Learning overview
- Estimated time
- 4 minutes
- Difficulty
- Beginner
- Prerequisites
- No prior experience required
- Learning outcome
- Explain python using a clear mental model · Apply python in practical Python work
- Last updated
- July 18, 2026
Protect a meaningful invariant
A useful class makes invalid state difficult to create and gives domain operations clear names. If a structure only stores fields, a dataclass may be enough.
from dataclasses import dataclass
from decimal import Decimal
@dataclass
class Account:
owner: str
balance: Decimal = Decimal("0")
def deposit(self, amount: Decimal) -> None:
if amount <= 0:
raise ValueError("Deposit must be positive")
self.balance += amountPrefer composition for changing collaborators
Pass storage, notification, or payment behavior into an object instead of inheriting from those services. Composition keeps responsibilities separate and makes test doubles straightforward.
Use inheritance for genuine substitutability
A subclass should honor the promises of its base type. Python resolves inherited attributes through method resolution order. Keep hierarchies shallow and consider protocols when callers only require a small behavioral interface.
Frequently asked questions
When should I create a Python class?
Create one when related state and behavior need a stable identity, invariant, lifecycle, or substitutable interface. Do not create a class merely to hold unrelated functions.
What does @dataclass provide?
It can generate initialization, representation, equality, ordering, and related methods from declared fields, reducing boilerplate for data-focused classes.
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