4.5. SummaryΒΆ

  1. The Overall Process for Creating Dynamic Web Pages

    Making dynamic web pages has a number of steps. I have suggested several ways of decoupling the parts, so you can alter the order, but if you are starting from nothing, you might follow the following sequence:

    1. Determine the inputs you want to work with and make a web form that makes it easy and obvious for the user to provide the data. You may initially want to have the form’s action URL be dumpcgi.cgi, so you can debug the form separately. Test with the local server. When everything seems OK, make sure to change the action URL to be the name of the CGI script you are writing. [Editing HTML Forms]
    2. It is easier to debug a regular Python program totally inside Idle than to mix the Idle editor and server execution. Particularly if the generation of output data is going to be complicated or there are lots of places you are planning to insert data into an output template, I suggest you write the processInput function with its output template first and test it without a server, as we did with additionWeb.py, providing either canned input in the main program, or taking input data from the keyboard, and saving the output page to a local file that you examine in your webbrowser. [Dynamically Created Static Local Pages from Python]
    3. When you are confident about your processInput function, put it in a program with the proper cgi skeleton, and add the necessary lines at the beginning of the main function to take all the CGI script input from the browser data. [adder.cgi]
    4. Be sure to check for syntax errors in Idle, for instance using Alt-X. Fix as necessary.
    5. Finally test the whole thing with the local server. Make sure the local server is running, and all the resources that you refer to are in the same folder as the local web server: Initial web page, web page templates, CGI script. Do not open the starting web page or CGI script in Idle or by finding it in your file system. You must run it in your browser with a URL that starts with http://localhost:8080/. In error, if you load a web page directly from your file system, it will not cause an obvious error - the dynamic actions will just not take place.
    6. If is does not work right:
      • If you get a page that uses your template, but it looks wrong, either fix your template or look for a logical error in your program. (If you had tested your processInput function in a regular Python program before, this should not happen.)
      • If the web page output shows an error description, see if you can pick any help out and go back and fix your code.
      • If you get nothing back in your web browser, make sure you had tested the final version of the code in Idle for syntax errors (Alt-X), and that you have the final error catching code in the CGI script, and that you used a URL that starts with http://localhost:8080/.
      • If all of the parts mentioned above are there, the problem may be with the server, not Python. Look in the local server window’s log output, and see if it points to a filename that it cannot find or ....
    7. If you have an account on a public server, it should not take much more work than just uploading your files to make your creation available to the whole world. You may have a public server with a different configuration than the Loyola server. If so see this note: [1]
  2. Markup: Plain text may be marked up to include formatting. The formatting may be easily interpreted only by a computer, or it may be more human readable. One form of human-readable markup is hypertext markup language (HTML). [Format of Web Page Markup]

    1. HTML markup involves tags enclosed in angle braces. Ending tags start with ‘/’. For instance <title>Computer Science</title>.

      1. Tags may be modified with attributes specified similar to Python string assignments, for example the text input field tag,

        <input value="red" name="color" type="radio">
        
    2. Modern editors allow HTML to be edited much like in a word processor. Two views of the data are useful: the formatted view and the source view, showing the raw HTML markup.

  3. Python and HTML: Since HTML is just a text string, it can easily be manipulated in Python, and read and written to text files. [Dynamically Created Static Local Pages from Python]

  4. The webbrowser module has a function open, that will open a file or web URL in the default browser: [Dynamically Created Static Local Pages from Python]

    webbrowser.open( filename )

  5. Common Gateway Interface (CGI). The sequence of events for generating a dynamic web page via CGI: [An Example in Operation]

    1. The data a user types is handled directly by the browser. It recognizes forms.
    2. The user presses a Submit button. An action is stored in the form saying what to do when the button is pressed.
    3. In the cases we consider in this tutorial, the action is given as a web resource, giving the location of a CGI script on some server. The browser sends the data that you entered to that web location.
    4. The server recognizes the page as an executable script, sees that it is a Python program, and executes it, using the data sent along from the browser form as input.
    5. The script runs, manipulates the input data into some results, and puts those results into the text of a web page that is the output of the program.
    6. The server captures this output from the program and send it back to the user’s browser as a new page to display.
    7. The results appear in the user’s browser.
  6. The cgi Module

    1. Create the object to process CGI input with [adder.cgi]

      form = cgi.FieldStorage()
      
    2. Extract the first value specified by the browser with name nameAttrib, or use default if no such value exists [adder.cgi]

      variable = form.getfirst( nameAttrib , default )

    3. Extract the list of all values specified by the browser associated with name nameAttrib [ref{More Advanced Examples]

      listVariable = form.getlist( nameAttrib )

      This case occurs if you have a number of checkboxes, all with the same name, but different values. The list may be empty.

  7. Local Python Servers.

    1. Python has modules for creating local testing servers that can handle static web pages and Python CGI scripts.[An Example in Operation]
    2. Different kinds of errors with CGI scripts are handled different ways by a local Python server. [Errors in CGI Scripts]
  8. A comparison of the various types of files used in web programming, listing the different ways to edit and use the files, is given in Editing and Testing Different Document Formats.

[1]The tutorial assumed a server configured as follows: html pages and CGI scripts can all be in the same directory, and the CGI scripts end with .cgi. This is the convention on Loyola’s Computer Science public server. Another common configuration is that scripts all go in a cgi-bin directory, where they just have the normal .py suffix. If you have a server with the latter configuration, your action URLs will be of the form cgi-bin/someScript.py. Depending on the server configuration the current directory may or may not be cgi-bin while the script executes. That may mean you need a path before the file names for your output templates, or your need to be careful what directory referenced files end up in. If you are making arrangements for your own site on a public server, be sure to check with your system administrator to find out what the conventions are.