formlib.py

from pylons import request

class FormException(Exception):
	def __init__(self, name, description, value):
		self.name, self.description, self.value = name, description, value

class FormEmptyException(FormException):
	def __str__(self):
		return 'Field %r (%s) is mandatory but empty.' % (self.name, self.description)

class FormInvalidException(FormException):
	def __str__(self):
		return 'Field %r (%s) failed validation.  Value: %r' % (self.name, self.description, self.value)

class Param(object):
	def validate_any(self, *args, **kwargs):
		return True
	
	def __init__(self, description, mandatory=True):
		self.description = description
		self.mandatory = mandatory
		self.validator = self.validate_any

class Form(object):
	def __init__(self):
		cls = self.__class__
		
		if not hasattr(cls, '__params__'):
			cls.__params__ = params = {}
			for name, value in ((name, getattr(cls, name)) for name in dir(cls)):
				if isinstance(value, str) and name[:2] != '__':
					value = Param(value)
				elif not isinstance(value, Param):
					continue
				
				if hasattr(cls, 'validate_' + name):
					validator = getattr(self, 'validate_' + name)
					if callable(validator):
						value.validator = validator
				params[name] = value
		else:
			params = cls.__params__
		
		for name in params:
			setattr(self, name, request.params.get(name, u''))
		
		for name, param in params.items():
			value = request.params.get(name, u'')
			
			if param.mandatory and value == u'':
				raise FormEmptyException(name, param.description, value)
			elif not param.validator(self, param, value):
				raise FormInvalidException(name, param.description, value)

testform.py

from formlib import *

class LoginForm(Form):
	email = 'Email'
	password = 'Password'
	return_to = Param('Return URI', mandatory=False)
	
	def validate_email(self, form, param, value):
		return '@' in value

# ...

try:
	form = LoginForm()
except FormEmptyException, e:
	return '%s required.' % e.description
except FormInvalidException, e:
	return '%s is malformed.' % e.description

if User.login(form.email, form.password):
	if form.return_to != u'':
		redirect(form.return_to)
	else:
		redirect_to(...)
else:
	return 'Login failed.'