Cursor rule

1---
2description: EEIK Python Standards — apply to all Python source files
3globs: ["**/*.py"]
4alwaysApply: true
5---
6 
7# Python & FastAPI Standards
8 
9**Applies To:** All Python source files
10**Enforced By:** `python-developer` agent, Ruff, mypy, CI gates
11**Reference:** `.claude/standards/python.md`, `.claude/standards/fastapi.md`
12 
13---
14 
15## The 8 Python Non-Negotiables
16 
17### 1 — Type Annotations on Everything
18 
19Every function and method must be fully annotated. `mypy --strict` must pass.
20 
21```python
22# ✅ CORRECT
23def find_orders(customer_id: str, statuses: list[str]) -> list[Order]:
24    ...
25 
26# ❌ VIOLATION — untyped
27def find_orders(customer_id, statuses):
28    ...
29```
30 
31### 2 — `logging` Not `print()`
32 
33```python
34# ✅ CORRECT
35import logging
36logger = logging.getLogger(__name__)
37logger.info("Order placed: order_id=%s", order.id)
38 
39# ❌ VIOLATION
40print(f"Order placed: {order.id}")
41```
42 
43### 3 — No Bare `except:`
44 
45```python
46# ✅ CORRECT
47try:
48    result = service.call()
49except ServiceUnavailableError as exc:
50    logger.error("Service call failed", exc_info=True)
51    raise
52 
53# ❌ VIOLATION — catches SystemExit, KeyboardInterrupt
54try:
55    result = service.call()
56except:
57    pass
58```
59 
60### 4 — No `import *`
61 
62```python
63# ✅ CORRECT
64from myapp.domain.orders import Order, OrderStatus
65 
66# ❌ VIOLATION
67from myapp.domain.orders import *
68```
69 
70### 5 — Pydantic / Dataclass for Domain Objects
71 
72```python
73# ✅ CORRECT
74from dataclasses import dataclass
75 
76@dataclass(frozen=True)
77class OrderId:
78    value: str
79 
80# ❌ VIOLATION — raw dict as domain object
81def process(order: dict) -> dict:
82    ...
83```
84 
85### 6 — Constructor Injection (No Global State)
86 
87```python
88# ✅ CORRECT — dependency injected via FastAPI Depends
89from typing import Annotated
90from fastapi import Depends
91 
92OrderServiceDep = Annotated[OrderService, Depends(get_order_service)]
93 
94@router.post("/orders")
95async def create_order(service: OrderServiceDep) -> OrderResponse:
96    ...
97 
98# ❌ VIOLATION — module-level instantiation
99db = create_engine(DATABASE_URL)  # global state
100```
101 
102### 7 — `async def` for All I/O
103 
104```python
105# ✅ CORRECT
106async def get_order(order_id: str) -> Order:
107    return await order_repo.find(order_id)
108 
109# ❌ VIOLATION — blocking call in async context
110async def get_order(order_id: str) -> Order:
111    import requests
112    resp = requests.get(f"/orders/{order_id}")  # blocks event loop
113```
114 
115### 8 — Settings via `BaseSettings`
116 
117```python
118# ✅ CORRECT
119from pydantic_settings import BaseSettings
120 
121class AppSettings(BaseSettings):
122    database_url: str
123    secret_key: str
124 
125# ❌ VIOLATION — hardcoded configuration
126DATABASE_URL = "postgresql://user:password@localhost/db"
127```
128 
129---
130 
131## FastAPI Route Rules
132 
133- All routes `async def` with explicit `response_model=`
134- Domain errors → global exception handler returning RFC 7807 `ProblemDetail`
135- Dependency injection via `Annotated[T, Depends(factory)]`
136- Never return raw `dict` from a route
137 
138```python
139# ✅ CORRECT FastAPI route
140@router.post("/v1/orders", response_model=OrderResponse, status_code=201)
141async def create_order(
142    body: CreateOrderRequest,
143    service: OrderServiceDep,
144) -> OrderResponse:
145    result = await service.place_order(body.to_command())
146    return OrderResponse.from_domain(result)
147```
148 
149---
150 
151## Anti-Patterns Checklist
152 
153| Anti-Pattern | Fix |
154|---|---|
155| `print()` | `logger.info(...)` |
156| Bare `except:` | `except SpecificError:` |
157| `import *` | Explicit imports |
158| Mutable default args | `None` sentinel |
159| Raw `dict` domain objects | `@dataclass(frozen=True)` or Pydantic |
160| Global DB/service instances | `Depends(factory)` per request |
161| Blocking I/O in `async def` | `await` with async library |
162| `time.sleep()` in tests | `asyncio.wait_for` or `tenacity` |
163