Python Idioms

Python Idioms

Pythonic patterns, anti-patterns, and quality rules for clean Python code.

Core Principles

Patterns vs Anti-Patterns

List/Dict Comprehensions

# GOOD: List comprehension
squares = [x**2 for x in range(10) if x % 2 == 0]

# BAD: Manual loop for simple transforms
squares = []
for x in range(10):
    if x % 2 == 0:
        squares.append(x**2)

Context Managers

# GOOD: Context manager for resources
with open('file.txt') as f:
    content = f.read()

# BAD: Manual resource management
f = open('file.txt')
try:
    content = f.read()
finally:
    f.close()

Unpacking

# GOOD: Tuple unpacking
first, *rest, last = items
x, y = point

# BAD: Index access
first = items[0]
last = items[-1]
x = point[0]
y = point[1]

Iteration

# GOOD: Direct iteration
for item in items:
    process(item)

# GOOD: Enumerate when index needed
for i, item in enumerate(items):
    print(f"{i}: {item}")

# BAD: Range-based iteration
for i in range(len(items)):
    process(items[i])

Dictionary Operations

# GOOD: dict.get() with default
value = data.get('key', default_value)

# GOOD: setdefault for initialization
cache.setdefault(key, []).append(item)

# BAD: Check and access
if 'key' in data:
    value = data['key']
else:
    value = default_value

String Formatting

# GOOD: f-strings (Python 3.6+)
message = f"Hello, {name}! You have {count} items."

# OK: .format() for dynamic templates
template = "Hello, {name}!"
message = template.format(name=name)

# BAD: % formatting or concatenation
message = "Hello, " + name + "!"
message = "Hello, %s!" % name

Boolean Checks

# GOOD: Truthiness
if items:  # Not empty
if not items:  # Empty
if value is None:  # Explicit None check

# BAD: Explicit comparisons
if len(items) > 0:
if items != []:
if value == None:

Anti-Patterns to Avoid

Mutable Default Arguments

# BAD: Mutable default
def add_item(item, items=[]):  # Items shared across calls!
    items.append(item)
    return items

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

Bare Except

# BAD: Catches everything including KeyboardInterrupt
try:
    risky_operation()
except:
    pass

# GOOD: Specific exceptions
try:
    risky_operation()
except (ValueError, TypeError) as e:
    logger.error(f"Operation failed: {e}")

Global State

# BAD: Global mutable state
_cache = {}
def get_data(key):
    global _cache
    if key not in _cache:
        _cache[key] = fetch(key)
    return _cache[key]

# GOOD: Class with instance state
class DataCache:
    def __init__(self):
        self._cache = {}

    def get(self, key):
        if key not in self._cache:
            self._cache[key] = self._fetch(key)
        return self._cache[key]

Using type() for Type Checks

# BAD: type() comparison
if type(obj) == list:
    process_list(obj)

# GOOD: isinstance for type checks
if isinstance(obj, (list, tuple)):
    process_sequence(obj)

Type Hints

# Function signatures
def process_items(items: list[str], max_count: int = 10) -> dict[str, int]:
    ...

# Optional types
from typing import Optional
def find_user(user_id: int) -> Optional[User]:
    ...

# Generic types
from typing import TypeVar, Generic
T = TypeVar('T')
class Container(Generic[T]):
    def __init__(self, value: T) -> None:
        self.value = value

Error Handling

# Custom exceptions with context
class ValidationError(Exception):
    def __init__(self, field: str, message: str):
        self.field = field
        super().__init__(f"{field}: {message}")

# Chained exceptions
try:
    parse_config(data)
except json.JSONDecodeError as e:
    raise ConfigError("Invalid config format") from e

Quality Checklist

Django-Specific

# GOOD: QuerySet optimization
users = User.objects.select_related('profile').filter(active=True)

# GOOD: Bulk operations
User.objects.bulk_create([User(name=n) for n in names])
User.objects.filter(old=True).update(status='archived')

# BAD: N+1 queries
for user in User.objects.all():
    print(user.profile.name)  # Query per user!

Flask-Specific

# GOOD: Application factory
def create_app(config=None):
    app = Flask(__name__)
    app.config.from_object(config or 'config.default')
    register_blueprints(app)
    return app

# GOOD: Request context
from flask import g, current_app
def get_db():
    if 'db' not in g:
        g.db = connect_db(current_app.config['DATABASE'])
    return g.db

Invocation — Python Anti-Idiom Checker (L3 Script)

After loading this skill, run the checker on any Python source file. Consume its output only — the script source never enters context.

Run via Bash (file argument):

python .claude/skills/code/python-idioms/scripts/python_lint.py path/to/file.py

Run via Bash (stdin — paste code or pipe):

cat path/to/file.py | python .claude/skills/code/python-idioms/scripts/python_lint.py -

The script outputs:

1. A JSON object with findings (list of anti-patterns with rule, severity, line, message) and a summary of counts by severity.
2. A human-readable markdown table sorted by severity descending.

Severity bands:

• HIGH — Correctness risk (mutable defaults, bare except)
• MEDIUM — Idiomatic debt (== None, range(len(...)), type() ==)
• LOW — Style/preference (informational)

Error handling: Script exits 1 on unparseable Python (syntax error in source) or unreadable file. Exit 0 even if findings are present — the agent decides how to act.

What the agent does with the output:

1. Lead remediation with HIGH findings first.
2. Reference MEDIUM findings as idiomatic improvements.
3. LOW findings are informational — use judgment whether to raise them.