How do I validate wtforms fields against one another?

Question

I have three identical SelectField inputs in a form, each with the same set of options. I can't use one multiple select.

I want to make sure that the user selects three different choices for these three fields.

In custom validation, it appears that you can only reference one field at a time, not compare the value of this field to others. How can I do that? Thanks!

Solution 1

You can override validate in your Form...

class MyForm(Form):
    select1 = SelectField('Select 1', ...)
    select2 = SelectField('Select 2', ...)
    select3 = SelectField('Select 3', ...)
    def validate(self):
        if not Form.validate(self):
            return False
        result = True
        seen = set()
        for field in [self.select1, self.select2, self.select3]:
            if field.data in seen:
                field.errors.append('Please select three distinct choices.')
                result = False
            else:
                seen.add(field.data)
        return result

Solution 2

You can use the form in your validation to get the value of other fields.

For example:

def validate_name(form, field):
    if form.other_variable.data == 'checked' and len(field.data) > 10:
        raise validation_error("say somgthing")

Solution 3

I wrote a small python library required to make cross-field validation like this easier. You can encode your validation logic declaratively as pairwise dependencies. So your form may look like:

from required import R, Requires, RequirementError

class MyForm(Form):

    VALIDATION = (
        Requires("select1", R("select1") != R("select2") +
        Requires("select2", R("select2") != R("select3") +
        Requires("select3", R("select3") != R("select1")
    )

    select1 = SelectField('Select 1', ...)
    select2 = SelectField('Select 2', ...)
    select3 = SelectField('Select 3', ...)

    def validate(self):
        data = {
            "select1": self.select1.data,
            "select2": self.select2.data,
            "select3": self.select3.data,
        }

        # you can catch the RequirementError
        # and append the error message to 
        # the form errors

        self.VALIDATION.validate(data)
        return result

You can take the VALIDATION object and append more validation rules or even put it in a separate module and import / reuse validation rules in different places.

Comments

[YPCrumble]

Thanks - why the line if not Form.validate(self):? That keeps firing when I try your suggestion and the validation doesn't run.

[FogleBird]

That's to do the default validation (as if you hadn't overridden validate)

[FogleBird]

The other steps are to perform validation across multiple fields, as you asked.

[Jimmy]

I just want to add, that I was confused by overriding here. Referring to O'Reilly Learning Python, I would call this extending the validate method. As you're still invoking the default behavior when you call if not Form.validate(self) And then the custom validation code runs. When I read that, it cleared things up for me. Still @FogleBird great answer

[Dan Passaro]

If you're on Python 3, better to write super().validate() than Form.validate(self). If you're on Python 2, use super(MyForm, self).validate().