Type Annotations - Python 3.11

Type Annotations - Python 3.11

This document provides complete, canonical type annotation guidance for Python 3.11.

Overview

Python 3.11 builds on 3.10's type syntax with the addition of the Self type (PEP 673), making method chaining and builder patterns significantly cleaner. All modern syntax from 3.10 continues to work.

What's new in 3.11:

• Self type for self-returning methods (PEP 673)
• Variadic generics with TypeVarTuple (PEP 646)
• Significantly improved error messages

Available from 3.10:

• Built-in generic types: list[T], dict[K, V], etc. (PEP 585)
• Union types with | operator (PEP 604)
• Optional with X | None

What you need from typing module:

• Self for self-returning methods (NEW)
• TypeVar for generic functions/classes
• Generic for generic classes
• Protocol for structural typing (rare - prefer ABC)
• TYPE_CHECKING for conditional imports
• Any (use sparingly)

Complete Type Annotation Syntax for Python 3.11

Basic Collection Types

✅ PREFERRED - Use built-in generic types:

names: list[str] = []
mapping: dict[str, int] = {}
unique_ids: set[str] = set()
coordinates: tuple[int, int] = (0, 0)

❌ WRONG - Don't use typing module equivalents:

from typing import List, Dict, Set, Tuple  # Don't do this
names: List[str] = []

Union Types

✅ PREFERRED - Use | operator:

def process(value: str | int) -> str:
    return str(value)

def find_config(name: str) -> dict[str, str] | dict[str, int]:
    ...

# Multiple unions
def parse(input: str | int | float) -> str:
    return str(input)

❌ WRONG - Don't use typing.Union:

from typing import Union
def process(value: Union[str, int]) -> str:  # Don't do this
    ...

Optional Types

✅ PREFERRED - Use X | None:

def find_user(id: str) -> User | None:
    """Returns user or None if not found."""
    if id in users:
        return users[id]
    return None

❌ WRONG - Don't use typing.Optional:

from typing import Optional
def find_user(id: str) -> Optional[User]:  # Don't do this
    ...

Self Type for Self-Returning Methods (NEW in 3.11)

✅ PREFERRED - Use Self for methods that return the instance:

from typing import Self

class Builder:
    def set_name(self, name: str) -> Self:
        self.name = name
        return self

    def set_value(self, value: int) -> Self:
        self.value = value
        return self

# Usage with type safety
builder = Builder().set_name("app").set_value(42)

❌ WRONG - Don't use bound TypeVar anymore:

from typing import TypeVar

T = TypeVar("T", bound="Builder")

class Builder:
    def set_name(self: T, name: str) -> T:  # Don't do this
        ...

When to use Self:

• Methods that return self
• Builder pattern methods
• Fluent interfaces with method chaining
• Factory classmethods

Self in classmethod:

from typing import Self

class Config:
    def __init__(self, data: dict[str, str]) -> None:
        self.data = data

    @classmethod
    def from_file(cls, path: str) -> Self:
        """Load config from file."""
        import json
        with open(path, encoding="utf-8") as f:
            data = json.load(f)
        return cls(data)

Generic Functions with TypeVar

✅ PREFERRED - Use TypeVar for generic functions:

from typing import TypeVar

T = TypeVar("T")

def first(items: list[T]) -> T | None:
    """Return first item or None if empty."""
    if not items:
        return None
    return items[0]

def identity(value: T) -> T:
    return value

Note: Python 3.12 introduces better syntax (PEP 695) for this pattern.

Generic Classes

✅ PREFERRED - Use Generic with TypeVar:

from typing import Generic, TypeVar

T = TypeVar("T")

class Stack(Generic[T]):
    """A generic stack data structure."""

    def __init__(self) -> None:
        self._items: list[T] = []

    def push(self, item: T) -> Self:  # Can combine with Self!
        self._items.append(item)
        return self

    def pop(self) -> T | None:
        if not self._items:
            return None
        return self._items.pop()

# Usage
int_stack = Stack[int]()
int_stack.push(42).push(43)  # Method chaining works!

Note: Python 3.12 introduces cleaner syntax for generic classes.

Constrained and Bounded TypeVars

✅ Use TypeVar constraints when needed:

from typing import TypeVar

# Constrained to specific types
Numeric = TypeVar("Numeric", int, float)

def add(a: Numeric, b: Numeric) -> Numeric:
    return a + b

# Bounded to base class
T = TypeVar("T", bound=BaseClass)

def process(obj: T) -> T:
    return obj

Callable Types

✅ PREFERRED - Use collections.abc.Callable:

from collections.abc import Callable

# Function that takes int, returns str
processor: Callable[[int], str] = str

# Function with no args, returns None
callback: Callable[[], None] = lambda: None

# Function with multiple args
validator: Callable[[str, int], bool] = lambda s, i: len(s) > i

Type Aliases

✅ Use simple assignment for type aliases:

# Simple alias
UserId = str
Config = dict[str, str | int | bool]

# Complex nested type
JsonValue = dict[str, "JsonValue"] | list["JsonValue"] | str | int | float | bool | None

def load_config() -> Config:
    return {"host": "localhost", "port": 8080}

Note: Python 3.12 introduces type statement for better alias support.

