581 Python 313 75fa5ef2

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# Type Annotations - Python 3.13

This document provides complete, canonical type annotation guidance for Python 3.13. Python 3.13
implements PEP 649 (Deferred Evaluation of Annotations), fundamentally changing how annotations are
evaluated.

## Overview

**The key change: forward references and circular imports work naturally without
`from __future__ import annotations`.**

All type features from previous versions (3.10-3.12) continue to work.

**What's new in 3.13:**

- PEP 649 deferred annotation evaluation
- Forward references work naturally (no quotes, no `from __future__`)
- Circular imports no longer cause annotation errors
- **DO NOT use `from __future__ import annotations`**

**Available from 3.12:**

- PEP 695 type parameter syntax: `def func[T](x: T) -> T`
- `type` statement for better type aliases

**Available from 3.11:**

- `Self` type for self-returning methods

## Universal Philosophy

**Code Clarity:**

- Types serve as inline documentation
- Make function contracts explicit
- Reduce cognitive load when reading code
- Help understand data flow without tracing through implementation

**IDE Support:**

- Enable autocomplete and intelligent suggestions
- Catch typos and attribute errors before runtime
- Support refactoring tools (rename, move, extract)
- Provide jump-to-definition for typed objects

**Bug Prevention:**

- Catch type mismatches during static analysis
- Prevent None-related errors with explicit optional types
- Document expected input/output without running code
- Enable early detection of API contract violations

## Consistency Rules

**All public APIs:**

- 🔴 MUST: Type all function parameters (except `self` and `cls`)
- 🔴 MUST: Type all function return values
- 🔴 MUST: Type all class attributes
- 🟡 SHOULD: Type module-level constants

**Internal code:**

- 🟡 SHOULD: Type function signatures where helpful for clarity
- 🟢 MAY: Type complex local variables where type isn't obvious
- 🟢 MAY: Omit types for obvious cases (e.g., `count = 0`)

## Basic Collection Types

✅ **PREFERRED** - Use built-in generic types:

```python
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:

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

**Why**: Built-in types are more concise, don't require imports, and are the modern Python standard
(available since 3.10).

## Union Types

✅ **PREFERRED** - Use `|` operator:

```python
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`:

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

## Optional Types

✅ **PREFERRED** - Use `X | None`:

```python
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`:

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

## Callable Types

✅ **PREFERRED** - Use `collections.abc.Callable`:

```python
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
```

## Interfaces: ABC vs Protocol

✅ **PREFERRED** - Use ABC for interfaces:

```python
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:

```python
from typing import Protocol

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

def render(obj: Drawable) -> None:
    obj.draw()
```

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

## Self Type for Self-Returning Methods (3.11+)

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

```python
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
```

## Generic Functions with PEP 695 (3.12+)

✅ **PREFERRED** - Use PEP 695 type parameter syntax:

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

def identity[T](value: T) -> T:
    """Return value unchanged."""
    return value

# Multiple type parameters
def zip_dicts[K, V](keys: list[K], values: list[V]) -> dict[K, V]:
    """Create dict from separate key and value lists."""
    return dict(zip(keys, values))
```

🟡 **VALID** - TypeVar still works:

```python
from typing import TypeVar

T = TypeVar("T")

def first(items: list[T]) -> T | None:
    if not items:
        return None
    return items[0]
```

**Note**: Prefer PEP 695 syntax for simple generics. TypeVar is still needed for constraints/bounds.

## Generic Classes with PEP 695 (3.12+)

✅ **PREFERRED** - Use PEP 695 class syntax:

```python
class Stack[T]:
    """A generic stack data structure."""

