Python Application Architecture

Modern Python application architecture following functional core / imperative shell pattern, Domain-Driven Design, and type-safe data modeling.

Core Principle: Functional Core / Imperative Shell

Separate pure business logic from side effects:

• Functional Core: Pure functions, business logic, no IO
• Imperative Shell: Coordinates external dependencies, handles side effects

See references/functional-core.md for detailed patterns and examples.

Layered Architecture

Follow bottom-up dependency flow:

Router/Handler → Service → Repository → Entity → Database

Each layer depends only on layers below.

Responsibilities:

• Entity: Domain models, validation, business rules, data transformations (fromRequest, toRecord, toResponse)
• Repository: Abstract storage interface, returns domain entities
• Service: Business workflows, orchestrates entities and repositories
• Router/Handler: HTTP handling, delegates to services

Domain Models

Entity Example

from dataclasses import dataclass
from uuid import UUID
from decimal import Decimal

@dataclass
class Order:
    """Entity - has identity and encapsulated behavior"""
    id: UUID
    customer_id: UUID
    total: Decimal
    status: str

    def apply_discount(self, rate: Decimal) -> None:
        """Business rule - encapsulated in entity"""
        if self.status == "pending":
            self.total = self.total * (1 - rate)

    @classmethod
    def from_request(cls, req, customer_id: UUID) -> "Order":
        """Transform API request → entity"""
        return cls(id=uuid4(), customer_id=customer_id, total=Decimal("0"), status="pending")

    def to_response(self):
        """Transform entity → API response"""
        return {"id": self.id, "total": self.total, "status": self.status}

Value Object Example

from dataclasses import dataclass

@dataclass(frozen=True)
class Money:
    """Value object - immutable, no identity"""
    amount: Decimal
    currency: str

    def add(self, other: "Money") -> "Money":
        if self.currency != other.currency:
            raise ValueError("Cannot add different currencies")
        return Money(self.amount + other.amount, self.currency)

See references/ddd.md for aggregates, bounded contexts, and domain services.

Repository Pattern

Abstract storage behind interface:

from abc import ABC, abstractmethod
from typing import Optional

class OrderRepository(ABC):
    """Abstract repository - interface only"""

    @abstractmethod
    def get(self, order_id: UUID) -> Optional[Order]:
        pass

    @abstractmethod
    def save(self, order: Order) -> None:
        pass

class PostgresOrderRepository(OrderRepository):
    """Concrete implementation"""

    def get(self, order_id: UUID) -> Optional[Order]:
        record = self.session.get(OrderRecord, order_id)
        return Order.from_record(record) if record else None

    def save(self, order: Order) -> None:
        record = order.to_record()
        self.session.merge(record)
        self.session.commit()

Data Modeling

• dataclasses: Domain models and internal logic (lightweight, standard library)
• Pydantic: API boundaries (validation, JSON schema, OpenAPI)
• Entity transformations: from_request(), to_response(), from_record(), to_record()

See references/data-modeling.md for validation patterns, Pydantic features, and transformation examples.

Best Practices

1. Pure functions first - Write business logic without IO dependencies
2. Entity encapsulation - Keep business rules inside entities
3. Repository abstraction - Hide storage details, work with domain entities
4. Validate at boundaries - Use Pydantic at API edges, simple validation in entities
5. Immutable value objects - Always use frozen=True
6. Single Responsibility - Each layer has one reason to change
7. Dependency direction - Always depend on abstractions, not implementations

Anti-Patterns

❌ Anemic Domain Model - Entities with only getters/setters, all logic in services
❌ Transaction Script - All logic in service layer, entities just data
❌ Leaky Abstraction - Repository exposing database details
❌ God Object - Entity with too many responsibilities
❌ Mixed Concerns - Business logic calling IO directly

For detailed examples, patterns, and decision trees, see the reference materials:

• references/functional-core.md - Core vs shell separation
• references/ddd.md - DDD patterns, aggregates, bounded contexts
• references/data-modeling.md - dataclasses, Pydantic, transformations