much for this extremely simple code, but wait until it gets complex and the Cookie module will be your friend. November 18, 2006 Retrieve the Cookie The returned cookie will be available as a string in the os.environ dictionary with the key ‘HTTP_COOKIE‘:<PRE class=program>cookie_string = os.environ.get(‘HTTP_COOKIE‘)</PRE> The load() method of the SimpleCookie object will parse that string rebuilding the object‘s mapping:<PRE class=program> cookie.load(cookie_string)</PRE> Complete code:<PRE class=program>#!/usr/bin/env python import Cookie, os, time cookie = Cookie.SimpleCookie() cookie[‘lastvisit‘] = str(time.time()) print cookie print ‘Content-Type: text/html\n‘ print ‘<html><body>‘ print ‘Server time is‘, time.asctime(time.localtime()), ‘‘ # The returned cookie is available in the os.environ dictionary cookie_string = os.environ.get(‘HTTP_COOKIE‘) # The first time the page is run there will be no cookies if not cookie_string: print ‘First visit or cookies disabled‘ else: # Run the page twice to retrieve the cookie print ‘The returned cookie string was "‘ + cookie_string + ‘"‘ # load() parses the cookie string cookie.load(cookie_string) # Use the value attribute of the cookie to get it lastvisit = float(cookie[‘lastvisit‘].value) print ‘Your last visit was at‘, print time.asctime(time.localtime(lastvisit)), ‘‘ print ‘</body></html>‘</PRE> When the client first loads the page there will be no cookie in the client‘s computer to be returned. The second time the page is requested then the cookie saved in the last run will be sent to the server. November 18, 2006 Morsels In the previous cookie retrieve program the lastvisit cookie value was retrieved through its value attribute:<PRE class=program> lastvisit = float(cookie[‘lastvisit‘].value)</PRE> When a new key is set for a SimpleCookie object a Morsel instance is created:<PRE class=shell>>>> import Cookie >>> import time >>> >>> cookie = Cookie.SimpleCookie() >>> cookie <SimpleCookie: > >>> >>> cookie[‘lastvisit‘] = str(time.time()) >>> cookie[‘lastvisit‘] <Morsel: lastvisit=‘1159535133.33‘> >>> >>> cookie[‘lastvisit‘].value ‘1159535133.33‘ </PRE> Each cookie, a Morsel instance, can only have a predefined set of keys: expires, path, commnent, domain, max-age, secure and version. Any other key will raise an exception.<PRE class=program>#!/usr/bin/env python import Cookie, time cookie = Cookie.SimpleCookie() # name/value pair cookie[‘lastvisit‘] = str(time.time()) # expires in x seconds after the cookie is output. # the default is to expire when the browser is closed cookie[‘lastvisit‘][‘expires‘] = 30 * 24 * 60 * 60 # path in which the cookie is valid. # if set to ‘/‘ it will valid in the whole domain. # the default is the script‘s path. cookie[‘lastvisit‘][‘path‘] = ‘/cgi-bin/‘ # the purpose of the cookie to be inspected by the user cookie[‘lastvisit‘][‘comment‘] = ‘holds the last user\‘s visit date‘ # domain in which the cookie is valid. always stars with a dot. # to make it available in all subdomains # specify only the domain like .my_site.com cookie[‘lastvisit‘][‘domain‘] = ‘.www.my_site.com‘ # discard in x seconds after the cookie is output # not supported in most browsers cookie[‘lastvisit‘][‘max-age‘] = 30 * 24 * 60 * 60 # secure has no value. If set directs the user agent to use # only (unspecified) secure means to contact the origin # server whenever it sends back this cookie cookie[‘lastvisit‘][‘secure‘] = ‘‘ # a decimal integer, identifies to which version of # the state management specification the cookie conforms. cookie[‘lastvisit‘][‘version‘] = 1 print ‘Content-Type: text/html\n‘ print ‘‘, cookie, ‘‘ for morsel in cookie: print ‘‘, morsel, ‘=‘, cookie[morsel].value print ‘<div style="margin:-1em auto auto 3em;">‘ for key in cookie[morsel]: print key, ‘=‘, cookie[morsel][key], ‘<br />‘ print ‘</div>‘</PRE> Notice that print cookie automaticaly formats the expire date. November 18, 2006 Sessions Sessions are the server side version of cookies. While