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

    def push(self, item: T) -> 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)
```

🟡 **VALID** - Generic with TypeVar still works:

```python
from typing import Generic, TypeVar

T = TypeVar("T")

class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: list[T] = []
    # ... rest of implementation
```

**Note**: PEP 695 is cleaner - no imports needed, type parameter scope is local to class.

## Type Parameter Bounds (3.12+)

✅ **Use bounds with PEP 695**:

```python
class Comparable:
    def compare(self, other: object) -> int:
        ...

def max_value[T: Comparable](items: list[T]) -> T:
    """Get maximum value from comparable items."""
    return max(items, key=lambda x: x)
```

## Constrained TypeVars (Still Use TypeVar)

✅ **Use TypeVar for specific type constraints**:

```python
from typing import TypeVar

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

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

❌ **WRONG** - PEP 695 doesn't support constraints:

```python
# This doesn't constrain to int|float
def add[Numeric](a: Numeric, b: Numeric) -> Numeric:
    return a + b
```

## Type Aliases with type Statement (3.12+)

✅ **PREFERRED** - Use `type` statement:

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

# Generic type alias
type Result[T] = tuple[T, str | None]

def process(value: str) -> Result[int]:
    try:
        return (int(value), None)
    except ValueError as e:
        return (0, str(e))
```

🟡 **VALID** - Simple assignment still works:

```python
UserId = str  # Still valid
Config = dict[str, str | int | bool]  # Still valid
```

**Note**: `type` statement is more explicit and works better with generics.

## Forward References and Circular Imports (NEW in 3.13)

✅ **CORRECT** - Just works naturally with PEP 649:

```python
# Forward reference - no quotes needed!
class Node:
    def __init__(self, value: int, parent: Node | None = None):
        self.value = value
        self.parent = parent

# Circular imports - just works!
# a.py
from b import B

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

# b.py
from a import A

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

# Recursive types - no future needed!
type JsonValue = dict[str, JsonValue] | list[JsonValue] | str | int | float | bool | None
```

❌ **WRONG** - Don't use `from __future__ import annotations`:

```python
from __future__ import annotations  # DON'T DO THIS in Python 3.13

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

**Why avoid `from __future__ import annotations` in 3.13:**

- Unnecessary - PEP 649 provides better default behavior
- Can cause confusion
- Masks the native 3.13 deferred evaluation
- Prevents you from leveraging improvements

## Complete Examples

### Tree Structure with Natural Forward References

```python
from typing import Self
from collections.abc import Callable

class Node[T]:
    """Tree node - forward reference works naturally in 3.13!"""

    def __init__(
        self,
        value: T,
        parent: Node[T] | None = None,  # Forward ref, no quotes!
        children: list[Node[T]] | None = None,  # Forward ref, no quotes!
    ) -> None:
        self.value = value
        self.parent = parent
        self.children = children or []

    def add_child(self, child: Node[T]) -> Self:
        """Add child and return self for chaining."""
        self.children.append(child)
        child.parent = self
        return self

    def find(self, predicate: Callable[[T], bool]) -> Node[T] | None:
        """Find first node matching predicate."""
        if predicate(self.value):
            return self

        for child in self.children:
            result = child.find(predicate)
            if result:
                return result

        return None

# Usage - all type-safe with no __future__ import!
root = Node[int](1)
root.add_child(Node[int](2)).add_child(Node[int](3))
```

### Generic Repository with PEP 695

```python
from abc import ABC, abstractmethod
from typing import Self

class Entity[T]:
    """Base class for entities."""

    def __init__(self, id: T) -> None:
        self.id = id

class Repository[T](ABC):
    """Generic repository interface."""

    @abstractmethod
    def get(self, id: str) -> T | None:
        """Get entity by ID."""

    @abstractmethod
    def save(self, entity: T) -> None:
        """Save entity."""

    @abstractmethod
    def delete(self, id: str) -> bool:
        """Delete entity, return True if deleted."""

class User(Entity[str]):
    def __init__(self, id: str, name: str) -> None:
        super().__init__(id)
        self.name = name

class UserRepository(Repository[User]):
    def __init__(self) -> None:
        self._users: dict[str, User] = {}

    def get(self, id: str) -> User | None:
        if id not in self._users:
            return None
        return self._users[id]

    def save(self, entity: User) -> None:
        self._users[entity.id] = entity

    def delete(self, id: str) -> bool:
        if id not in self._users:
            return False
        del self._users[id]
        return True
```

