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.. comment (set Emacs mode) -*- doctest -*-
>>> import sys
>>> import formencode
FormEncode Validation
:author: Ian Bicking <>
:version: |release|
:date: |today|
.. contents::
Validation (which encompasses conversion as well) is the core function
of FormEncode. FormEncode really tries to *encode* the values from
one source into another (hence the name). So a Python structure can
be encoded in a series of HTML fields (a flat dictionary of strings).
A HTML form submission can in turn be turned into a the original
Python structure.
Using Validation
In FormEncode validation and conversion happen simultaneously.
Frequently you cannot convert a value without ensuring its validity,
and validation problems can occur in the middle of conversion.
The basic metaphor for validation is **to_python** and
**from_python**. In this context "Python" is meant to refer to "here"
-- the trusted application, your own Python objects. The "other" may
be a web form, an external database, an XML-RPC request, or any data
source that is not completely trusted or does not map directly to
Python's object model. :meth:`to_python` is the process of taking
external data and preparing it for internal use, :meth:`from_python`
generally reverses this process (:meth:`from_python` is usually the less
interesting of the pair, but provides some important features).
The core of this validation process is two methods and an exception::
>>> import formencode
>>> from formencode import validators
>>> validator = validators.Int()
>>> validator.to_python("10")
>>> validator.to_python("ten")
Traceback (most recent call last):
Invalid: Please enter an integer value
``"ten"`` isn't a valid integer, so we get a :class:`formencode.Invalid`
exception. Typically we'd catch that exception, and use it for some
sort of feedback. Like:
.. comment (fake raw_input):
>>> raw_input_input = []
>>> def raw_input(prompt):
... value = raw_input_input.pop(0)
... print ('%s%s' % (prompt, value))
... return value
>>> raw_input_input.extend(['ten', '10'])
>>> raw_input_input.extend(['bob', ''])
>>> import six
>>> if not six.PY2: # Python 3
... input = raw_input
>>> def get_integer():
... while 1:
... try:
... value = raw_input('Enter a number: ')
... return validator.to_python(value)
... except formencode.Invalid as e:
... print (e)
>>> get_integer()
Enter a number: ten
Please enter an integer value
Enter a number: 10
We can also generalize this kind of function::
>>> def valid_input(prompt, validator):
... while 1:
... try:
... value = raw_input(prompt)
... return validator.to_python(value)
... except formencode.Invalid as e:
... print (e)
>>> valid_input('Enter your email: ', validators.Email())
Enter your email: bob
An email address must contain a single @
Enter your email:
:class:`Invalid` exceptions generally give a good, user-readable error
message about the problem with the input. Using the exception gets
more complicated when you use compound data structures (dictionaries
and lists), which we'll talk about later__.
.. __: `Compound Validators`_
We'll talk more about these individual validators later, but first
we'll talk about more complex validation than just integers or
individual values.
.. _Schemas:
Available Validators
There's lots of validators. The best way to read about the individual
validators available in the :mod:`formencode.validators` module is to read
about :mod:`~formencode.validators` and :mod:`~formencode.national`.
Compound Validators
While validating single values is useful, it's only a *little* useful.
Much more interesting is validating a set of values. This is called a
For instance, imagine a registration form for a website. It takes the
following fields, with restrictions:
* ``first_name`` (not empty)
* ``last_name`` (not empty)
* ``email`` (not empty, valid email)
* ``username`` (not empty, unique)
* ``password`` (reasonably secure)
* ``password_confirm`` (matches password)
There's a couple validators that aren't part of FormEncode, because
they'll be specific to your application::
>>> # We don't really have a database of users, so we'll fake it:
>>> usernames = []
>>> class UniqueUsername(formencode.FancyValidator):
... def _convert_to_python(self, value, state):
... if value in usernames:
... raise formencode.Invalid(
... 'That username already exists',
... value, state)
... return value
.. note::
The class :class:`formencode.FancyValidator` is the superclass
for most validators in FormEncode, and it provides a number of useful
features that most validators can use -- for instance, you can pass
``strip=True`` into any of these validators, and they'll strip
whitespace from the incoming value before any other validation.
