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A validator simply takes an input, verifies it fulfills some criterion, such as a maximum length for a string and returns. Or, if the validation fails, raises a :class:`~wtforms.validators.ValidationError`. This system is very simple and flexible, and allows you to chain any number of validators on fields.

Built-in validators

Custom validators

We will step through the evolution of writing a length-checking validator similar to the built-in :class:`Length` validator, starting from a case-specific one to a generic reusable validator.

Let's start with a simple form with a name field and its validation:

class MyForm(Form):
    name = TextField('Name', [Required()])

    def validate_name(form, field):
        if len( > 50:
            raise ValidationError('Name must be less than 50 characters')

Above, we show the use of an :ref:`in-line validator <inline-validators>` to do validation of a single field. In-line validators are good for validating special cases, but are not easily reusable. If, in the example above, the name field were to be split into two fields for first name and surname, you would have to duplicate your work to check two lengths.

So let's start on the process of splitting the validator out for re-use:

def my_length_check(form, field):
    if len( > 50:
        raise ValidationError('Field must be less than 50 characters')

class MyForm(Form):
    name = TextField('Name', [Required(), my_length_check])

All we've done here is move the exact same code out of the class and as a function. Since a validator can be any callable which accepts the two positional arguments form and field, this is perfectly fine, but the validator is very special-cased.

Instead, we can turn our validator into a more powerful one by making it a factory which returns a callable:

def length(min=-1, max=-1):
    message = 'Must be between %d and %d characters long.' % (min, max)

    def _length(form, field):
        l = and len( or 0
        if l < min or max != -1 and l > max:
            raise ValidationError(message)

    return _length

class MyForm(Form):
    name = TextField('Name', [Required(), length(max=50)])

Now we have a configurable length-checking validator that handles both minimum and maximum lengths. When length(max=50) is passed in your validators list, it returns the enclosed _length function as a closure, which is used in the field's validation chain.

This is now an acceptable validator, but we recommend that for reusability, you use the pattern of allowing the error message to be customized via passing a message= parameter:

class Length(object):
    def __init__(self, min=-1, max=-1, message=None):
        self.min = min
        self.max = max
        if not message:
            message = u'Field must be between %i and %i characters long.' % (min, max)
        self.message = message

    def __call__(self, form, field):
        l = and len( or 0
        if l < self.min or self.max != -1 and l > self.max:
            raise ValidationError(self.message)

length = Length

In addition to allowing the error message to be customized, we've now converted the length validator to a class. This wasn't necessary, but we did this to illustrate how one would do so. Because fields will accept any callable as a validator, callable classes are just as applicable. For complex validators, or using inheritance, you may prefer this.

We aliased the Length class back to the original length name in the above example. This allows you to keep API compatibility as you move your validators from factories to classes, and thus we recommend this for those writing validators they will share.

Setting flags on the field with validators

Sometimes, it's useful to know if a validator is present on a given field, like for use in template code. To do this, validators are allowed to specify flags which will then be available on the :attr:`field's flags object <wtforms.fields.Field.flags>`. Some of the built-in validators such as :class:`Required` already do this.

To specify flags on your validator, set the field_flags attribute on your validator. When the Field is constructed, the flags with the same name will be set to True on your field. For example, let's imagine a validator that validates that input is valid BBCode. We can set a flag on the field then to signify that the field accepts BBCode:

# class implementation
class ValidBBCode(object):
    field_flags = ('accepts_bbcode', )

    pass # validator implementation here

# factory implementation
def valid_bbcode():
    def _valid_bbcode(form, field):
        pass # validator implementation here

    _valid_bbcode.field_flags = ('accepts_bbcode', )
    return _valid_bbcode

Then we can check it in our template, so we can then place a note to the user:

{{ field(rows=7, cols=70) }}
{% if field.flags.accepts_bbcode %}
    <div class="note">This field accepts BBCode formatting as input.</div>
{% endif %}

Some considerations on using flags:

  • Flags can only set boolean values, and another validator cannot unset them.
  • If multiple fields set the same flag, its value is still True.
  • Flags are set from validators only in :meth:`Field.__init__`, so inline validators and extra passed-in validators cannot set them.