Sage Cell Server has migrated to Python 3

I recently discovered that many of the SageMath cells in the web version of Applied Discrete Structures (ADS) were no longer working as they had in the past. Then I found out why.  Apparently, 2020 is the Y2K of the Python Universe.  ... and I was oblivious to it!

The Python 2 language, i.e. Python 2.7.x, is officially being discontinued on January 1, 2020 (first planned for 2015) after which security patches and other improvements will not be released for it.^[29][30] With Python 2's end-of-life, only Python 3.5.x^[31] and later will be supported.

Due to differences in the two versions of Python, there are two main impacts on the code on the current web version of ADS.

• "print expr" produces an error and needs to be replaced with "print(expr)"
• map produces an iterator instead of a list.  This is fixed by wrapping  list( ) around the map expression if the original output is as desired.  More about iterators.
• keys of a dictionary are are also an iterator and using list is the fix in this situation too.