title: In Google App Engine, how to check input validity of Key created by urlsafe?
body: <p>Suppose I create a key from user input websafe url</p>
  
  <pre><code>key = ndb.Key(urlsafe=some_user_input)
  </code></pre>
  
  <p>How can I check if the <code>some_user_input</code> is valid?</p>
  
  <p>My current experiment shows that statement above will throw <code>ProtocolBufferDecodeError (Unable to merge from string.)</code> exception if the <code>some_user_input</code> is invalid, but could not find anything about this from the API. Could someone kindly confirm this, and point me some better way for user input validity checking instead of catching the exception?</p>
  
  <p>Thanks a lot!</p>