.. _validation:

FormEncode @validate, and TurboGears Validation
===============================================

When using TurboGears, your controller methods get their arguments
built from the various GET, POST, and URL mechanisms provided by
TurboGears. The only downside is that all the arguments will be
strings and you'd like them converted to their normal Python datatype:
numbers to ``int``, dates to ``datetime``, etc.

This conversion functionality is provided by the `FormEncode`_ package
and is applied to your methods using the ``@validate()``
decorator. FormEncode provides both validation and conversion as a
single step, reasoning that you frequently need to validate something
before you can convert it or that you'll need to convert something
before you can really validate it.

The ``@validate()`` decorator can evaluate both widget-based forms and
the standard form arguments so they are not dependent on widgets at
all.

Furthermore, the ``@validate()`` decorator is not really required at
all.  It just provides a convenience so that you can assume that you
have the right kind of data inside your controller methods. This helps
separate validation logic from application logic about what to do with
valid data.

If you don't put a ``@validate()`` decorator on your method, you'll
simply have to do the string conversion in your controller.

Validating arguments (without form widgets)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When not using forms, the story gets a bit more complex. Basically,
you need to specify which validator goes with which argument using the
``validators`` keyword argument. Here's a simple example:

.. code-block:: python
    
    from formencode import validators
    
    @expose('json')
    @validate(validators={"a":validators.Int(), "b":validators.Email})
    def two_validators(self, a=None, b=None, *args):
        errors = [{key, value} in pylons.c.form_errors.iteritems()]
        values =  pylons.c.form_values
        return dict(a=a, b=b, errors=str(errors), values=str(values))

The dictionary passed to validators maps the incoming field names to
the appropriate FormEncode validators, ``Int`` in this example.

If there's a validation error, TurboGears calls the error_handler if
it exists, but it always adds form_errors and form_values to the
tmpl_context, so they will be available there for the rest of the
request.  In this case if there are validation errors, we grab both
the error messages and the original `unvalidated` values and return
them in the error message.

FormEncode provides a number of useful pre-made validators for you to
use: they are available in the ``formencode.validators`` module.

For most validators, you can pass keyword arguments for more specific
constraints.

