Dp Composite

In this tutorial, you'll learn about How to Fix Composite Errors. We cover key concepts, practical examples, and best practices.

Fix composite errors when leaf and composite treated differently requiring type checking.

Quick Fix

Wrong

class File:
    def __init__(self,n): self.name=n
    def ls(self): print(self.name)
class Dir:
    def __init__(self,n):
        self.name=n; self.children=[]
    def ls(self):
        print(self.name)
        for c in self.children:
            if isinstance(c,File): c.ls()
            else: c.ls()  # still works but type-specific logic elsewhere

Client must check type. Adding new Component type requires updating all call sites.

Right

class Component:
    def __init__(self,n): self.name=n
    def ls(self): print(self.name)
class File(Component): pass
class Dir(Component):
    def __init__(self,n):
        super().__init__(n); self.children=[]
    def add(self,c): self.children.append(c)
    def ls(self):
        print(self.name); [c.ls() for c in self.children]

root=Dir('root'); root.add(File('a.txt')); root.ls(). Leaf and Dir treated uniformly.

Prevention

Composite lets clients treat individual objects and compositions uniformly. Same interface for leaf and composite.

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