Pylons, FormEncode and external validation - validation

Question

I'm building a web frontend to a server-side application, using Pylons 1.0.
Right now I'm writing the first form, and I'm facing a problem concerning validation.. Using FormEncode and the #validate decorator I can easily validate the user input from a client-side perspective, but when I submit the data to the server, it may perform additional checks and eventually throw back exceptions that I need to show to the user.
My question: is there a concise way to integrate/emulate this exception handling into the FormEncode/validate flow? For example, redisplay the form with filled fields and an error message, as would happen if the exception had come from the #validate itself?
Here's what I have at the moment:
def edit(self, id):
return render('/edit_user.mako')
#validate(schema=form.UserForm(), form="edit")
def add_user(self):
if request.POST:
u = helpers.load_attributes(User(), self.form_result)
try:
model.save_to_server(u)
except MyBaseException, exc:
helpers.flash(unicode(exc))
return self.edit()
In this way, in case of a server-side exception I can see the "flash" message but the form of course will have empty fields :/

Answer 1

I like to implement:
from formencode import htmlfill
def create(self):
if request.params:
try:
Post.validate(request.paramse)
post = helpers.load_attributes(Post(), request.params)
model.save_to_server(post)
flash('OK', 'success')
redirect(...)
except InvalidException as e:
for key, message in e.unpack_errors().iteritems():
flash(message, 'error')
return htmlfill.render(render('/blogs/create.html'), request.params)
where my Post.validate:
#staticmethod
def validate(data):
schema = PostSchema()
schema.to_python(data)
In this way, if is the first time (request.params empty) html fills form with nothing, when user send datas html fills form with request.params

Answer 2

Another way (inspired by this answer) is to write a decorator similar to #validate that would catch the desired exceptions and use htmlfill to display their message:
def handle_exceptions(form):
def wrapper(func, self, *args, **kwargs):
try:
return func(self, *args, **kwargs)
except MyBaseException, e:
request = self._py_object.request
errors = { "exception" : unicode(e) }
params = request.POST
decoded = params.mixed()
request.environ['REQUEST_METHOD'] = 'GET'
self._py_object.tmpl_context.form_errors = errors
request.environ['pylons.routes_dict']['action'] = form
response = self._dispatch_call()
# If the form_content is an exception response, return it
if hasattr(response, '_exception'):
return response
htmlfill_kwargs2 = {}
htmlfill_kwargs2.setdefault('encoding', request.charset)
return htmlfill.render(response, defaults=params, errors=errors,
**htmlfill_kwargs2)
return decorator(wrapper)
The decorator would be used like:
#handle_exceptions("edit")
#validate(schema=form.UserForm(), form="edit")
def add_user(self):
if request.POST:
u = helpers.load_attributes(User(), self.form_result)
model.save_to_server(u)