title: Pass label for field to WTForms __init__
body: <p>I want to pass the label for a form field as an argument to the form's <code>__init__</code>. Right now I get a <code>NameError: name 'self' is not defined</code> error. </p>
  
  <pre><code>class MyForm(FlaskForm):
      def __init__(self, label_t, **kw):
          super(MyForm, self).__init__(**kw)
          self.label_t = label_t
  
      name = StringField(self.label_t, validators=[DataRequired()])
  
  f = MyForm("test1", csrf_enabled=False)
  </code></pre>
  
  <p>I also tried putting the name variable into the init function, 
  but I get <code>AttributeError: 'UnboundField' object has no attribute '__call__'</code>.</p>
  
  <pre><code>class MyForm(FlaskForm):
      def __init__(self, label_t, **kw):
          super(MyForm, self).__init__(**kw)
          self.label_t = label_t
          self.name = StringField(self.label_t, validators=[DataRequired()])
  </code></pre>
  
  <p>How can I pass the label for a field to <code>__init__</code>?</p>