This overrides the internal :meth:`_convert_to_python()` method:
:class:`formencode.FancyValidator` adds a number of extra features, and then
calls the internal :meth:`_convert_to_python` method, which is the method you'll
typically write. Contrary to the external method :meth:`to_python`, its
only concern is the conversion part, not the validation part. If further
validation is necessary, this can be done in two other internal methods,
either :meth:`_validate_python()` or :meth:`_validate_other()`. We will give
an example for that later. When a validator finds an error, it raises an
exception (:class:`formencode.Invalid`), with the error message and the
value and "state" objects. We'll talk about state_ later. Here's the
other custom validator, that checks passwords against words in the
standard Unix word file::
>>> class SecurePassword(formencode.FancyValidator):
... words_filename = '/usr/share/dict/words'
... def _convert_to_python(self, value, state):
... f = open(self.words_filename)
... lower = value.strip().lower()
... for line in f:
... if line.strip().lower() == lower:
... raise formencode.Invalid(
... 'Please do not base your password on a '
... 'dictionary term', value, state)
... return value
And here's a schema::
>>> class Registration(formencode.Schema):
... first_name = validators.ByteString(not_empty=True)
... last_name = validators.ByteString(not_empty=True)
... email = validators.Email(resolve_domain=True)
... username = formencode.All(validators.PlainText(),
... UniqueUsername())
... password = SecurePassword()
... password_confirm = validators.ByteString()
... chained_validators = [validators.FieldsMatch(
... 'password', 'password_confirm')]
Like any other validator, a :class:`Registration` instance will have the
:meth:`to_python` and :meth:`from_python` methods. The input should be a
dictionary (or a Paste MultiDict), with keys like ``"first_name"``,
``"password"``, etc. The validators you give as attributes will be
applied to each of the values of the dictionary. *All* the values
will be validated, so if there are multiple invalid fields you will
get information about all of them.
Most validators (anything that subclasses
:class:`formencode.FancyValidator`) will take a certain standard
set of constructor keyword arguments. See
:class:`formencode.api.FancyValidator` for more -- here we use
Another notable validator is :class:`formencode.compound.All` -- this
is a *compound validator* -- that is, it's a validator that takes
validators as input. Schemas are one example; in this case :class:`All`
takes a list of validators and applies each of them in turn.
:class:`formencode.compound.Any` is its compliment, that uses the
first passing validator in its list.
.. _pre_validators:
.. _chained_validators:
:attr:`chained_validators` are validators that are run on the entire
dictionary after other validation is done (:attr:`pre_validators` are
applied before the schema validation). chained_validators will also
allow for multiple validators to fail and report to the error_dict
so, for example, if you have an email_confirm and a password_confirm
fields and use FieldsMatch on both of them as follows:
>>> chained_validators = [
... validators.FieldsMatch('password',
... 'password_confirm'),
... validators.FieldsMatch('email',
... 'email_confirm')]
This will leave the error_dict with both password_confirm and
email_confirm error keys, which is likely the desired behavior
for web forms.
