I am using Python 2.7.11 for the coding challenge

admins-MacBook-Air:insight-data-challenge admin$ python
Python 2.7.11 (default, Jan 22 2016, 08:29:18) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

I used the parser.parse method of the python-dateutil module to parse date and time in the raw tweet json and used the SortedListWithKey collection from the sortedcontainers module to maintain an ordered collection of tweets based on their created_at timestamps. Please use pip to install the required python modules by running the command pip install -r requirements.txt

(venv) admins-MacBook-Air:insight-data-challenge admin$ pip install -r requirements.txt 
Collecting python-dateutil==2.5.2 (from -r requirements.txt (line 1))
  Using cached python_dateutil-2.5.2-py2.py3-none-any.whl
Collecting six==1.10.0 (from -r requirements.txt (line 2))
  Using cached six-1.10.0-py2.py3-none-any.whl
Collecting sortedcontainers==1.4.4 (from -r requirements.txt (line 3))
Installing collected packages: six, python-dateutil, sortedcontainers
Successfully installed python-dateutil-2.5.2 six-1.10.0 sortedcontainers-1.4.4

The following classes in src/utils/twitter.py help calculate the average degree of a vertex in a Twitter hashtag graph for the last 60 seconds, and update this each time a new tweet appears

1. Tweet Class - Represents a Tweet with has some attributes defined
2. TweetWithRawData Class - Derived class of Tweet, which contains the raw text
3. TwitterNode Class - Represents a Twitter Vertex or Node on the Graph
4. TwitterNodeGraph Class - Represents a graph with Twitter Vertices or Nodes and Edges between them
5. TwitterNodeGraphWithSlidingWindow Class - Similar to TwitterNodeGraph but has a sliding window

I have added tests in insight_testsuite/tests

Additionally, I have unittests defined in src/test/ directory to validate code in src/utils/twitter.py I would have like to clear them up, but didn't have enough time.