4.6. Prototype

• Create new object by copying an existing object

The Prototype design pattern is a creational design pattern that allows cloning objects, even complex ones, without coupling to their specific classes. All prototype classes have a common interface that makes it possible to clone objects.

4.6.1. Problem

>>> class User:
...     def __init__(self, username, password, is_user, is_staff, is_admin):
...         self.username = username
...         self.password = password
...         self.is_user = is_user
...         self.is_staff = is_staff
...         self.is_admin = is_admin
>>>
>>>
>>> alice = User(
...     username='alice',
...     password='secret',
...     is_user=True,
...     is_staff=True,
...     is_admin=False,
... )
>>>
>>> bob = User(
...     username=alice.username,
...     password=alice.password,
...     is_user=alice.is_user,
...     is_staff=alice.is_staff,
...     is_admin=alice.is_admin,
... )

4.6.2. Solution

>>> class User:
...     def __init__(self, username, password, is_user, is_staff, is_admin):
...         self.username = username
...         self.password = password
...         self.is_user = is_user
...         self.is_staff = is_staff
...         self.is_admin = is_admin
...
...     def clone(self):
...         data = vars(self)
...         return User(**data)
>>>
>>>
>>> alice = User(
...     username='alice',
...     password='secret',
...     is_user=True,
...     is_staff=True,
...     is_admin=False,
... )
>>>
>>> bob = alice.clone()

The clone method creates a new instance of the User class, and those two instances are different objects:

>>> alice
<__main__.User object at 0x105f69160>
>>>
>>> bob
<__main__.User object at 0x105f3efd0>

But, they have the same data:

>>> vars(alice)
{'username': 'alice', 'password': 'secret', 'is_user': True, 'is_staff': True, 'is_admin': False}
>>>
>>> vars(bob)
{'username': 'alice', 'password': 'secret', 'is_user': True, 'is_staff': True, 'is_admin': False}

4.6.3. Rationale

• You can customize object on cloning

>>> class User:
...     def __init__(self, username, password, is_user, is_staff, is_admin):
...         self.username = username
...         self.password = password
...         self.is_user = is_user
...         self.is_staff = is_staff
...         self.is_admin = is_admin
...
...     def clone(self, **kwargs):
...         data = vars(self) | kwargs
...         return User(**data)
>>>
>>>
>>> alice = User(
...     username='alice',
...     password='secret',
...     is_user=True,
...     is_staff=True,
...     is_admin=False,
... )
>>>
>>> bob = alice.clone(username='bob', password='qwerty')

The clone method creates a new instance of the User class, and those two instances are different objects:

>>> alice
<__main__.User object at 0x105f69160>
>>>
>>> bob
<__main__.User object at 0x105f3efd0>

But, because we passed new values to the clone method, they have different data - some fields are overwritten:

>>> vars(alice)
{'username': 'alice', 'password': 'secret', 'is_user': True, 'is_staff': True, 'is_admin': False}
>>>
>>> vars(bob)
{'username': 'bob', 'password': 'qwerty', 'is_user': True, 'is_staff': True, 'is_admin': False}

4.6.4. Case Study

Problem:

class User:
    def __init__(self, firstname, lastname, email):
        self.firstname = firstname
        self.lastname = lastname
        self.email = email


if __name__ == '__main__':
    a = User('Mark', 'Watney', 'mwatney@nasa.gov')
    b = User( a.firstname, a.lastname, a.email)

Solution:

class User:
    def __init__(self, firstname, lastname, email):
        self.firstname = firstname
        self.lastname = lastname
        self.email = email

    def copy(self):
        return User(
            self.firstname,
            self.lastname,
            self.email
        )

if __name__ == '__main__':
    a = User('Mark', 'Watney', email='mwatney@nasa.gov')
    b = a.copy()

Diagram:

4.6.5. Use Case - 1

>>> data = [1, 2, 3]
>>> a = data
>>> b = data.copy()
>>>
>>>
>>> data
[1, 2, 3]
>>>
>>> a
[1, 2, 3]
>>>
>>> b
[1, 2, 3]

>>> data.append(4)
>>>
>>>
>>> data
[1, 2, 3, 4]
>>>
>>> a
[1, 2, 3, 4]
>>>
>>> b
[1, 2, 3]

4.6.6. Use Case - 2

from typing import Literal
from dataclasses import dataclass, field
from datetime import datetime


