Composition vs Inheritance

The Idea

Inheritance says a child is its parent, forever, decided at import time. Composition says an object has parts it can call, chosen when you build it — or later. Prefer composition: parts recombine, and a change to one part does not ripple through a family of subclasses.

Real-World Example

A camera rig versus a camcorder. The rig is a body, a lens, a gimbal and a mic that clip together, so a low-light shoot swaps one piece. The camcorder decided all four at the factory, and the only upgrade is a new camcorder.

The Code

class Belt:
    def apply(self, item): return item + " +moved"
class Stamper:
    def apply(self, item): return item + " +stamped"

class Line:                              # HAS-A list of stages; it IS-A nothing
    def __init__(self, *stages): self.stages = list(stages)
    def run(self, item):
        for s in self.stages: item = s.apply(item)
        return item

line = Line(Belt())
print(line.run("blank"))                 # blank +moved
line.stages.append(Stamper())            # new behaviour at runtime, no subclass
print(line.run("blank"))                 # blank +moved +stamped

class Wrapper:
  def apply(self, item): return "[" + item + "]"

class Line:
  def __init__(self, *stages): self.stages = list(stages)
  def run(self, item):
      for s in self.stages: item = ___
      return item

print(Line(Wrapper(), Wrapper()).run("x"))   # should print [[x]]