title: Dynamic form fields in flask.request.form
body: <p>I've looked through the documentation, but for the life of me, I can't figure out how the <code>request.form</code> object in Flask is populated.  <a href="http://flask.pocoo.org/docs/api/#flask.Request.form">The documentation</a> says that it's filled with parsed form data from POST or PUT requests, but my form is dynamic so I don't necessarily know what fields exist when the POST request is sent - though I want to make sure I add the information from these fields to the database.</p>
  
  <p>Some of the fields in the form are always there, but there will also be any number of extra fields from a list of about 60.  How should I go about figuring out which of these additional fields are in the request and how should I get the data from them?</p>
  
  <p>EDIT: My specific problem has been solved, but it's still worth asking how the request.form dictionary is populated.  I found out the hard way that if a checkbox input is unchecked, there is no key added to the dictionary under its name, and trying to get the value of a key that does not exist from the dictionary results in a rather confusing and cryptic <code>HTTP 400 BAD REQUEST</code> error.</p>