@dataclass
class User:
    firstname: str
    lastname: str
    username: str
    password: str
    email: str
    last_login: datetime | None
    role: Literal['admin', 'user', 'guest']
    groups: list[str] = field(default_factory=list)

    def clone(self):
        return User(
            firstname = self.firstname,
            lastname = self.lastname,
            username = self.username,
            password = self.password,
            email = self.email,
            last_login = self.last_login,
            role = self.role,
            groups = self.groups)


mark = User(
    firstname='Mark',
    lastname='Watney',
    username='mwatney',
    password='Ares3',
    email='mwatney@nasa.gov',
    last_login=None,
    role='admin',
    groups=['admins', 'users'],
)

melissa = mark.clone()

print(melissa)
# User(firstname='Mark', lastname='Watney', username='mwatney', password='Ares3', email='mwatney@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

melissa.firstname = 'Melissa'
melissa.lastname = 'Lewis'
melissa.username = 'mlewis'
melissa.email = 'mlewis@nasa.gov'

print(melissa)
# User(firstname='Melissa', lastname='Lewis', username='mlewis', password='Ares3', email='mlewis@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

4.6.7. Use Case - 3

from typing import Literal
from dataclasses import dataclass, field
from datetime import datetime


@dataclass
class User:
    firstname: str
    lastname: str
    username: str
    password: str
    email: str
    last_login: datetime | None
    role: Literal['admin', 'user', 'guest']
    groups: list[str] = field(default_factory=list)

    def clone(self):
        return User(**vars(self))


mark = User(
    firstname='Mark',
    lastname='Watney',
    username='mwatney',
    password='Ares3',
    email='mwatney@nasa.gov',
    last_login=None,
    role='admin',
    groups=['admins', 'users'],
)

melissa = mark.clone()

print(melissa)
# User(firstname='Mark', lastname='Watney', username='mwatney', password='Ares3', email='mwatney@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

melissa.firstname = 'Melissa'
melissa.lastname = 'Lewis'
melissa.username = 'mlewis'
melissa.email = 'mlewis@nasa.gov'

print(melissa)
# User(firstname='Melissa', lastname='Lewis', username='mlewis', password='Ares3', email='mlewis@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

4.6.8. Use Case - 4

from typing import Literal
from dataclasses import dataclass, field
from datetime import datetime


@dataclass
class User:
    firstname: str
    lastname: str
    username: str
    password: str
    email: str
    last_login: datetime | None
    role: Literal['admin', 'user', 'guest']
    groups: list[str] = field(default_factory=list)

    def clone(self, **kwargs):
        values = vars(self) | kwargs
        return User(**values)


mark = User(
    firstname='Mark',
    lastname='Watney',
    username='mwatney',
    password='Ares3',
    email='mwatney@nasa.gov',
    last_login=None,
    role='admin',
    groups=['admins', 'users'],
)

melissa = mark.clone(
    firstname='Melissa',
    lastname='Lewis',
    username='mlewis',
    email='mlewis@nasa.gov',
)

print(melissa)
# User(firstname='Melissa', lastname='Lewis', username='mwatney', password='Ares3', email='mwatney@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

4.6.9. Use Case - 5

from typing import Literal
from dataclasses import dataclass, field
from datetime import datetime


@dataclass
class User:
    firstname: str
    lastname: str
    username: str
    password: str
    email: str
    last_login: datetime | None
    role: Literal['admin', 'user', 'guest']
    groups: list[str] = field(default_factory=list)

    def clone(self, **kwargs):
        values = vars(self) | kwargs
        cls = self.__class__
        return cls(**values)

@dataclass
class Admin(User):
    pass


mark = Admin(
    firstname='Mark',
    lastname='Watney',
    username='mwatney',
    password='Ares3',
    email='mwatney@nasa.gov',
    last_login=None,
    role='admin',
    groups=['admins', 'users'],
)

melissa = mark.clone(
    firstname='Melissa',
    lastname='Lewis',
    username='mlewis',
    email='mlewis@nasa.gov',
)

print(melissa)
# Admin(firstname='Melissa', lastname='Lewis', username='mwatney', password='Ares3', email='mwatney@nasa.gov', last_login=None, role='admin', groups=['admins', 'users'])

