_:n9a2b20aff6dd4a6bb0dea39861c7a860xb1
<http://schema.org/text>
"I am not sure what \"pres = pres or set()\" this is doing?  Is it passing in logic into a variable?\n```courses.py\ncourses = {'count': 2,\n           'title': 'Django Basics',\n           'prereqs': [{'count': 3,\n                     'title': 'Object-Oriented Python',\n                     'prereqs': [{'count': 1,\n                               'title': 'Python Collections',\n                               'prereqs': [{'count':0,\n                                         'title': 'Python Basics',\n                                         'prereqs': []}]},\n                              {'count': 0,\n                               'title': 'Python Basics',\n                               'prereqs': []},\n                              {'count': 0,\n                               'title': 'Setting Up a Local Python Environment',\n                               'prereqs': []}]},\n                     {'count': 0,\n                      'title': 'Flask Basics',\n                      'prereqs': []}]}\ndef prereqs(data, pres=None):\n    pres = pres or set()\n```"
<https://teamtreehouse.com/community/prerequisite-challenge-pres-pres-or-set>