Since a :class:`formencode.schema.Schema` is just another kind of
validator, you can nest these indefinitely, validating dictionaries of
.. _SimpleFormValidator:
Another way to do simple validation of a complete form is with
:class:`formencode.schema.SimpleFormValidator`. This class wraps a simple
function that you write. For example::
>>> from formencode.schema import SimpleFormValidator
>>> def validate_state(value_dict, state, validator):
... if value_dict.get('country', 'US') == 'US':
... if not value_dict.get('state'):
... return {'state': 'You must enter a state'}
>>> ValidateState = SimpleFormValidator(validate_state)
>>> ValidateState.to_python({'country': 'US'}, None)
Traceback (most recent call last):
Invalid: state: You must enter a state
The :func:`validate_state` function (or any validation function) returns
any errors in the form (or it may raise Invalid directly). It can
also modify the :obj:`value_dict` dictionary directly. When it returns
None this indicates that everything is valid. You can use this with a
:class:`Schema` by putting :class:`ValidateState` in :attr:`pre_validators`
(all validation will be done before the schema's validation, and if there's
an error the schema won't be run). Or you can put it in
:attr:`chained_validators` and it will be run *after* the schema. If the
schema fails (the values are invalid) then :class:`ValidateState` will not
be run, unless you set :attr:`validate_partial_form` to True (like
``ValidateState = SimpleFormValidator(validate_state,
validate_partial_form=True)``. If you validate a partial form you
should be careful that you handle missing keys and other
possibly-invalid values gracefully.
.. _ForEach:
You can also validate lists of items using
:class:`formencode.foreach.ForEach`. For example, let's say we have a
form where someone can edit a list of book titles. Each title has an
associated book ID, so we can match up the new title and the book it
is for::
>>> class BookSchema(formencode.Schema):
... id = validators.Int()
... title = validators.ByteString(not_empty=True)
>>> validator = formencode.ForEach(BookSchema())
The :obj:`validator` we've created will take a list of dictionaries as
input (like ``[{"id": "1", "title": "War & Peace"}, {"id": "2",
"title": "Brave New World"}, ...]``). It applies the :class:`BookSchema`
to each entry, and collects any errors and reraises them. Of course,
when you are validating input from an HTML form you won't get well
structured data like this (we'll talk about that later__).
.. __: `HTTP/HTML Form Input`_
Writing Your Own Validator
We gave a brief introduction to creating a validator earlier
(:class:`UniqueUsername` and :class:`SecurePassword`). We'll discuss
that a little more. Here's a more complete implementation of
>>> import re
>>> class SecurePassword(validators.FancyValidator):
... min = 3
... non_letter = 1
... letter_regex = re.compile(r'[a-zA-Z]')
... messages = {
... 'too_few': 'Your password must be longer than %(min)i '
... 'characters long',
... 'non_letter': 'You must include at least %(non_letter)i '
... 'characters in your password',
... }
... def _convert_to_python(self, value, state):
... # _convert_to_python gets run before _validate_python.
... # Here we strip whitespace off the password, because leading
... # and trailing whitespace in a password is too elite.
... return value.strip()
... def _validate_python(self, value, state):
... if len(value) < self.min:
... raise formencode.Invalid(self.message("too_few", state,
... min=self.min),
... value, state)
... non_letters = self.letter_regex.sub('', value)
... if len(non_letters) < self.non_letter:
... raise formencode.Invalid(self.message("non_letter", state,
... non_letter=self.non_letter),
... value, state)
With all validators, any arguments you pass to the constructor will be
used to set instance variables. So :class:`SecureValidator(min=5)` will
be a minimum-five-character validator. This makes it easy to also
subclass other validators, giving different default values.
Unlike the previous implementation we use the already mentioned
:meth:`_validate_python` method, which is another internal method
:class:`FancyValidator` allows us to override. :meth:`_validate_python`
doesn't have any return value, it simply raises an exception if it
needs to. It validates the value *after* it has been converted
(by :meth:`_convert_to_python`). :meth:`_validate_other` validates before
conversion, but that's usually not that useful. The external method
:meth:`to_python` cares about the extra features such as the
:attr:`if_empty` parameter, and uses the internal methods to do the
actual conversion and validation; first it calls :meth:`_validate_other`,
then :meth:`_convert_to_python` and at last :meth:`_validate_python`.
The use of ``self.message(...)`` is meant to make the messages easy to
format for different environments, and replacable (with translations,
or simply with different text). Each message should have an
identifier (``"min"`` and ``"non_letter"`` in this example). The
keyword arguments to :meth:`message` are used for message substitution.
See Messages_ for more.
