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   print('%s %.2f' % ("hue =",swSky1.hue))
   print('{0} {1:.2f}'.format("hue =",swSky1.hue))
   print("hue =", format(swSky1.hue,'.2f'))

The three lines above give the same result ````hue = 220.00````.
The first line is the old way, “s” means string and “f” means float. The second line is the new way. 0 in {} refer to the first variable and 1:.2f in {} refers to the second variable swSky1.hue and format it to 2 digits behind decimal point.

Reference:
- Using % and .format() for great good!: https://pyformat.info/

py37-matplotlib has the following notes:
The default backend is the interactive Mac OS X backend. Different backends can be specified using the ~/.matplotlib/matplotlibrc file. More details regarding backends can be found in the matplotlib FAQ: https://matplotlib.org/tutorials/introductory/usage.html#backends

    File "main.py", line 15
    self.sat = (cMax - cMin)/(1-abs(2*self.lum-1))
                                                 ^
    TabError: inconsistent use of tabs and spaces in indentation

Convert all tab to space will solve this problem.