My fancy hello world app on Google App Engine

helloworld.py

import webapp2

mainForm="""
<form method='post' action='#'>
    <label>What world do you come from?
        <br>
        <input name='world' value='%(world)s' placeholder='Minecraft World'>
    </label>

    <label>What's your name?
        <input name='name' value='%(name)s'>
    </label>

    <label>How old are you?
        <input name='age' value='%(age)s'>
    </label>
    <br>
    <br>
    <br>
    <div style='color:red'>%(error)s</div>
    <input type='submit'>
</form>
"""

class MainPage(webapp2.RequestHandler):
    def get(self):
        #self.response.headers['Content-Type'] = 'text/plain'
        #self.response.write('Hello, World!')
        #self.response.write(form)
        self.write_form()

    # writes the form with an optional error message
    # can also take in form inputs from previous submission to populate the
    # value attributes so as to preserve user input on error
    def write_form(self, error='', world='', name='', age=''):
        import cgi
        self.response.write(mainForm % {'error': cgi.escape(error),
                                        'world': cgi.escape(world),
                                        'name': cgi.escape(name),
                                        'age': cgi.escape(age),
                                        })

    def post(self):
        w = self.request.get('world').split(' ')[0]
        n = self.request.get('name')
        a = self.request.get('age')
        # validate form
        err=None
        worlds=('Minecraft', 'Hello Kitty', 'Disney', 'Fantasy')
        if w not in worlds:
            err="Sorry %(n)s I've never heard of your %(w)s world!" %{'n':n, 'w':w}
            strw=', '.join(w+' world' for w in worlds)
            err+="\nI've only heard of " + strw
        elif not a.isdigit():
            err="Sorry %s, '%s' doesn't seem like a valid age to me" %(n, a)
        elif int(a)>18:
            err="Sorry %s, I'm afraid you're too old to enjoy this site..." %n
        elif int(a)<=6:
            err="Sorry %s, this site is not for kindergarteners" %n
        if err:
            self.write_form(err,w,n,a)
        else: # redirect to success page
            from urllib import urlencode
            self.redirect('/hello?'+urlencode({'world':w, 'name':n, 'age':a}))

# success page; prints a customized greeting
class GreetHandler(webapp2.RequestHandler):
    def get(self):
        #self.response.write(self.request)
        greeting="""
        <h1>Oh hai dear %s from %s World!</h1>
        """
        n = self.request.get('name')
        w = self.request.get('world')
        self.response.write(greeting %(n, w))

app = webapp2.WSGIApplication([
    ('/', MainPage),
    ('/hello', GreetHandler),
    ], debug=True)