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

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 -v myfile.py`

# %% Imports

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

# %% Data

# %% Result