Function Design Conventions
Quick Reference
| Principle | Pattern |
|---|---|
| Explicit dependencies | Pass as parameters, no global state |
| Single responsibility | One function = one task |
| Pure when possible | Return new values, don't mutate inputs |
| Guard clauses | Validate early, return/raise immediately |
| Stable return types | Same type regardless of input |
| Errors via exceptions | Reserve None for "not found" only |
| Command-query separation | Action OR data, not both |
| Keyword-only args | Use * for optional/multiple params |
| No mutable defaults | Use None, create inside function |
| Async prefix | async_ for async functions |
Core Principles
Explicit Dependencies
# INCORRECT - hidden dependency on global state
_current_user: User | None = None
def get_permissions() -> list[str]:
return _current_user.permissions
# CORRECT - explicit dependency
def get_permissions(user: User) -> list[str]:
return user.permissions
Single Responsibility
# INCORRECT - does two things
def validate_and_save_user(user: User) -> bool:
if not user.email or "@" not in user.email:
return False
db.save(user)
return True
# CORRECT - separate concerns
def is_valid_email(email: str) -> bool:
return bool(email and "@" in email)
def save_user(user: User) -> None:
if not is_valid_email(user.email):
raise ValidationError("Invalid email address")
db.save(user)
Pure Functions When Possible
# INCORRECT - modifies input
def normalize_scores(scores: list[float]) -> None:
max_score = max(scores)
for i in range(len(scores)):
scores[i] /= max_score
# CORRECT - returns new value
def normalize_scores(scores: list[float]) -> list[float]:
max_score = max(scores)
return [s / max_score for s in scores]
Guard Clauses (Return Early)
# INCORRECT - deeply nested
def process_order(order: Order | None) -> Receipt:
if order is not None:
if order.items:
if order.payment_verified:
return generate_receipt(order)
else:
raise PaymentError("Payment not verified")
else:
raise ValidationError("Order has no items")
else:
raise ValidationError("Order is required")
# CORRECT - guard clauses
def process_order(order: Order | None) -> Receipt:
if order is None:
raise ValidationError("Order is required")
if not order.items:
raise ValidationError("Order has no items")
if not order.payment_verified:
raise PaymentError("Payment not verified")
return generate_receipt(order)
Return Type Stability
# INCORRECT - inconsistent return types
def find_user(user_id: int) -> User | None | bool:
if user_id < 0:
return False
user = db.get(user_id)
return user
# CORRECT - consistent return type
def find_user(user_id: int) -> User:
if user_id < 0:
raise ValueError("user_id must be >=0; got {user_id}")
return db.get(user_id)
Exceptions Over None for Errors
# INCORRECT - None conflates "not found" with "error"
def load_config(path: Path) -> Config | None:
if not path.exists():
return None
try:
return Config.from_file(path)
except ParseError:
return None
# CORRECT - exceptions for errors, None only for "not found"
def load_config(path: Path) -> Config:
if not path.exists():
raise FileNotFoundError(f"Config file not found: {path}")
try:
return Config.from_file(path)
except ParseError as e:
raise ConfigurationError(f"Invalid config format: {e}") from e
Command-Query Separation
# INCORRECT - does both
def get_next_id() -> int:
global _counter
_counter += 1 # Side effect (command)
return _counter # Returns value (query)
# CORRECT - separate command and query
class IdGenerator:
def __init__(self) -> None:
self._counter = 0
def next(self) -> int:
"""Return the next ID (query only)."""
return self._counter + 1
def advance(self) -> None:
"""Increment the counter (command only)."""
self._counter += 1
def take(self) -> int:
"""Get next ID and advance. Clearly named to indicate both."""
id = self.next()
self.advance()
return id
Keep Functions Small and Focused
# INCORRECT - too much happening
def process_document(doc: Document) -> ProcessedDocument:
# Validate
if not doc.content:
raise ValueError("Empty document")
if len(doc.content) > MAX_LENGTH:
raise ValueError("Document too long")
# Extract metadata
title = doc.content.split("\n")[0]
word_count = len(doc.content.split())
# Transform content
cleaned = doc.content.lower().strip()
tokens = cleaned.split()
# ... 50 more lines
# CORRECT - composed of focused functions
def process_document(doc: Document) -> ProcessedDocument:
validate_document(doc)
metadata = extract_metadata(doc)
tokens = tokenize_content(doc.content)
return ProcessedDocument(metadata=metadata, tokens=tokens)
Parameter Guidelines
Limit Positional Parameters
# INCORRECT - too many positional parameters
def create_user(name: str, email: str, age: int, role: str, dept: str) -> User:
...
# CORRECT - group related parameters
@dataclass
class UserInput:
name: str
email: str
age: int
role: str
department: str
def create_user(input: UserInput) -> User:
...
Keyword-Only Arguments
Use * to force keyword arguments for optional parameters or functions with 3+ parameters.
# CORRECT - keyword-only after *
def fetch_data(
url: str,
*, # Everything after this must be keyword-only
timeout: float = 30.0,
retries: int = 3,
headers: dict[str, str] | None = None,
) -> Response:
...
# Callers must be explicit
response = fetch_data("https://api.example.com", timeout=60.0, retries=5)
No Mutable Default Arguments
# INCORRECT - mutable default (shared across calls!)
def add_item(item: str, items: list[str] = []) -> list[str]:
items.append(item)
return items
add_item("a") # Returns ["a"]
add_item("b") # Returns ["a", "b"] - BUG!
# CORRECT - None default with internal creation
def add_item(item: str, items: list[str] | None = None) -> list[str]:
if items is None:
items = []
items.append(item)
return items
Async Functions
Prefix async functions with async_ to make the async nature visible at call sites.
# CORRECT - async prefix makes it clear
async def async_fetch_user(user_id: int) -> User:
return await client.get(f"/users/{user_id}")
async def async_process_batch(items: list[Item]) -> list[Result]:
return await asyncio.gather(*[async_process(item) for item in items])
# Usage is clear about async nature
user = await async_fetch_user(123)
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