Integrating purchase cancellation into the web flow

The first order of business is to add a Cancel button to the last page in the web flow described earlier. We add this to the /path/to/repo/www/chapter_05/templates/purchase.html HTML template:

    <form action="/chapter_05/purchase.py" method="post">
    <input type="submit" name="cancel" value="Cancel" /></form>

We then modify purchase.py to scan for this extra input in the form post data. The main changes involve changing the message inserts to account for a cancellation. We also scan for the presence of cancel in the form:

if cust :
    html_out.addInsert('%name%', cust.getFullName())
    lastPurch  = purch_service.fetchLastPurchaseForCust(cust.getKey())
    if not lastPurch :
        html_out.addInsert('%purchase%', 'No recent purchase found')
    else :
        mostRecent = html_out.buildLastPurchase(lastPurch)
        html_out.addInsert...