iktomi.forms.convs.Converter instances are responsible for all staff related to data validation and convertation.

Converters implement copy interface.

Value convertation

Converter subclasses should define methods for transformations in two directions:

  • to_python method accepts unicode value of user info, and returns value converted to python object of defined type. If the value can not be converted, it raises iktomi.forms.convs.ValidationError.
  • Filters and validators provide extra validation and are called after to_python method. See below for details.
  • from_python method accepts python object and returns corresponding unicode string.


The common way for converter to indicate that given value can not be accepted is to raise ValidationError from to_python method:

def to_python(self, value):
    if not self.match(value):
        raise ValidationError(error_message)
    return value

If ValidationError was raised, the error is added to form.errors dictionary. The key is current field’s input name and the value is

In the case ValidationError occured, converter returns field’s initial/last valid value. In other words, the value is reverted to it’s last valid state, basically it is initial state.

Raise error for other field

Sometimes you need to show error not on a field which is validating, or show error on multiple fields on the same condition. In this case ValidationError.by_field argument can be used.

Just pass in by_field kwarg a dict where a key is input_name of any field in the form and a value is error message:

raise convs.ValidationError(by_field={
        'name': 'Provide a name or a nickname',
        'nickname': 'Provide a name or a nickname'})

Relative field input names can be used. If a name starts with a dot, conv.field.get_field will be used to get target field. If it starts with two dots, conv.field.parent.get_field will be used, three dots - conv.field.parent.parent.get_field, etc.

Why not to set form.errors[field.input_name] directly? Trust me, it is not good idea! One reason is conv.accept silent mode, used to fill in initial values of the field and sometimes can be used to call converter as a function without form attached and error handling. Other reasons?

Error messages redefinition

Default converters support redefinition of default error messages.

Set error_<type> parameter to your own message template, for example:

           error_regex='Should match YY.MM.DD',
           error_required='Do not forget this field!')

Require Check

The most used feature is require check. If the converter has require attribute set to True, it checks whether to_python result is an empty value:


Empty values are empty string, emty list, empty dict and None. If the result value is equal to one of these values, ValidationError with conv.error_required is raised.

ListOf and Multiple Values

Multiple values with same key in raw data are supported by ListOf converter.

ListOf gets all values by field’s key from raw data MultiDict, applies ListOf.conv to each of them, and collects non-empty results into a list. If ValidationError was raised in ListOf.conv, the value is also ignored:

class MyForm(Form):

    fields = [

# ids=1&ids=2&ids=x =>
# {"ids" [1, 2]}

The multiple property of fields and widgets having ListOf converter, is equal to True.

Filters and validators:

Additional validation and simple one-way convertation can be made by validators:

      Char(strip, length(0, 100), required=True))

Filters are functions performing additional validation and convertation after to_python() method. The interface of filters is following:

def filter_value(conv, value):
    if wrong(value):
        raise ValidationError(..)
    new_value = do_smth(value)
    return new_value

convs.Char(filter_value, required=True)

Validators are shortcuts to filters that do no convertations, but only do assertions:

def validate(conv, value):
    return is_valid(value)

Both filters and validators can be passed to converter as positional arguments and will be applied after to_python() method and required check in order they are mentioned.


Iktomi implements very basic internationalization support. There are N_ and M_ markers for single and plural translatable strings respectively.

There is no complex mechanics with threadlocals or other things allowing to transparently “in place” and lazy translate these strings. Iktomi by default supports only translation of ValidationError messages before they are added in form.errors.

For single messages env.gettext is called, and for plural ones env.ngettext is called. You must provide these methods in your Application subclass.

Here is an example of how plural merker works. Dictionary formatting with % is used and a key M_.count_field is used as count indicator to ngettext:

message = M_(u'must be less than %(max)d symbol',
             u'must be less than %(max)d symbols',
def validate(conv, value):
    max_length = get_max_length(conv)
    if len(value) > max_length:
        message = message % dict(max=max_length)
        raise convs.ValidationError(message)
    return value

Converters for Aggregate Fields

Collective validation

FieldSet and FieldBlock converters are good place to implement a complex validation rules, including data from more than one field.

You can implement them in to_python method of converter or in a validator. To access a value of child field just get it from actual dict by a field name:

#def validate(conv, value):
def to_python(self, value):
    if value['field1'] == value['field2']:
        raise ValidationError('values must not be equal')
    return value

ValidationError.by_field feature also can be useful here.

Custom FieldSet Value Type

To get a custom object as a clean value of FieldSet, you can define own Converter subclass implementing transformations from an object to dictionary (in from_python method) and from dictionary to an object (in to_python).

The most basic example of converter of this kind:

class ObjConv(Converter):

    def from_python(self, value):
        result = {}
        # in case there are nested FieldBlock fields, always use field.field_names
        # to get a list of fields directly contained in the value
        field_names = sum([x.field_names for x in self.field.fields], [])
        for field_name in field_names:
            result[field_name] = getattr(value, field_name)
        return result

    def to_python(self, value):
        return self.model(** value)

You can see iktomi.unstable.forms.convs.ModelDictConv as an example of custom FieldSet converter.

Converter implementations

Examples of converters are Int, Char, Html, Bool, Date, etc.