For what it's worth, GenerateRandomColour does not access any instance variables and therefore needn't even be a member of the class Population. If you wanted it there for encapsulation purposes, consider making it a staticmethod like so:
@staticmethod()
def GenerateRandomColour():
rn.seed()
colour = rn.choice(['C', 'P', 'Z'])
return colour
Just something to consider...
LL
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