diamond problem:
python 2:
Use depth first traversal, method resolution order (MRO): D, B, A, C, A.
Problem:
python 3:
Use C3 Linearization Algorithm, MRO: D, B, C, A
C3 linearization
Private Attributes and Methods只能在那個class下使用。
語法:前面加兩個底線(後面不用)
Example: