title: WTForms inline validation of more than 1 field
body: <p>I am using WTForms validation and I want to perform an operation on two fields and then return one error message.</p>
  
  <p>I know that I can do this:</p>
  
  <pre><code>class SignupForm(Form):
      age = IntegerField(u'Age')
  
      def validate_age(form, field):
          if field.data &lt; 13:
              raise ValidationError("We're sorry, you must be 13 or older to register")
  </code></pre>
  
  <p><br></p>
  
  <p>I realise that "validate_age" function is linked to the age field but I want to do something like this:</p>
  
  <pre><code>class SignupForm(Form):
      lowerage = IntegerField(u'LowerAge')
      upperage = IntegerField(u'UpperAge')
  
      def validate_ages(form, lowerage, upperage):
          if lowerage.data &lt; 13 &amp;&amp; upperage.data &gt; 65:
              raise ValidationError("We're sorry, you must be aged between 13 and 65older to register")
  </code></pre>
  
  <p><br></p>
  
  <p>I realise that code totally wouldn't work because of the way WTForms works but I want to perform custom validation on two fields and return one error message. Is this possible and if so how would it be done? 
  Thanks</p>