6.3. Inheritance Composition

>>> class UserPermissions:
...     pass
>>>
>>> class AdminPermissions:
...     pass
>>>
>>>
>>> class Account:
...     def __init__(self, permissions):
...         self.permissions = permissions

>>> alice = Account(UserPermissions())
>>> bob = Account(AdminPermissions())

6.3.1. Case Study

SetUp:

>>> import json
>>> from datetime import date

Naive example:

>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
... }
>>>
>>> json.dumps(data)
'{"firstname": "Alice", "lastname": "Apricot"}'

Let's add a birthdate:

>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
...     'birthdate': date(2000, 1, 1),
... }
>>>
>>> json.dumps(data)
Traceback (most recent call last):
TypeError: Object of type date is not JSON serializable
when serializing dict item 'birthdate'

If you want to use your better version of encoder (for example which can encode date object. You can create a class which inherits from default json.JSONEncoder and overwrite .default() method. Here's the solution:

>>> class MyEncoder(json.JSONEncoder):
...     def default(self, obj):
...         if isinstance(obj, date):
...             return obj.isoformat()
>>>
>>>
>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
...     'birthdate': date(2000, 1, 1),
... }
>>>
>>> json.dumps(data, cls=MyEncoder)
'{"firstname": "Alice", "lastname": "Apricot", "birthdate": "2000-01-01"}'

6.3.2. Assignments

# %% About
# - Name: Inheritance Composition A
# - 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. Define class `UserPermissions`
# 2. Define class `AdminPermissions`
# 3. Compose `Account` from `UserPermissions`, `AdminPermissions`
# 4. Use composition
# 5. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj klasę `UserPermissions`
# 2. Zdefiniuj klasę `AdminPermissions`
# 3. Skomponuj `Account` z klas `UserPermissions`, `AdminPermissions`
# 4. Użyj kompozycji
# 5. Uruchom doctesty - wszystkie muszą się powieść

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

>>> from inspect import isclass

>>> assert isclass(UserPermissions)
>>> assert isclass(AdminPermissions)
>>> assert isclass(Account)

>>> result = Account()
Traceback (most recent call last):
TypeError: Account.__init__() missing 1 required positional argument: 'permissions'

>>> result = Account(UserPermissions())
>>> assert hasattr(result, 'permissions'), \
'Account instance must have `permissions` attribute'

>>> assert type(result.permissions) is UserPermissions

>>> result = Account(AdminPermissions())
>>> assert hasattr(result, 'permissions'), \
'Account instance must have `permissions` attribute'

>>> assert type(result.permissions) is AdminPermissions
"""

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

# %% Imports

# %% Types
UserPermissions: type
AdminPermissions: type
Account: type

# %% Data

# %% Result