It is used for preventing users from entering incorrect data

Example 1

                  from odoo.exceptions import ValidationError
                  .... 
                  .... 

                  @api.constrains('date_end')
                  def _check_date_end(self):
                      for record in self:
                          if record.date_end < fields.Date.today():
                              raise ValidationError("The end date cannot be set in the past")

                          

                @api.constrains('name', 'description')
def _check_description(self):
for record in self:
    
if record.name == record.description:
        
raise ValidationError("Fields name and description must be different")

the constraints specified to the field ‘age’ in a model check if the age is greater than 30 or not.

@api.constrains('age')
def _check_something(self):
    for record in self:
        if record.age > 30:
            raise ValidationError(“Age is greater than”)

        

        from odoo.exceptions import ValidationError
@api.constrains(‘amount’)
def _check_something(self):
    for record in self:
        if record.amount < 0:
            raise ValidationError(“Amount cannot be negative”)

        

        @api.constrains(field1, field2)
def _check_values(self):
    for record in self:
        if record.field1 == record.field2:
            raise ValidationError("Fields field1 and field2 must be not be same")

        

        @api.constrains('instructor_id', 'attendee_ids')
    def _check_instructor_not_in_attendees(self):
        for r in self:
            if r.instructor_id and r.instructor_id in r.attendee_ids:
                raise exceptions.ValidationError("A session's instructor can't be an attendee")

            

            from openerp.exceptions import ValidationError

                @api.constraints('product_qty', 'cost_unit')
                def _check_something(self):
                    for record in self:
                        if record.product_qty < 1:
                            raise ValidationError("Qty must be more than 0")