🎉 Hooray! After 3 years of work, I've finally released the ebook on design patterns! Check it out »

Prototype in Python

Prototype is a creational design pattern that allows cloning objects, even complex ones, without coupling to their specific classes.

All prototype classes should have a common interface that makes it possible to copy objects even if their concrete classes are unknown. Prototype objects can produce full copies since objects of the same class can access each other’s private fields.

Usage of the pattern in Python



Usage examples: The Prototype pattern is available in Python out of the box with a Cloneable interface.

Identification: The prototype can be easily recognized by a clone or copy methods, etc.

Conceptual Example

This example illustrates the structure of the Prototype design pattern. It focuses on answering these questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

main.py: Conceptual Example

from __future__ import annotations
from datetime import datetime
from copy import deepcopy
from typing import Any

class Prototype:
    The example class that has cloning ability. We'll see how the values of
    field with different types will be cloned.

    def __init__(self) -> None:
        self._primitive = None
        self._component = None
        self._circular_reference = None

    def primitive(self) -> Any:
        return self._primitive

    def primitive(self, value: Any) -> None:
        self._primitive = value

    def component(self) -> object:
        return self._component

    def component(self, value: object) -> None:
        self._component = value

    def circular_reference(self) -> ComponentWithBackReference:
        return self._circular_reference

    def circular_reference(self, value: ComponentWithBackReference) -> None:
        self._circular_reference = value

    def clone(self) -> Prototype:
        self.component = deepcopy(self.component)

        # Cloning an object that has a nested object with backreference requires
        # special treatment. After the cloning is completed, the nested object
        # should point to the cloned object, instead of the original object.
        self.circular_reference = deepcopy(self.circular_reference)
        self.circular_reference.prototype = self
        return deepcopy(self)

class ComponentWithBackReference:
    def __init__(self, prototype: Prototype):
        self._prototype = prototype

    def prototype(self) -> Prototype:
        return self._prototype

    def prototype(self, value: Prototype) -> None:
        self._prototype = value

if __name__ == "__main__":
    # The client code.
    p1 = Prototype()
    p1.primitive = 245
    p1.component = datetime.now()
    p1.circular_reference = ComponentWithBackReference(p1)

    p2 = p1.clone()

    if p1.primitive is p2.primitive:
        print("Primitive field values have been carried over to a clone. Yay!")
        print("Primitive field values have not been copied. Booo!")

    if p1.component is p2.component:
        print("Simple component has not been cloned. Booo!")
        print("Simple component has been cloned. Yay!")

    if p1.circular_reference is p2.circular_reference:
        print("Component with back reference has not been cloned. Booo!")
        print("Component with back reference has been cloned. Yay!")

    if p1.circular_reference.prototype is p2.circular_reference.prototype:
        print("Component with back reference is linked to original object. Booo!", end="")
        print("Component with back reference is linked to the clone. Yay!", end="")

Output.txt: Execution result

Primitive field values have been carried over to a clone. Yay!
Simple component has been cloned. Yay!
Component with back reference has been cloned. Yay!
Component with back reference is linked to the clone. Yay!

Prototype in Other Languages

Design Patterns: Prototype in Java Design Patterns: Prototype in C# Design Patterns: Prototype in C++ Design Patterns: Prototype in PHP Design Patterns: Prototype in Ruby Design Patterns: Prototype in Swift Design Patterns: Prototype in TypeScript