How to change priority of WTForms validators? - validation

Question

tl;dr: I need to change the order in which WTForms validators validate the user input. How do I do that?
Details:
Flask code:
class SampleForm(Form):
user_email = user_email_field
...
#api.route('/sample-route')
class ClassName(Resource):
#api.expect(sample_payload)
#api.marshal_with(sample_response)
def post(self):
form = SampleForm(formdata=MultiDict(api.payload))
if not form.validate():
return {"form_errors": form.errors}, 400
...
WTForms validation field:
user_email_field = EmailField('Email Address',[
validators.InputRequired(Errors.REQUIRED_FIELD),
validators.Length(min=5, max=256),
validators.Email(Errors.INVALID_EMAIL_ADDRESS),
])
Problem is, user_email is checked by validators in the wrong order. I send a request with the following body:
{
"user_email": ""
}
I get this response:
{
"form_errors": {
"user_email": [
"'' is too short"
]
}
}
As you see, despite being 2nd in the list of validators, validators.Length() kicks in before everything else.
If I comment it out in the validation field like that:
user_email_field = EmailField('Email Address',[
validators.InputRequired(Errors.REQUIRED_FIELD),
# validators.Length(min=5, max=256),
validators.Email(Errors.INVALID_EMAIL_ADDRESS),
])
then the exact same request will yield a desired response:
{
"errors": null,
"success": null,
"form_errors": {
"user_email": [
"REQUIRED_FIELD"
]
}
}
However, this is not a working solution because, of course, then eMail won't be checked for its length.
Question:
How do I change the priority of these validators? How do I make WTForms always check the user input with validators.InputRequired() FIRST and with validators.Length() SECOND and not the other way around?

Answer 1

As per your question:
You can use custom validators for that purpose. Then you can change the order of validation by if, elif, else logic within.
class Test(Form):
test_field = StringField('Test_Field')
submit = SubmitField()
def validate_test_field(self, test_field):
try:
if len(test_field.data) == 0:
raise wtforms.validators.ValidationError('Input is required for this field!')
elif len(test_field.data) not in range(5, 257):
raise wtforms.validators.ValidationError(f'Error in input length.')
except TypeError as e:
raise wtforms.validators.ValidationError('Input is required for this field!')
Check the documentation for custom validators here