Python Anti Patterns

Python Anti-Patterns Checklist

A reference checklist of common mistakes and anti-patterns in Python code. Review this before finalizing implementations to catch issues early.

Note: This skill focuses on what to avoid. For guidance on positive patterns and architecture, see the python-patterns skill.

Core Concepts

1. Centralize Cross-Cutting Concerns

Timeouts, retries, and configuration should live in one place, not scattered across every call site.

2. Separate Layers

Keep I/O, business logic, and API concerns in distinct layers. Don't mix SQL into business functions or leak ORM models to API consumers.

3. Handle Failures Explicitly

Catch specific exceptions, preserve partial results in batch operations, and validate inputs at boundaries.

4. Use the Type System

Annotate all public functions, use typed collections, and let static analysis catch bugs before runtime.

Infrastructure Anti-Patterns

Scattered Timeout/Retry Logic

# BAD: Timeout logic duplicated everywhere
def fetch_user(user_id):
    try:
        return requests.get(url, timeout=30)
    except Timeout:
        logger.warning("Timeout fetching user")
        return None

def fetch_orders(user_id):
    try:
        return requests.get(url, timeout=30)
    except Timeout:
        logger.warning("Timeout fetching orders")
        return None

Fix: Centralize in decorators or client wrappers.

# GOOD: Centralized retry logic
@retry(stop=stop_after_attempt(3), wait=wait_exponential())
def http_get(url: str) -> Response:
    return requests.get(url, timeout=30)

Double Retry

# BAD: Retrying at multiple layers
@retry(max_attempts=3)  # Application retry
def call_service():
    return client.request()  # Client also has retry configured!

Fix: Retry at one layer only. Know your infrastructure's retry behavior.

Hard-Coded Configuration

# BAD: Secrets and config in code
DB_HOST = "prod-db.example.com"
API_KEY = "sk-12345"

def connect():
    return psycopg.connect(f"host={DB_HOST}...")

Fix: Use environment variables with typed settings.

# GOOD
from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    db_host: str = Field(alias="DB_HOST")
    api_key: str = Field(alias="API_KEY")

settings = Settings()

Architecture Anti-Patterns

Exposed Internal Types

# BAD: Leaking ORM model to API
@app.get("/users/{id}")
def get_user(id: str) -> UserModel:  # SQLAlchemy model
    return db.query(UserModel).get(id)

Fix: Use DTOs/response models.

# GOOD
@app.get("/users/{id}")
def get_user(id: str) -> UserResponse:
    user = db.query(UserModel).get(id)
    return UserResponse.from_orm(user)

Mixed I/O and Business Logic

# BAD: SQL embedded in business logic
def calculate_discount(user_id: str) -> float:
    user = db.query("SELECT * FROM users WHERE id = ?", user_id)
    orders = db.query("SELECT * FROM orders WHERE user_id = ?", user_id)
    # Business logic mixed with data access
    if len(orders) > 10:
        return 0.15
    return 0.0

Fix: Repository pattern. Keep business logic pure.

# GOOD
def calculate_discount(user: User, orders: list[Order]) -> float:
    # Pure business logic, easily testable
    if len(orders) > 10:
        return 0.15
    return 0.0

Error Handling Anti-Patterns

Bare Exception Handling

# BAD: Swallowing all exceptions
try:
    process()
except Exception:
    pass  # Silent failure - bugs hidden forever

Fix: Catch specific exceptions. Log or handle appropriately.

# GOOD
try:
    process()
except ConnectionError as e:
    logger.warning("Connection failed, will retry", error=str(e))
    raise
except ValueError as e:
    logger.error("Invalid input", error=str(e))
    raise BadRequestError(str(e))

Ignored Partial Failures

# BAD: Stops on first error
def process_batch(items):
    results = []
    for item in items:
        result = process(item)  # Raises on error - batch aborted
        results.append(result)
    return results

Fix: Capture both successes and failures.