a cookie persists data (or state) at the client, sessions do it at the server. Sessions have the advantage that the data do not travel the network thus making it both safer and faster although this not entirely true as shown in the next paragraph The session state is kept in a file or in a database at the server side. Each session is identified by an id or session id (SID). To make it possible to the client to identify himself to the server the SID must be created by the server and sent to the client and then sent back to the server whenever the client makes a request. There is still data going through the net, the SID. The server can send the SID to the client in a link‘s query string or in a hidden form field or as a Set-Cookie header. The SID can be sent back from the client to the server as a query string parameter or in the body of the HTTP message if the post method is used or in a Cookie HTTP header. If a cookie is not used to store the SID then the session will only last until the browser is closed, or the user goes to another site breaking the POST or query string transmission, or in other words, the session will last only until the user leaves the site. November 18, 2006 Cookie Based SID A cookie based session has the advantage that it lasts until the cookie expires and, as only the SID travels the net, it is faster and safer. The disadvantage is that the client must have cookies enabled. The only particularity with the cookie used to set a session is its value:<PRE class=program># The sid will be a hash of the server time sid = sha.new(repr(time.time())).hexdigest()</PRE> The hash of the server time makes an unique SID for each session.<PRE class=program>#!/usr/bin/env python import sha, time, Cookie, os cookie = Cookie.SimpleCookie() string_cookie = os.environ.get(‘HTTP_COOKIE‘) # If new session if not string_cookie: # The sid will be a hash of the server time sid = sha.new(repr(time.time())).hexdigest() # Set the sid in the cookie cookie[‘sid‘] = sid # Will expire in a year cookie[‘sid‘][‘expires‘] = 12 * 30 * 24 * 60 * 60 # If already existent session else: cookie.load(string_cookie) sid = cookie[‘sid‘].value print cookie print ‘Content-Type: text/html\n‘ print ‘<html><body>‘ if string_cookie: print ‘Already existent session‘ else: print ‘New session‘ print ‘SID =‘, sid, ‘‘ print ‘</body></html>‘</PRE> In every page the existence of the cookie must be tested. If it does not exist then redirect to a login page or just create it if a login or a previous state is not required. November 18, 2006 Query String SID Query string based session:<PRE class=program>#!/usr/bin/env python import sha, time, cgi, os sid = cgi.FieldStorage().getfirst(‘sid‘) if sid: # If session exists message = ‘Already existent session‘ else: # New session # The sid will be a hash of the server time sid = sha.new(repr(time.time())).hexdigest() message = ‘New session‘ qs = ‘sid=‘ + sid print """\ Content-Type: text/html\n <html><body> %s SID = %s <a href="./set_sid_qs.py?sid=%s">reload</a> </body></html> """ % (message, sid, sid)</PRE> To mantain a session you will have to append the query string to all the links in the page. Save this file as set_sid_qs.py and run it two or more times. Try to close the browser and call the page again. The session is gone. The same happens if the page address is typed in the address bar. November 18, 2006 Hidden Field SID The hidden form field SID is almost the same as the query string based one, sharing the same problems.<PRE class=program>#!