Other Validator Usage
Validators use instance variables to store their customization
information. You can use either subclassing or normal instantiation
to set these. These are (effectively) equivalent::
>>> plain = validators.Regex(regex='^[a-zA-Z]+$')
>>> # and...
>>> class Plain(validators.Regex):
... regex = '^[a-zA-Z]+$'
>>> plain = Plain()
You can actually use classes most places where you could use an
instance; :meth:`.to_python()` and :meth:`.from_python()` will create
instances as necessary, and many other methods are available on both
the instance and the class level.
When dealing with nested validators this class syntax is often easier
to work with, and better displays the structure.
.. _FancyValidator:
There are several options that most validators support (including your
own validators, if you subclass from :class:`formencode.FancyValidator`):
If set, then this value will be returned if the input evaluates
to false (empty list, empty string, None, etc), but not the 0 or
False objects. This only applies to ``.to_python()``.
If true, then if an empty value is given raise an error.
(Both with ``.to_python()`` and also ``.from_python()``
if ``.validate_python`` is true).
If true and the input is a string, strip it (occurs before empty
If set, then when this validator would raise Invalid during
``.to_python()``, instead return this value.
If set, when the Python value (converted with
``.from_python()``) is invalid, this value will be returned.
If True (the default), then ``._validate_python()`` and
``._validate_other()`` will not be called when
``.from_python()`` is used.
Typically when a field is missing the schema will raise an
error. In that case no validation is run -- so things like
``if_invalid`` won't be triggered. This special attribute (if
set) will be used when the field is missing, and no error will
occur. (``None`` or ``()`` are common values)
All the validators receive a magic, somewhat meaningless ``state``
argument (which defaults to ``None``). It's used for very little in
the validation system as distributed, but is primarily intended to be
an object you can use to hook your validator into the context of the
larger system.
For instance, imagine a validator that checks that a user is permitted
access to some resource. How will the validator know which user is
logged in? State! Imagine you are localizing it, how will the
validator know the locale? State! Whatever else you need to pass in,
just put it in the state object as an attribute, then look for that
attribute in your validator.
Also, during compound validation (a :class:`formencode.schema.Schema`
or :class:`formencode.foreach.ForEach`) the state (if not None) will
have more instance variables added to it. During a :class:`Schema`
(dictionary) validation the instance variable ``key`` and
``full_dict`` will be added -- ``key`` is the current key (i.e.,
validator name), and ``full_dict`` is the rest of the values being
validated. During a :class:`ForEeach` (list) validation, ``index`` and
``full_list`` will be set.
Invalid Exceptions
Besides the string error message, :class:`formencode.Invalid`
exceptions have a few other instance variables:
The input to the validator that failed.
The associated state_.
The error message (``str(exc)`` returns this)
If the exception happened in a ``ForEach`` (list) validator, then
this will contain a list of ``Invalid`` exceptions. Each item
from the list will have an entry, either None for no error, or an
If the exception happened in a :class:`Schema` (dictionary) validator,
then this will contain :class:`Invalid` exceptions for each failing
field. Passing fields not be included in this dictionary.
This method returns a set of lists and dictionaries containing
strings, for each error. It's an unpacking of :attr:`error_list`,
:attr:`error_dict` and :attr:`msg`. If you get an Invalid exception
from a :class:`Schema`, you probably want to call this method on the
exception object.
.. _Messages:
Messages, Language Customization
All of the error messages can be customized. Each error message has a
key associated with it, like ``"too_few"`` in the registration
example. You can overwrite these messages by using you own ``messages
= {"key": "text"}`` in the class statement, or as an argument when you
call a class. Either way, you do not lose messages that you do not
define, you only overwrite ones that you specify.
Messages often take arguments, like the number of characters, the
invalid portion of the field, etc. These are always substituted as a
dictionary (by name). So you will use placeholders like ``%(key)s``
for each substitution. This way you can reorder or even ignore
placeholders in your new message.