# GOOD
def process_batch(items) -> BatchResult:
    succeeded = {}
    failed = {}
    for idx, item in enumerate(items):
        try:
            succeeded[idx] = process(item)
        except Exception as e:
            failed[idx] = e
    return BatchResult(succeeded, failed)

Missing Input Validation

# BAD: No validation
def create_user(data: dict):
    return User(**data)  # Crashes deep in code on bad input

Fix: Validate early at API boundaries.

# GOOD
def create_user(data: dict) -> User:
    validated = CreateUserInput.model_validate(data)
    return User.from_input(validated)

Resource Anti-Patterns

Unclosed Resources

# BAD: File never closed
def read_file(path):
    f = open(path)
    return f.read()  # What if this raises?

Fix: Use context managers.

# GOOD
def read_file(path):
    with open(path) as f:
        return f.read()

Blocking in Async

# BAD: Blocks the entire event loop
async def fetch_data():
    time.sleep(1)  # Blocks everything!
    response = requests.get(url)  # Also blocks!

Fix: Use async-native libraries.

# GOOD
async def fetch_data():
    await asyncio.sleep(1)
    async with httpx.AsyncClient() as client:
        response = await client.get(url)

Type Safety Anti-Patterns

Missing Type Hints

# BAD: No types
def process(data):
    return data["value"] * 2

Fix: Annotate all public functions.

# GOOD
def process(data: dict[str, int]) -> int:
    return data["value"] * 2

Untyped Collections

# BAD: Generic list without type parameter
def get_users() -> list:
    ...

Fix: Use type parameters.

# GOOD
def get_users() -> list[User]:
    ...

Testing Anti-Patterns

Only Testing Happy Paths

# BAD: Only tests success case
def test_create_user():
    user = service.create_user(valid_data)
    assert user.id is not None

Fix: Test error conditions and edge cases.

# GOOD
def test_create_user_success():
    user = service.create_user(valid_data)
    assert user.id is not None

def test_create_user_invalid_email():
    with pytest.raises(ValueError, match="Invalid email"):
        service.create_user(invalid_email_data)

def test_create_user_duplicate_email():
    service.create_user(valid_data)
    with pytest.raises(ConflictError):
        service.create_user(valid_data)

Over-Mocking

# BAD: Mocking everything
def test_user_service():
    mock_repo = Mock()
    mock_cache = Mock()
    mock_logger = Mock()
    mock_metrics = Mock()
    # Test doesn't verify real behavior

Fix: Use integration tests for critical paths. Mock only external services.

Language-Level Anti-Patterns

Mutable Default Arguments

# BAD: Default list is shared across all calls
def append_to(item, items=[]):
    items.append(item)
    return items

append_to(1)  # [1]
append_to(2)  # [1, 2] -- not [2]!

Fix: Use None and create a new object inside the function.

# GOOD
def append_to(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

type() vs isinstance()

# BAD: Fails for subclasses
if type(obj) == list:
    process(obj)

Fix: Use isinstance, which respects inheritance.

# GOOD
if isinstance(obj, list):
    process(obj)

== None vs is None

# BAD: == can be overridden by __eq__
if value == None:
    process()

Fix: Use the identity check is None.

# GOOD
if value is None:
    process()

Wildcard Imports

# BAD: Pollutes namespace, hides where names come from
from os.path import *

Fix: Import only what you need.

# GOOD
from os.path import join, exists

Quick Review Checklist

Before finalizing code, verify:

• No scattered timeout/retry logic (centralized)
• No double retry (app + infrastructure)
• No hard-coded configuration or secrets
• No exposed internal types (ORM models, protobufs)
• No mixed I/O and business logic
• No bare except Exception: pass
• No ignored partial failures in batches
• No missing input validation
• No unclosed resources (using context managers)
• No blocking calls in async code
• All public functions have type hints
• Collections have type parameters
• Error paths are tested
• Edge cases are covered
• No mutable default arguments (def f(x=[]))
• Using isinstance() not type() for type checks
• Using is None not == None
• No wildcard imports (from module import *)

Common Fixes Summary