When from future import annotations is Needed

Use from __future__ import annotations when you encounter:

Forward references (class referencing itself):

from __future__ import annotations

class Node:
    def __init__(self, value: int, parent: Node | None = None):
        self.value = value
        self.parent = parent

Circular type imports:

# a.py
from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from b import B

class A:
    def method(self) -> B:
        ...

Complex recursive types:

from __future__ import annotations

JsonValue = dict[str, JsonValue] | list[JsonValue] | str | int | float | bool | None

Interfaces: ABC vs Protocol

✅ PREFERRED - Use ABC for interfaces:

from abc import ABC, abstractmethod

class Repository(ABC):
    @abstractmethod
    def get(self, id: str) -> User | None:
        """Get user by ID."""

    @abstractmethod
    def save(self, user: User) -> None:
        """Save user."""

🟡 VALID - Use Protocol only for structural typing:

from typing import Protocol

class Drawable(Protocol):
    def draw(self) -> None: ...

# Any object with draw() method matches
def render(obj: Drawable) -> None:
    obj.draw()

Dignified Python prefers ABC because it makes inheritance and intent explicit.

Complete Examples

Builder Pattern with Self

from typing import Self

class QueryBuilder:
    """SQL query builder with fluent interface."""

    def __init__(self) -> None:
        self._select: list[str] = ["*"]
        self._from: str | None = None
        self._where: list[str] = []
        self._limit: int | None = None

    def select(self, *columns: str) -> Self:
        """Specify columns to select."""
        self._select = list(columns)
        return self

    def from_table(self, table: str) -> Self:
        """Specify table to query."""
        self._from = table
        return self

    def where(self, condition: str) -> Self:
        """Add WHERE condition."""
        self._where.append(condition)
        return self

    def limit(self, n: int) -> Self:
        """Set LIMIT."""
        self._limit = n
        return self

    def build(self) -> str:
        """Build final SQL query."""
        if not self._from:
            raise ValueError("FROM table not specified")

        parts = [f"SELECT {', '.join(self._select)}"]
        parts.append(f"FROM {self._from}")

        if self._where:
            parts.append(f"WHERE {' AND '.join(self._where)}")

        if self._limit:
            parts.append(f"LIMIT {self._limit}")

        return " ".join(parts)

# Usage with type-safe method chaining
query = (
    QueryBuilder()
    .select("id", "name", "email")
    .from_table("users")
    .where("active = true")
    .where("age > 18")
    .limit(10)
    .build()
)

Factory Methods with Self

from typing import Self
from pathlib import Path
import json

class Config:
    """Application configuration with multiple factory methods."""

    def __init__(self, data: dict[str, str | int]) -> None:
        self.data = data

    @classmethod
    def from_json(cls, path: Path) -> Self:
        """Load configuration from JSON file."""
        if not path.exists():
            raise FileNotFoundError(f"Config not found: {path}")

        with path.open(encoding="utf-8") as f:
            data = json.load(f)
        return cls(data)

    @classmethod
    def from_env(cls) -> Self:
        """Load configuration from environment variables."""
        import os
        data = {
            k.lower(): v
            for k, v in os.environ.items()
            if k.startswith("APP_")
        }
        return cls(data)

    @classmethod
    def default(cls) -> Self:
        """Create default configuration."""
        return cls({"host": "localhost", "port": 8080})

    def with_override(self, key: str, value: str | int) -> Self:
        """Return new config with overridden value."""
        new_data = self.data.copy()
        new_data[key] = value
        return type(self)(new_data)

# All factory methods return correct type
config = Config.from_json(Path("config.json"))
dev_config = config.with_override("debug", True)

Type Checking Rules

What to Type

✅ MUST type:

• All public function parameters (except self, cls)
• All public function return values
• All class attributes (public and private)
• Module-level constants

🟡 SHOULD type:

• Internal function signatures
• Complex local variables

🟢 MAY skip:

• Simple local variables where type is obvious (count = 0)
• Lambda parameters in short inline lambdas
• Loop variables in short comprehensions

Running Type Checker

uv run ty check

All code should pass type checking without errors.

Type Checking Configuration

Configure ty in pyproject.toml:

[tool.ty.environment]
python-version = "3.11"

Common Patterns

Checking for None

✅ CORRECT - Check before use:

def process_user(user: User | None) -> str:
    if user is None:
        return "No user"
    return user.name

Dict.get() with Type Safety

✅ CORRECT - Handle None case:

def get_port(config: dict[str, int]) -> int:
    port = config.get("port")
    if port is None:
        return 8080
    return port

List Operations

✅ CORRECT - Check before accessing:

def first_or_default(items: list[str], default: str) -> str:
    if not items:
        return default
    return items[0]

Migration from Python 3.10

If upgrading from Python 3.10:

1. Replace bound TypeVar with Self for self-returning methods:

   • Old: T = TypeVar("T", bound="ClassName")
   • New: from typing import Self and use -> Self

2. Enjoy improved error messages (no code changes needed)

3. All existing 3.10 syntax continues to work

References

• PEP 673: Self Type
• PEP 646: Variadic Generics
• Python 3.11 What's New