title: Can webargs / Marshmallow in Python modify a field, not just validate it?
body: <p>I'm using Flask with flask-restful and <a href="https://webargs.readthedocs.io/en/latest/" rel="nofollow noreferrer">webargs</a> (which uses <a href="https://marshmallow.readthedocs.io/en/latest/" rel="nofollow noreferrer">Marshmallow</a> as its backend).  Currently I'm able to pull in fields I want with this:</p>
  <pre><code>class AddGroup(Resource):
      args = {
          'name': fields.Str(missing=None),
          'phone': fields.Str(missing=None),
      }
  
      @use_args(args)
      def get(self, args):
          name = args['name'].strip()
          # ... some GET-related code ...
  
      @use_args(args)
      def post(self, args):
          name = args['name'].strip()
          # ... some POST-related code ...
  </code></pre>
  <p>So far so good.  But what I'd really like to do is make sure that <code>args['name']</code> comes into the various methods (&quot;post&quot;, &quot;get&quot;, etc.) with whitespace already stripped, so I don't have to process each variable manually each time.  (stripping whitespace is just an example - it may be some other simple or complex transformation)</p>
  <p>Is there a way (either by overriding the String field, defining my own field, or whatever) that will allow the <code>args</code> to be pre-processed before they arrive in the class methods?</p>