/usr/bin/env python import sha, time, cgi, os sid = cgi.FieldStorage().getfirst(‘sid‘) if sid: # If session exists message = ‘Already existent session‘ else: # New session # The sid will be a hash of the server time sid = sha.new(repr(time.time())).hexdigest() message = ‘New session‘ qs = ‘sid=‘ + sid print """\ Content-Type: text/html\n <html><body> %s SID = %s <form method="post"> <input type="hidden" name=sid value="%s"> <input type="submit" value="Submit"> </form> </body></html> """ % (message, sid, sid)</PRE> November 18, 2006 A Session Class Since session state maintenance happens in many pages a session class is in order. Save the next program as session.py and import it in all pages in which session state is required. The session object will try to create a directory named session in the document root if it doesn‘t exist. If it can‘t you will have to create it yourself and chmod it to 2770.<PRE class=program>import sha, shelve, time, Cookie, os class Session(object): def __init__(self, expires=None, cookie_path=None): string_cookie = os.environ.get(‘HTTP_COOKIE‘, ‘‘) self.cookie = Cookie.SimpleCookie() self.cookie.load(string_cookie) if self.cookie.get(‘sid‘): sid = self.cookie[‘sid‘].value # Clear session cookie from other cookies self.cookie.clear() else: self.cookie.clear() sid = sha.new(repr(time.time())).hexdigest() self.cookie[‘sid‘] = sid if cookie_path: self.cookie[‘sid‘][‘path‘] = cookie_path session_dir = os.environ[‘DOCUMENT_ROOT‘] + ‘/session‘ if not os.path.exists(session_dir): try: os.mkdir(session_dir, 02770) # If the apache user can‘t create it create it manualy except OSError, e: errmsg = """%s when trying to create the session directory. \ Create it as ‘%s‘""" % (e.strerror, os.path.abspath(session_dir)) raise OSError, errmsg self.data = shelve.open(session_dir + ‘/sess_‘ + sid, writeback=True) os.chmod(session_dir + ‘/sess_‘ + sid, 0660) # Initializes the expires data if not self.data.get(‘cookie‘): self.data[‘cookie‘] = {‘expires‘:‘‘} self.set_expires(expires) def close(self): self.data.close() def set_expires(self, expires=None): if expires == ‘‘: self.data[‘cookie‘][‘expires‘] = ‘‘ elif isinstance(expires, int): self.data[‘cookie‘][‘expires‘] = expires self.cookie[‘sid‘][‘expires‘] = self.data[‘cookie‘][‘expires‘]</PRE> An example of the session class usage:<PRE class=program>#!/usr/bin/env python import cgitb; cgitb.enable() # Some hosts will need to have document_root appended # to sys.path to be able to find user modules import sys, os sys.path.append(os.environ[‘DOCUMENT_ROOT‘]) import session, time sess = session.Session(expires=365*24*60*60, cookie_path=‘/‘) # expires can be reset at any moment: sess.set_expires(‘‘) # or changed: sess.set_expires(30*24*60*60) # Session data is a dictionary like object lastvisit = sess.data.get(‘lastvisit‘) if lastvisit: message = ‘Welcome back. Your last visit was at ‘ + \ time.asctime(time.gmtime(float(lastvisit))) else: message = ‘New session‘ # Save the current time in the session sess.data[‘lastvisit‘] = repr(time.time()) print """\ %s Content-Type: text/plain\n sess.cookie = %s sess.data = %s %s """ % (sess.cookie, sess.cookie, sess.data, message) sess.close() November 18, 2006 Mod_python Mod_python is an Apache module only. Mod_python version 3.x requires Apache 2.x and Python 2.2.1. For Apache 1.3 only mod_python 2.x can be used. This tutorial requires mod_python 3.1+ <H3>Much faster than CGI</H3> For every requested CGI page the Python interpreter must be launched. With mod_python Apache starts one main interpreter for the main server and one subinterpreter for each virtual host and these interpreters will be reused for all requests. They will never finish until the server is shut down. <H3>Mod_python handlers</H3> Apache processes requests in phases. A handler is a function that processes a particular phase of a request. Handlers are provided by Apache and by its modules, like mod_python. A number of handlers are available in mod_python. Most of them are low level replacements for Apache handlers but two, Publisher and PSP, are high level and will be covered here. The PSP handler processes text documents with Python code embedded between server tags and transform it in pure Python code, much like PHP and ASP. If you are used to these and don‘t wan‘t to learn a new web programming paradigm then PSP is your choice. The Publisher handler allows access to functions and variables within a module via URLs. It avoids the spagetthi coding style thus giving the code a much more application like appearance. November 18, 2006 Apache Configuration There are two options to configure the Publisher handler. <H3>SetHandler directive</H3> This will be used in this tutorial. If you want to follow it exactly use it yourserf and avoid plenty of confusion. In this configuration all files in a directory will be handled by the Publisher.