Checking all checkboxes with Selenium

I am trying to check all checkboxes on a page similarily to http://ift.tt/2htoZ09.

I try to do this with

checkboxes = driver.find_elements_by_class_name('gwt-CheckBox')
for checkbox in checkboxes:
    if not checkbox.is_selected():
        checkbox.click()

however, all checkboxes give False for is_selected() so the ones that are already selected get switched off and the ones that are not selected get switched on. Below I have an example of the a checked CheckBox class (Library Name) and an unchecked Checkbox class (Paired nominal length).

How can I check if these are already selected?

  <div style="position: relative; display: inline-block; vertical-align: top; float: left;">
   <span class="gwt-CheckBox">
    <input id="gwt-uid-27" tabindex="0" type="checkbox" value="on"/>
    <label for="gwt-uid-27">
     Library name
    </label>
   </span>
  </div>
 </div>
 <div class="holderFp" style="width: 100%; position: relative; overflow: hidden; display: block;">
  <div style="position: relative; display: inline-block; vertical-align: top; float: left; width: 25%;">
   <span class="gwt-CheckBox">
    <input id="gwt-uid-28" tabindex="0" type="checkbox" value="on"/>
    <label for="gwt-uid-28">
     Paired nominal length
    </label>
   </span>
  </div>