Python 확장 내장형 구현 방법 분석

class Set:
  def __init__(self, value=[]): # Constructor
    self.data = [] # Manages a list
    self.concat(value)
  def intersect(self, other): # other is any sequence
    res = [] # self is the subject
    for x in self.data:
      if x in other: # Pick common items
        res.append(x)
    return Set(res) # Return a new Set
  def union(self, other): # other is any sequence
    res = self.data[:] # Copy of my list
    for x in other: # Add items in other
      if not x in res:
        res.append(x)
    return Set(res)
  def concat(self, value): # value: list, Set...
    for x in value: # Removes duplicates
      if not x in self.data:
        self.data.append(x)
  def __len__(self):     return len(self.data) # len(self)
  def __getitem__(self, key): return self.data[key] # self[i]
  def __and__(self, other):  return self.intersect(other) # self & other
  def __or__(self, other):  return self.union(other) # self | other
  def __repr__(self):     return 'Set:' + repr(self.data) # print()
if __name__ == '__main__':
  x = Set([1, 3, 5, 7])
  print(x.union(Set([1, 4, 7]))) # prints Set:[1, 3, 5, 7, 4]
  print(x | Set([1, 4, 6])) # prints Set:[1, 3, 5, 7, 4, 6]