Available Validators
~~~~~~~~~~~~~~~~~~~~

* Attribute
* Bool
* CIDR
* ConfirmType
* Constant
* CreditCardExpires
* CreditCardSecurityCode
* CreditCardValidator
* DateConverter
* DateTime
* DateValidator
* DictConverter
* Email
* Empty
* False
* FancyValidator
* FieldStorageUploadConverter
* FieldsMatch
* FileUploadKeeper
* FormValidator
* IDeclarative
* IPhoneNumberValidator
* ISchema
* IValidator
* Identity
* IndexListConverter
* Int
* Interface
* Invalid
* MACAddress
* MaxLength
* MinLength
* NoDefault
* NotEmpty
* Number
* OneOf
* PhoneNumber
* PlainText
* PostalCode
* Regex
* RequireIfMissing
* RequireIfPresent
* Set
* SignedString
* StateProvince
* String
* StringBool
* StringBoolean
* StripField
* TimeConverter
* True
* URL
* UnicodeString
* Validator
* Wrapper

For the absolute most up-to date list of available validators, check
the `FormEncode validators`_ module. You can also create your own
validators or build on existing validators by inheriting from one of
the defaults.

See the FormEncode documentation for how this is done.

.. _`FormEncode validators`: http://formencode.org/module-formencode.validators.html#classes

You can also compose ``compound`` validators with logical operations,
the FormEncode compound module provides `All` (all must pass), 
`Any` (any one must pass) and `Pipe` (all must pass with the results of 
each validator passed to the next item in the Pipe).  You can use these 
like so::

    from formencode.compound import All
    ...
    the_validator=All(
        validators.NotEmpty(),
        validators.UnicodeString(),
    )

Validating widget-based forms
-----------------------------

The simplest way to use ``@validate()`` is to pass in a reference to a
widgets-based form:

.. code-block:: python

    @validate(projectname.forms.a_form)

The widgets system will take care of building a schema to handle the
data conversions and you'll wind up with the ``int`` or ``datetime``
objects you specified when building the form. When paired with the
`validate` decorator, you can handle the common case of building a
form, validating it, redisplaying the form if there are errors, and
converting a valid form into the proper arguments in only a few lines
of Python.

You can also pass the form using a keyword argument:

.. code-block:: python

    @validate(form=projectname.forms.a_form)
    
You might also want to tell TurboGears to pass off handling of invalid
data to a different controller.  To do that you just pass the method
you want called to @validate via the error_handler param:

.. code-block:: python

    @validate(forms.myform, error_handler=process_form_errors)

The method in question will be called, with the unvalidated data as
its parameters.  And error validation messages will be stored in
pylons.tmpl_context.

Here's a quick example of how this all works:

.. code-block:: python

    @expose('json')
    @validate(form=myform)
    def process_form_errors(self, **kwargs):
        #add error messages to the kwargs dictionary and return it
        kwargs['errors'] = pylons.tmpl_context.form_errors
        return dict(kwargs)
    
    @expose('json')
    @validate(form=myform, error_handler=process_form_errors)
    def send_to_error_handler(self, **kwargs):
        return dict(kwargs)

If there's a validation error in myform, the send_to_error_handler
method will never get called.  Instead process_form_errors will get
called, and the validation error messages can be picked up from the
form_errors value of the template context object
(pylons.tmpl_context).

Schema validation
-----------------

Sometimes you need more power and flexibility than you can get from
validating individual form fields.  Fortunately FormEncode provides
just the thing for us -- Schema validators.

If you want to do multiple-field validation, reuse validators or just
clean up your code, validation ``Schema``s are the way to go. You
create a validation schema by inheriting from
``formencode.schema.Schema`` and pass the newly created ``Schema``
as the ``validators`` argument instead of passing a dictionary.

Create a schema:

.. code-block:: python

    class PwdSchema(schema.Schema):
        pwd1 = validators.String(not_empty=True)
        pwd2 = validators.String(not_empty=True)
        chained_validators = [validators.FieldsMatch('pwd1', 'pwd2')]

Then you can use that schema in @validate rather than a dictionary of
validators::

    @expose()    
    @validate(validators=PwdSchema())
    def password(self, pwd1, pwd2):
        if pylons.c.form_errors:
            return "There was an error"
        else:
            return "Password ok!"

Besides noticing our brilliant security strategy, please notice the
``chained_validators`` part of the schema that guarantees a pair of
matching fields.

Again, for information about ``Invalid`` exception objects, creating
your own validators, schema and FormEncode in general, refer to the
`FormEncode Validator`_ documentation and don't be afraid to check the
``Formencode.validators`` source. It's often clearer than the 
documentation.

Note that Schema validation is rigorous by default, in particular, you 
must declare *every* field you are going to pass into your controller 
or you will get validation errors.  To avoid this, add::

    class MySchema( schema.Schema ):
        allow_extra_fields=True

to your schema declaration.

.. _`FormEncode Validator`: http://formencode.org/docs/Validator.html

Converting URL strings to Python types manually
-----------------------------------------------

You can always use e.g. Python's ``int()`` method to convert a string
to an integer and use a try/except block to catch errors in the
conversion process:

.. code-block:: python

  from tg import controllers, expose
  class Root(controllers.RootController):

  #...
      # return the result of x+y
      @expose()
      def addnum(self, x, y)
          try:
             return str(int(x)+int(y))
          except:
             return 'value is not valid'
    
This isn't that hard, but it quickly becomes unwieldy when you start
converting large numbers of arguments. Moreover, you still have the
problem of propagating the errors back to your users. In the end, it's
usually far simpler to use the validation framework.

.. _FormEncode: http://formencode.org/

