There is a lot of data available to our program in
addition to the
POST data. The App
Engine environment makes this data available to our application for each
incoming request so that we can do things differently based on this
Variables describing the server environment (SERVER_SOFTWARE or SERVER_NAME)
Variables describing the request data (REQUEST_METHOD, HTTP_USER_AGENT, or CONTENT_TYPE)
Variables describing the browser environment variables (HTTP_USER_AGENT, HTTP_ACCEPT, and so on)
You can find documentation about these parameters at http://hoohoo.ncsa.uiuc.edu/cgi/in.html. This is a very old website that describes the Common Gateway Interface (CGI), which was the way that the very first web servers passed input data from an HTTP request into application code running on the server.
We will write an application that reads and dumps out all the information available to our application. We call this the “dumper” program because it just looks at its input and dumps it out.
The dumper program consists of a very simple app.yaml file and a single index.py Python file that contains the complete code of our App Engine program.
ae-02-dumperversion: 1 runtime: python api_version: 1 handlers: - url: /.* script: index.py
import os import sys print 'Content-Type: text/html' print '' print '<form method="post" action="/" >' print 'Zap Data: <input type="text" name="zap"><br/>' print 'Zot Data: <input type="text" name="zot"><br/>' print '<input type="submit">' print '</form>' print '<pre>' print 'Environment keys:' print '' for param in os.environ.keys(): print param, ':', os.environ[param] print '' print 'Data' count = 0 for line in sys.stdin: count = count + 1 print line if count > 100: break print '</pre>'
Let’s walk through each of the parts of the dumper program’s index.py.
print 'Content-Type: text/html' print ''
When you select View Source on an HTML page, you are not shown the response headers because they are not part of the HTML document. However, with the Firebug plug-in, you can see the HTTP response headers under the Net tab, as shown in Figure 4-12.
The next set of
POST data to our program.
This form now has two input text areas named
zot, so we can see what happens with multiple
print '<form method="post" action="/" >' print 'Zap Data: <input type="text" name="zap"><br/>' print 'Zot Data: <input type="text" name="zot"><br/>' print '<input type="submit">' print '</form>'
The form is quite basic, with two text fields and a Submit button. The next lines of the program read in a set of variables passed to our program as a Python dictionary. These are the environment or CGI variables. They are a combination of the server configuration as well as information about the particular request itself.
We iterate through the keys in the dictionary and then print the
keys and values, separated by a colon character, using a Python
print '<pre>' print 'Environment keys:' print '' for param in os.environ.keys(): print param, ':', os.environ[param] print ''
The output from this section is as follows:
HTTP_REFERER : http://www.appenginelearn.com/ SERVER_SOFTWARE : Development/1.0 SCRIPT_NAME : REQUEST_METHOD :
GETHTTP_KEEP_ALIVE : 300 SERVER_PROTOCOL : HTTP/1.0 QUERY_STRING : CONTENT_LENGTH : HTTP_ACCEPT_CHARSET : ISO-8859-1,utf-8;q=0.7,*;q=0.7 HTTP_USER_AGENT : Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:18.104.22.168) Gecko/2008120121 Firefox/3.0.5 HTTP_CONNECTION : keep-alive SERVER_NAME : localhost REMOTE_ADDR : 127.0.0.1 PATH_TRANSLATED : /Users/csev/Desktop/teach/appengine/apps/ae-02-dumper/index.py SERVER_PORT : 8080 AUTH_DOMAIN : gmail.com CURRENT_VERSION_ID : 1.1 HTTP_HOST : localhost:8080 TZ : UTC USER_EMAIL : HTTP_ACCEPT : text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 APPLICATION_ID : ae-02-dumper GATEWAY_INTERFACE : CGI/1.1 HTTP_ACCEPT_LANGUAGE : en-us,en;q=0.5 CONTENT_TYPE : application/x-www-form-urlencoded HTTP_ACCEPT_ENCODING : gzip,deflate PATH_INFO : /
Because this is an HTTP
request, there is no data to print.
You can consult the CGI documentation for the details on each of the previously mentioned variables: http://hoohoo.ncsa.uiuc.edu/cgi/in.html.
When we are programming at the CGI level, we are using the old mystical ways of the early web server programs. We won’t use this pattern for much longer, but it is good to start by understanding the low-level details and then delegate the handling of those details to a web framework.
The last part of the index.py
program dumps out up to the first 100 lines of
POST data, if the data exists:
print 'Data' count = 0 for line in sys.stdin: count = count + 1 print line if count > 100: break
According to CGI rules, the
POST data is presented to the application via
its standard input. In Python, we can read through the predefined file
handle sys.stdin to access our
POST data using a Python
If you look at the bottom of the initial output of the program,
you will see that there is no
data because when you navigate to
http://localhost:8080, the browser issues an HTTP
GET request for the initial document
POST data dumping code,
we must enter some data into the Zap and Zot input fields and click the
Submit button, as shown in Figure 4-13.
When we click Submit, our browser sends a
POST request, which we can immediately see in
REQUEST_METHOD variable change
Environment keys: HTTP_REFERER : http://www.appenginelearn.com/ SERVER_SOFTWARE : Development/1.0 SCRIPT_NAME : REQUEST_METHOD :
POSTHTTP_KEEP_ALIVE : 300 SERVER_PROTOCOL : HTTP/1.0 QUERY_STRING : ...
And if we scroll down to the bottom of the output, we can see the
GATEWAY_INTERFACE : CGI/1.1 HTTP_ACCEPT_LANGUAGE : en-us,en;q=0.5 CONTENT_TYPE :
application/x-www-form-urlencodedHTTP_ACCEPT_ENCODING : gzip,deflate PATH_INFO : / Data zap=Some+Data&zot=Some+More+Data
To make parsing easier, the
POST data is encoded by escaping spaces and
special characters. Each parameter starts with an ampersand (&) to
distinguish its new parameters from the data of the previous parameter.
To make sense of this input, we would have to parse the input data using
string parsing and then unescape the data to get back to the actual data
that was typed into the form.
There are two ways to encode
POST data. The easy way to encode the
POST data is called
“application/x-www-form-urlencoded”; this approach concatenates all the
data into a single line of input as shown earlier. The more complex way
POST data is called
multipart/form-data and is described in the next section.