Python 3 11

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

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

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)

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

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

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

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)

from typing import TypeVar

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

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

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)

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

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!

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

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

# 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}

from __future__ import annotations

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

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

if TYPE_CHECKING:
    from b import B

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

from __future__ import annotations

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

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."""

from typing import Protocol

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

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

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()
)

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)

uv run ty check

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

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

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

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