<PRE class=program><Directory /path/to/publisher/directory> SetHandler mod_python PythonHandler mod_python.publisher PythonDebug On </Directory></PRE> The lines above must be included in the Apache httpd.conf file or in the .htaccess file. The <Directory> directive must not be used inside the .htacces file. Instead use only the three internal lines. The first line, SetHandler mod_python, tells Apache that all the files in that directory will be handled by mod_python, regardless of the file extension. The second line, PythonHandler mod_python.publisher, is a mod_python directive. The mod_python handler to be used here is the Publisher. The third line, PythonDebug On, sets mod_python to send error messages to the standard output, quite convenient for debugging. Should be deleted when in production. This configuration will be used in the next pages. It enables a traversal algorithm a bit more convenient than the configuration shown next. That is, it allows the index page of the site to be executed when no page is declared in the URL as in http://mysite.tld. Also there is no need to write the .py extension. If there is the need to serve static files from the same directory add this:<PRE class=program><Files ~ "\.(gif|html|jpg|png)$"> SetHandler default-handler </Files></PRE> <H3>AddHandler directive</H3> The second option to configure the Publisher handler is changing the SetHandler directive to:<PRE class=program>AddHandler mod_python .py</PRE> With this directive all other file types will be handled by their default handlers without the need to set them. But it will be necessary to use the .py extension in the URIs. November 18, 2006 Hello World If the next script is saved as hello.py in the /dir/path directory under the document root then we can execute it with: http://my_site.tld/dir/path/hello.py or http://my_site.tld/dir/path/hello<PRE class=program>def index(): s = """\ <html> <body> <h2>Hello World!</h2> </body> </html> """ return s</PRE> The document root is the default directory where Apache looks for the requested files. Find out what it is looking for the DocumentRoot directive in the httpd.conf file. The /dir/path is the path to the directory configured to be handled by the Publisher. November 18, 2006 URI Traversal If the previous hello.py script was saved as index.py then it could be executed as any of: http://my_site.tld/dir/path/ http://my_site.tld/dir/path/index http://my_site.tld/dir/path/index.py The Publisher handler will execute the module named in the URI. If there is no module in the URI it will look for index.py. The following script has two functions and it is possible to choose which one will be executed.<PRE class=program>s = """\ <html><body> <h2>Hello %s!</h2> </body></html> """ def index(): return s % ‘World‘ def everybody(): return s % ‘everybody‘</PRE> The function to be called can be passed in the URI. If the previous script was saved as index.py the everybody() function could be executed as any of: http://my_site.tld/dir/path/index.py/everybody http://my_site.tld/dir/path/index/everybody http://my_site.tld/dir/path/everybody If no function is found in the URI then the function called by default is the index() function. November 18, 2006 Objects Visibility It is not possible to publish a module. Either an object within the module is present in the URI or the module has an index function. Otherwise the Publisher will return a "404 Not Found" to the client. If the next script is saved as my_module.py the only way to execute it is: http://my_site.tld/dir/path/my_module/main http://my_site.tld/dir/path/my_module.py/main Global variables are also visible: http://my_site.tld/dir/path/my_module.py/s http://my_site.tld/dir/path/my_module/s<PRE class=program>s = """\ <html><body> <h2>I can‘t be published as a module!