apply-prototype-pattern

name	apply-prototype-pattern
description	Use when creating a new object by copying an existing instance is cheaper or simpler than constructing from scratch — especially when object initialization is expensive or when the exact type of object to create is determined at runtime.
source	Gamma, Helm, Johnson, Vlissides, "Design Patterns: Elements of Reusable Object-Oriented Software" (1994) pp. 117–126; Python built-in copy module (copy/deepcopy); JavaScript Object.create() and spread syntax; Java Cloneable interface
tags	["design-patterns","creational","prototype","oop","developer","cloning","performance"]
related	["apply-abstract-factory-pattern","apply-builder-pattern"]

Apply Prototype Pattern

Create new objects by copying an existing prototype rather than constructing from scratch.

Why This Is Best Practice

Adopted by: JavaScript (the prototype-based object model is the language's core inheritance mechanism; Object.create() is a direct implementation), Python (copy module's copy() and deepcopy() are standard library prototype operations), Java (Cloneable interface and Object.clone()), and Unity game engine (prefab instantiation is a prototype clone operation at the engine level). Impact: GoF documents the prototype's primary benefit: when the cost of creating an object is significantly greater than the cost of copying one, prototypes reduce construction cost to memcopy speed. In game engines (Unity), instantiating thousands of enemies from a single prefab prototype is an order-of-magnitude faster than constructing each from scratch with physics body initialization. Why best: The alternative — reconstructing the full object state every time — is wasteful when the state was expensive to compute. The prototype also eliminates coupling to constructor signatures: the client copies the object without knowing its concrete class.

Sources: Gamma et al. (1994) pp. 117–126; Python copy module documentation; Unity Manual: Instantiating Prefabs

Steps

Step 1: Add a clone() method to the prototype interface

from abc import ABC, abstractmethod
import copy

class Prototype(ABC):
    @abstractmethod
    def clone(self) -> "Prototype": ...

Step 2: Implement clone() using shallow or deep copy

class GameEnemy(Prototype):
    def __init__(self, name: str, health: int, position: tuple, inventory: list):
        self.name = name
        self.health = health
        self.position = position
        self.inventory = inventory

    def clone(self) -> "GameEnemy":
        return copy.deepcopy(self)   # deep copy: inventory list is independent

Use deepcopy when the prototype contains mutable nested objects that each clone should own independently. Use copy (shallow) only when nested objects are shared by design or are immutable.

Step 3: Create a prototype registry for named prototypes

class EnemyRegistry:
    def __init__(self):
        self._prototypes: dict[str, GameEnemy] = {}

    def register(self, name: str, prototype: GameEnemy) -> None:
        self._prototypes[name] = prototype

    def create(self, name: str) -> GameEnemy:
        prototype = self._prototypes.get(name)
        if prototype is None:
            raise KeyError(f"no prototype registered for '{name}'")
        return prototype.clone()

registry = EnemyRegistry()
registry.register("orc", GameEnemy("Orc", 100, (0, 0), ["sword"]))

# Later — create many orcs cheaply
orc1 = registry.create("orc")
orc2 = registry.create("orc")

Step 4: Mutate the clone, not the prototype

After cloning, modify only the clone's instance-specific state:

orc1.position = (10, 5)
orc1.health = 80   # damaged variant
# registry's prototype is unchanged

When NOT to Use

• Simple objects with cheap construction — if new Object() is nearly free, cloning adds complexity with no benefit.
• Objects with non-copyable resources — file handles, network connections, and database connections cannot be meaningfully cloned. Use a factory instead.
• Deep object graphs with circular references — deepcopy handles these but can be slow and surprising. Test explicitly.

Common Mistakes

Shallow copy when deep copy is needed. If the prototype holds a list, a shallow copy shares the list between prototype and clone. Mutating the clone's list mutates the prototype. Use deepcopy unless sharing is intentional.

Modifying the prototype instead of the clone. The registry prototype is the template. Any mutation to it affects all future clones. Treat registered prototypes as read-only after registration.

Implementing clone() by calling the constructor. Calling ConcreteProduct(self.field1, self.field2, ...) in clone() defeats the pattern — it re-couples clone() to the constructor signature. Use copy.deepcopy(self) or equivalent.