4.6.10. Use Case - 6

>>> import copy
>>> from dataclasses import dataclass, field
>>>
>>>
>>> @dataclass
... class Account:
...     username: str
...
...     def clone(self):
...         return copy.deepcopy(self)
>>>
>>>
>>> @dataclass
... class User(Account):
...     permissions: list = field(default_factory=lambda: ["read", "comment"])
...     is_active: bool = True
...
...     def __post_init__(self):
...         self.account_type = "user"
>>>
>>>
>>> @dataclass
... class Admin(Account):
...     permissions: list = field(default_factory=lambda: ["read", "write", "delete", "admin"])
...     is_active: bool = True
...
...     def __post_init__(self):
...         self.account_type = "admin"
>>>
>>>
>>> # Main
>>> def main():
...     # Create prototype instances
...     alice = User(username="alice")
...     bob = Admin(username="bob")
...
...     print(f"Original alice: {alice}")
...     print(f"Original bob: {bob}")
...
...     # Create clones
...     alice_clone = alice.clone()
...     bob_clone = bob.clone()
...
...     # Verify clones are separate objects but with same data
...     print(f"\nCloned alice: {alice_clone}")
...     print(f"Cloned bob: {bob_clone}")
...
...     # Modify clones
...     alice_clone.permissions.append("upload")
...     bob_clone.username = "bob_admin"
...
...     # Show that originals are unchanged
...     print(f"\nAfter modifications:")
...     print(f"Original alice: {alice}")
...     print(f"Original bob: {bob}")
...     print(f"Modified alice clone: {alice_clone}")
...     print(f"Modified bob clone: {bob_clone}")

4.6.11. Assignments

# %% About
# - Name: DesignPatterns Creational PrototypeDate
# - Difficulty: easy
# - Lines: 5
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Create class `Date` with:
#    - `year: int`
#    - `month: int`
#    - `day: int`
#    - method `.clone()`
# 2. Method `.clone()` returns another `Date` with the same values
# 3. Do not use `vars(self)`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Stwórz klasę `Date` z:
#    - `year: int`
#    - `month: int`
#    - `day: int`
#    - metodą `.clone()`
# 2. Metoda `.clone()` zwraca kolejny `Date` z tymi samymi wartościami
# 3. Nie używaj `vars(self)`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from pprint import pprint

>>> date = Date(1969, 7, 21)
>>> result = date.clone()

>>> result.year
1969
>>> result.month
7
>>> result.day
21
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`

# %% Imports
from dataclasses import dataclass

# %% Types
from typing import Callable
Date: type
clone: Callable[[object], object]

# %% Data

# %% Result
@dataclass
class Date:
    year: int
    month: int
    day: int

# %% About
# - Name: DesignPatterns Creational PrototypeTime
# - Difficulty: easy
# - Lines: 2
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Create class `Time` with:
#    - `hour: int`
#    - `minute: int`
#    - `second: int`
#    - `microsecond: int`
#    - method `.clone()`
# 2. Method `.clone()` returns another `Time` with the same values
# 3. Use `vars(self)`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Stwórz klasę `Time` z:
#    - `hour: int`
#    - `minute: int`
#    - `second: int`
#    - `microsecond: int`
#    - metodą `.clone()`
# 2. Metoda `.clone()` zwraca kolejny `Time` z tymi samymi wartościami
# 3. Użyj `vars(self)`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from pprint import pprint

>>> time = Time(2, 56, 15)
>>> result = time.clone()

>>> result.hour
2
>>> result.minute
56
>>> result.second
15
>>> result.microsecond
0
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`

# %% Imports
from dataclasses import dataclass

# %% Types
from typing import Callable
Date: type
clone: Callable[[object], object]

# %% Data

# %% Result
@dataclass
class Time:
    hour: int = 0
    minute: int = 0
    second: int = 0
    microsecond: int = 0

# %% About
# - Name: DesignPatterns Creational PrototypeDragon
# - Difficulty: easy
# - Lines: 6
# - Minutes: 8

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Create class `Dragon`
# 2. Dragon has attributes:
#    - `name: str`
#    - `position: tuple[int,int]` default `(0, 0)`
#    - `health: int` random from 50 to 100
#    - `gold: int` random from 1 to 100
#    - method `.clone()`
# 3. Method `.clone()` returns another `Dragon` with the same values
# 4. Use `random.randint()` to generate pseudorandom numbers
# 5. Run doctests - all must succeed

# %% Polish
# 1. Stwórz klasę `Dragon`
# 2. Dragon ma atrybuty:
#    - `name: str`
#    - `position: tuple[int,int]` domyślnie `(0, 0)`
#    - `health: int` losowe od 50 do 100
#    - `gold: int` losowe od 1 do 100
#    - metodę `.clone()`
# 3. Metoda `.clone()` zwraca kolejnego `Dragon` z tymi samymi wartościami
# 4. Użyj `random.randint()` do generowania pseudolosowych liczb
# 5. Uruchom doctesty - wszystkie muszą się powieść

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from pprint import pprint
>>> from random import seed
>>> seed(0)

>>> dragon = Dragon('Wawelski')
>>> result = dragon.clone()

>>> result.name
'Wawelski'

>>> result.health
74

>>> result.gold
98

>>> result.position
(0, 0)
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`

# %% Imports
from dataclasses import dataclass, field
from random import randint, seed

# %% Types
from typing import Callable
Dragon: type
clone: Callable[[object], object]

# %% Data
seed(0)

# %% Result
@dataclass
class Dragon:
    ...