## General Best Practices

**Prefer specificity:**

```python
# ✅ GOOD - Specific
def get_config() -> dict[str, str | int]:
    ...

# ❌ WRONG - Too vague
def get_config() -> dict:
    ...
```

**Use Union sparingly:**

```python
# ✅ GOOD - Union only when necessary
def process(value: str | int) -> str:
    ...

# ❌ WRONG - Too permissive
def process(value: str | int | list | dict) -> str | None | list:
    ...
```

**Be explicit with None:**

```python
# ✅ GOOD - Explicit optional
def find_user(id: str) -> User | None:
    ...

# ❌ WRONG - Implicit None return
def find_user(id: str) -> User:
    return None  # Type checker error!
```

**Avoid Any when possible:**

```python
# ✅ GOOD - Specific type
def serialize(obj: User | Config) -> str:
    ...

# ❌ WRONG - Defeats purpose of types
from typing import Any
def serialize(obj: Any) -> str:
    ...
```

## When to Use Types

**Always type:**

- Public function signatures (parameters + return)
- Class attributes (including private ones)
- Function parameters that cross module boundaries
- Return values that aren't immediately obvious

**Type when helpful:**

- Complex local variables
- Closures and nested functions
- Lambda expressions used as callbacks

**Can skip:**

- Obvious cases: `count = 0`, `name = "example"`
- Trivial private helpers
- Test fixture setup code (if types add no clarity)

## Type Checking with ty

Dignified Python uses ty for static type checking:

```bash
# Check all files
ty check

# Check specific file
ty check src/mymodule.py

# Check with specific Python version
ty check --python-version 3.13
```

**Configuration** (in `pyproject.toml`):

```toml
[tool.ty.environment]
python-version = "3.13"
```

## Anti-Patterns

**❌ Don't ignore type errors with `# type: ignore`**

```python
# ❌ WRONG - Hiding type error
result = unsafe_function()  # type: ignore

# ✅ CORRECT - Fix the type error
result: Expected = cast(Expected, unsafe_function())
```

**❌ Don't use bare Exception in type hints**

```python
# ❌ WRONG - No value from typing exception
def risky() -> str | Exception:
    ...

# ✅ CORRECT - Let exceptions bubble
def risky() -> str:
    ...  # Raises ValueError on error
```

**❌ Don't over-type simple cases**

```python
# ❌ WRONG - Obvious from context
def add_numbers(a: int, b: int) -> int:
    result: int = a + b  # Unnecessary type annotation
    return result

# ✅ CORRECT - Type only signature
def add_numbers(a: int, b: int) -> int:
    result = a + b  # Type is obvious
    return result
```

## Migration from 3.10/3.11

If migrating from Python 3.10/3.11:

1. **Remove `from __future__ import annotations`** - No longer needed
2. **Consider upgrading to PEP 695 syntax** - Cleaner generics
3. **Use `type` statement for aliases** - More explicit than assignment
4. **Remove quoted forward references** - They work naturally now

```python
# Python 3.10/3.11
from __future__ import annotations
from typing import TypeVar, Generic

T = TypeVar("T")

class Node(Generic[T]):
    def __init__(self, value: T, parent: "Node[T] | None" = None):
        ...

# Python 3.13
from typing import Self

class Node[T]:
    def __init__(self, value: T, parent: Node[T] | None = None):
        ...
```

## What typing imports are still needed?

**Very rare:**

- `TypeVar` - Only for constrained/bounded type variables
- `Any` - Use sparingly when type truly unknown
- `Protocol` - Structural typing (prefer ABC)
- `TYPE_CHECKING` - Conditional imports to avoid circular dependencies

**Never needed:**

- `List`, `Dict`, `Set`, `Tuple` - Use built-in types
- `Union` - Use `|` operator
- `Optional` - Use `X | None`
- `Generic` - Use PEP 695 class syntax