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CS108 Lecture 22: Web Applications: Forms

HTML Forms Processing Form Inputs The Common Gateway Interface CGI processing in Python Aaron Stevens

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What You’ll Learn Today

 Using HTML forms to collect data from a client and send it to your web application

  Accessing user input from inside your python web application: the Common Gateway Interface

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HTTP

HyperText Transfer Protocol http://www.ltg.ed.ac.uk/~ht/WhatAreURIs/

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Data-Driven Web Applications

http://www.windowsecurity.com/img/upl/web-apps11165227941888.gif

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HTML Forms

HTML Forms are a way to provide input to the webserver:

Forms are created with the <FORM> tag.

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HTML Forms

Here is the code that created that form:

Rendered Display:

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Linking Forms to Actions

HTML Forms send data to the webserver, as specified by a parameter called action:

action tells the form to which URL to send the HTTP request and data.

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Using Google Search as CGI

Google Search responds to HTTP requests with minimal form data: http://www.google.com/search?q=<value>

action = http://www.google.com/search q = the query value

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HTML Form Fields

We need to specify each form field’s name, type, and value:

!type=text

Computer Science Processing HTML Form Fields

The application waiting to process a form is called a Common Gateway Interface or CGI.   Requires a non-trivial script on the server to process the

data. Some easy CGIs to which we can send FORM data:   Send a query to Google search, maps, etc.   Send the form data to email: http://cs-webapps.bu.edu/util/formmailer.py http://www.response-o-matic.com/

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Email form fields

Example: contact form

Here’s an application you can use to send form data to email: http://cs-webapps.bu.edu/util/formmailer.py Must include a field named “mailto”, with a value for your email address.

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HTML Forms

HTML Forms are a way to provide input to the webserver:

Forms are created with the <FORM> tag.

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How CGI Works Web Browser

(client) Web App (server)

GET /

GET + name

RESP: form

RESP: welcome + name

If “name” NOT in CGI data? Show form

If “name” in CGI data: do welcome

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HTML Forms

Here is the code that created that form:

HTML/GUI Display:

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Processing CGI Form Fields

Common Gateway Interface or CGI is a way for web applications to send data to the server. Python’s CGI module provides access to a dict of all form fields and their values.

Use the cgi.FieldStorage() function to get this dict, to which the code refers to as form.

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Processing CGI Form Fields

Notice a few things:  Check that a field exists before ‘reading’ it.  Notice the item in the dict is an object of type cgi.MiniFieldStorage. The data we want is the .value attribute.

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Using CGI Data in the HTML Response After we ‘read’ the CGI Fields, we can use in our program/output:

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Example: Web Calculator

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Example: Web Calculator

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Example: Web Calculator

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Computer Science Example: Web Calculator

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Example: Web Calculator

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We must handle two different cases:

Web Browser (client)

Web App (server)

GET /

RESP: login form

Got form data? NO: send form

GET + login form

RESP: welcome

Got form data? YES: read out name

and send welcome message

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More HTML Forms Fields

Reference: http://www.w3schools.com/HTML/html_forms.asp for more syntax, etc.

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type=radio!

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type=checkbox

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<textarea>

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<select>

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Computer Science Reading Other CGI Form Data

Here are a variety of CGI Form Fields:

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Reading Other CGI Data

All form fields are presented to Python as Type MiniFieldStorage. This code: Produces this HTML output:

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Reading Other CGI Data

These data are key-value pairs:

Notice that the checkboxes might produce a list of MiniFieldStorage objects.   Or a single MiniFieldStorage if only one checkbox

was selected!

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Reading a CGI Data list If needed, check type of form fields… And process as a list of MiniFieldStorage Notice output obtained by indexing into this list:

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Computer Science Organizing a Python Web App

Divide and Conquer Strategy:   Separate the processing from Input/Output   Separate each “action” to is own function(s)

Test each part of the application by itself!

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Separate Processing from Input/Output

Write functions to do the computation.   unit test with a non-web, console application.

Examples: calculatePMT, dollarFormat, wordCount, encode/decode/ciphering, etc.

Always test computation first, and then begin work on presentation.

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Many functions make simple programs Put each web interaction into its own function.   Print a web form.   Display an output.   Got multiple paths/outputs -- one function each!

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What You Learned Today

  The HTML <FORM> tag   Accessing CGI field data in Python   Be mindful of what type of data you are

expecting. When in doubt, use introspection!  Divide and conquer strategy for web application

development.

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Computer Science Announcements and To Do List

  Readings:   HTML Forms tutorial: http://www.w3schools.com/HTML/

  !