</h2> </body></html> """ def main(): return s main()</PRE> Only the objects inside the module are visible. It is not possible to call the module like: http://my_site.tld/dir/path/my_module http://my_site.tld/dir/path/my_module.py <H3>Hide objects from the web</H3> As objects inside a module can be published it may be necessary to hide some of them. If an object name starts with an underscore it will be hidden from the web.<PRE class=program># The _s variable is not visible _s = """\ <html><body> <h2>%s</h2> </body></html> """ def f(): x = ‘y‘ return s % ‘f() function‘ # The v variable is visible v = _s % ‘v variable‘</PRE> In the example above the _s variable is not visible, that is it can‘t be shown with: http://my_site.tld/dir/path/objects.py/_s November 18, 2006 Request Object The Publisher passes a Request object to every function it calls that declares it as an argument. This script will retrieve most of its attributes:<PRE class=program>from cgi import escape from urllib import unquote # The Publisher passes the Request object to the function def index(req): s = """\ <html><head> <style type="text/css"> td {padding:0.2em 0.5em;border:1px solid black;} table {border-collapse:collapse;} </style> </head><body> <table cellspacing="0" cellpadding="0">%s</table> </body></html> """ attribs = ‘‘ # Loop over the Request object attributes for attrib in dir(req): attribs += ‘<tr><td>%s</td><td>%s</td></tr>‘ attribs %= (attrib, escape(unquote(str(req.__getattribute__(attrib))))) return s % (attribs)</PRE> <A href="http://webpython.codepoint.net/files/other/request.html">View the output</A> It is not necessary to have the Request object as an argument in the function if it will not be used. The Publisher knows when it is necessary for the object to be passed. November 18, 2006 Code License All code, and only the code, in webpython.codepoint.net is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. All code in webpython.codepoint.net is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. Copyright (C) 2006 Clodoaldo Pinto Neto cpn@codepoint.net Preamble The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. 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November 18, 2006 Recommended Books <TABLE style="PADDING-RIGHT: 1em; PADDING-LEFT: 1em; PADDING-BOTTOM: 1em; MARGIN: auto; PADDING-TOP: 1em; TEXT-ALIGN: center" cellSpacing=0 cellPadding=0 border=0 ID="Table1"> <TBODY> <TR> <TD style="WIDTH: 220px"><A href="http://www.amazon.com/gp/product/1565925092?ie=UTF8&tag=fhstat-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=1565925092"> <IMG style="BORDER-RIGHT: gray 1px solid; BORDER-TOP: gray 1px solid; BORDER-LEFT: gray 1px solid; BORDER-BOTTOM: gray 1px solid" height=240 src="http://webpython.codepoint.net/files/http://www.softxml.com/images/http_guide.jpg" width=190></A> <IMG style="MARGIN: 0px; BORDER-TOP-STYLE: none! important; BORDER-RIGHT-STYLE: none! important; BORDER-LEFT-STYLE: none! important; BORDER-BOTTOM-STYLE: none! important" height=1 alt="" src="http://www.assoc-amazon.com/e/ir?t=fhstat-20&l=as2&o=1&a=1565925092" width=1 border=0> </TD> <TD style="WIDTH: 220px"><A href="http://www.amazon.com/gp/product/0596100469?ie=UTF8&tag=fhstat-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=0596100469"> <IMG height=240 src="http://webpython.codepoint.net/files/http://www.softxml.com/images/python_nutshell.jpg" width=160></A> <IMG style="MARGIN: 0px; BORDER-TOP-STYLE: none! important; BORDER-RIGHT-STYLE: none! important; BORDER-LEFT-STYLE: none! important; BORDER-BOTTOM-STYLE: none! important" height=1 alt="" src="http://www.assoc-amazon.com/e/ir?t=fhstat-20&l=as2&o=1&a=0596100469" width=1 border=0> </TD></TR></TBODY></TABLE>