title: top-level marshmallow schema validation
body: <p>From <a href="http://marshmallow.readthedocs.org/en/latest/quickstart.html#validation" rel="noreferrer">Marshmallow#validation</a>, I know I can register validators on specific fields in a Schema. If a validator fails, errors in :</p>
  
  <pre><code>data, errors = MySchema().load({"some":"data})
  </code></pre>
  
  <p>will include error information for any field which has failed validators :</p>
  
  <pre><code>errors
  # =&gt; some error message for the field that failed 
  </code></pre>
  
  <p>My question : Is it possible to validate at the Schema level (rather than at individual field level) and still return an error in the above way? </p>
  
  <p>As an arbitrary example, say I wanted to validate that you tried to <code>MySchema().load()</code> n distinct keys.
  I currently have a <code>@pre_load</code> method which checks the structure of the input and <code>raise ValidationError('message')</code> if the data is ill-formed, but I would like to return it as result.errors like field validation does. What are my options?</p>