Pyhon/Odoo11: Error while validating constraint '<' not supported between instances of 'str' and 'datetime.date'

Question

I am new to python and not sure how to fix this error. Please guide me. I want to have validation for the date field where if the date field has past date which is lesser than the current date then it should give an error message and shouldn't save the value despite I click the 'Save' button.

I get the error like I mentioned in the title.

Here's the code

    from odoo import api, fields, models
    from odoo.exceptions import ValidationError
    from datetime import datetime    

    now = datetime.now()
    curdat = now.strftime("%x")

    @api.multi
    @api.constrains('date', 'curdat')
    def date_constrains(self):
        if self.date < self.curdat:
            raise ValidationError('Date must be today date or greater than today date...')