Flask-WTForms throws error for IntegerField instead of failing validation

Question

When I create a form using wtf_forms and Flask-WTF and use the IntegerField input, I can't use it in combination with the Length validator

If I remove the Length restriction then it works fine. Surely I should be able to apply a Length validation to an IntegerField?

Python Code.

from flask_wtf import Form
from wtforms import TextField, PasswordField, IntegerField, validators

class RegistrationForm(Form):
    firstname = TextField('First Name', [validators.Required()])
    lastname = TextField('Last Name', [validators.Required()])
    telephone = IntegerField('Telephone', [validators.Length(min=10, max=10, message="Telephone should be 10 digits (no spaces)")])

TypeError
TypeError: object of type 'int' has no len()

Traceback (most recent call last)
File "C:\Python27\lib\site-packages\flask\app.py", line 1701, in __call__
return self.wsgi_app(environ, start_response)
File "C:\Python27\lib\site-packages\flask\app.py", line 1689, in wsgi_app
response = self.make_response(self.handle_exception(e))
File "C:\Python27\lib\site-packages\flask\app.py", line 1687, in wsgi_app
response = self.full_dispatch_request()
File "C:\Python27\lib\site-packages\flask\app.py", line 1360, in full_dispatch_request
rv = self.handle_user_exception(e)
File "C:\Python27\lib\site-packages\flask\app.py", line 1358, in full_dispatch_request
rv = self.dispatch_request()
File "C:\Python27\lib\site-packages\flask\app.py", line 1344, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "C:\index.py", line 45, in submit
if form.validate_on_submit():
File "C:\Python27\lib\site-packages\flask_wtf\form.py", line 156, in validate_on_submit
return self.is_submitted() and self.validate()
File "C:\Python27\lib\site-packages\wtforms\form.py", line 271, in validate
return super(Form, self).validate(extra)
File "C:\Python27\lib\site-packages\wtforms\form.py", line 130, in validate
if not field.validate(self, extra):
File "C:\Python27\lib\site-packages\wtforms\fields\core.py", line 175, in validate
stop_validation = self._run_validation_chain(form, chain)
File "C:\Python27\lib\site-packages\wtforms\fields\core.py", line 195, in _run_validation_chain
validator(form, self)
File "C:\Python27\lib\site-packages\wtforms\validators.py", line 91, in __call__
l = field.data and len(field.data) or 0
TypeError: object of type 'long' has no len()

Answer 1

The error below means that you are trying to check the length of an integer which is not allowed by python. If you want to check length, then it must be a string. IntegerField() however by definition is an integer

object of type 'int' has no len()

You need to create something like below. NumberRange takes a range of numbers.

IntegerField('Telephone', [validators.NumberRange(min=0, max=10)])

Alternatively,I suggest you use a FormField and define your own telephone field. There is an exact example here to create a telephone field:

http://wtforms.simplecodes.com/docs/0.6.1/fields.html#wtforms.fields.FormField