I wonder if there is an open source CMS coded in Python that is as big as Drupal (or Joomla or Wordpress).
You are looking for Django. (Edited to say: OK, it's a web application framework, but there's lots of overlap. Django-CMS is maybe more what you want, but is in no way as big as Drupal or Joomla.)
Plone is an open source python powered CMS.
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