title: How can I do form-level validation in WTForms?
body: <p>I'm building lots of forms in an application with wtforms. I have a need to perform, and present the user with, "form-level" validation errors.</p>
  
  <p>There is no documented/supported way to do this. That is fine. </p>
  
  <p>I know I can override the forms validate method, perform my checks there, and put my additional validation errors on the _errors property of the form.</p>
  
  <p>The problem with this approach is that form._errors also contains all of the field-level validation errors (which also, of course, are accessible via the errors property of each field).</p>
  
  <p>So, I want to have a clean, API consistent way to return <em>only</em> form-level validation errors. I can hack this myself on the _error property, but I'm interested in other solutions to this problem.</p>
  
  <p>How would you do form-level validation in wtforms?</p>