When you are creating a validator, for maximum flexibility you should
use the :meth:`message` method, like::
messages = {
'key': 'my message (with a %(substitution)s)',
def _validate_python(self, value, state):
raise formencode.Invalid(self.message('key', state, substitution='apples'),
value, state)
Localization of Error Messages (i18n)
When a failed validation occurs FormEncode tries to output the error
message in the appropriate language. For this it uses the standard
`gettext <>`_ mechanism of
python. To translate the message in the appropriate message FormEncode
has to find a gettext function that translates the string. The
language to be translated into and the used domain is determined by
the found gettext function. To serve a standard translation mechanism
and to enable custom translations it looks in the following order to
find a gettext (``_``) function:
1. method of the :obj:`state` object
2. function :func:`__builtin__._`. This function is only used when::
Validator.use_builtin_gettext == True #True is default
3. formencode builtin :func:`_stdtrans` function
for standalone use of FormEncode. The language to use is determined
out of the locale system (see gettext documentation). Optionally you
can also set the language or the domain explicitly with the
formencode.api.set_stdtranslation(domain="FormEncode", languages=["de"])
Formencode comes with a Domain ``FormEncode`` and the corresponding
messages in the directory
4. Custom gettext function and addtional parameters
If you use a custom gettext function and you want FormEncode to
call your function with additional parameters you can set the
Available languages
All available languages are distributed with the code. You can see the
currently available languages in the source under the directory
If your language is not present yet, please consider contributing a
translation (where ``<lang>`` is you language code)::
$ svn co
$ easy_install Babel
$ python init_catalog -l <lang>
$ emacs formencode/i18n/<lang>/LC_MESSAGES/FormEncode.po # or whatever
# editor you prefer make the translation
$ python compile_catalog -l <lang>
Then test, and send the PO and MO files to
See also `the Python internationalization documents
Optionally you can also add a test of your language to
``tests/``. An Example of a language test::
ne = formencode.validators.NotEmpty()
def test_de():
_test_lang("de", u"Bitte einen Wert eingeben")
And the test for your language::
def test_<lang>():
_test_lang("<lang>", u"<translation of Not Empty Text in the language <lang>")
HTTP/HTML Form Input
The validation expects nested data structures; specifically
:class:`formencode.schema.Schema` and
:class:`formencode.foreach.ForEach` deal with these structures well.
HTML forms, however, do not produce nested structures -- they produce
flat structures with keys (input names) and associated values.
Validator includes the module :mod:`formencode.variabledecode`,
which allows you to encode nested dictionary and list structures into
a flat dictionary.
To do this it uses keys with ``"."`` for nested dictionaries, and
``"-int"`` for (ordered) lists. So something like:
| key | value |
| names-1.fname | John |
| names-1.lname | Doe |
| names-2.fname | Jane |
| names-2.lname | Brown |
| names-3 | Tim Smith |
| action | save |
| action.option | overwrite |
| action.confirm | yes |
Will be mapped to::
{'names': [{'fname': "John", 'lname': "Doe"},
{'fname': "Jane", 'lname': 'Brown'},
"Tim Smith"],
'action': {None: "save",
'option': "overwrite",
'confirm': "yes"},
In other words, ``'a.b'`` creates a dictionary in ``'a'``, with
``'b'`` as a key (and if ``'a'`` already had a value, then that value
is associated with the key ``None``). Lists are created with keys
with ``'-int'``, where they are ordered by the integer (the integers
are used for sorting, missing numbers in a sequence are ignored).
:class:`formencode.variabledecode.NestedVariables` is a validator that
decodes and encodes dictionaries using this algorithm. You can use it
with a Schema's `pre_validators`_ attribute.
Of course, in the example we use the data is rather eclectic -- for
instance, Tim Smith doesn't have his name separated into first and
last. Validators work best when you keep lists homogeneous. Also, it
is hard to access the ``'action'`` key in the example; storing the
options (action.option and action.confirm) under another key would be