Modernize Type Hints: PEP 585 & PEP 604

[zekunlou]

Background

Python 3.7 introduced typing module aliases (List, Dict, Tuple, Optional, Union, etc.) as a workaround for built-in types not supporting generic subscript syntax at runtime. This is now superseded by two PEPs:

• PEP 585 (Python 3.9): Built-in types (list, dict, tuple, ...) became directly generic, making typing.List etc. redundant. These aliases are soft-deprecated in 3.9 (which end of life on Oct 2025), emit deprecation warnings from 3.12, and will be removed in Python 3.14.
• PEP 604 (Python 3.10): Introduces the X | Y union syntax, replacing the verbose Union[X, Y] and Optional[X] (X | None).

Changes

• Drop from typing import List, Dict, Tuple, Optional, Union across the codebase; replace with built-in equivalents.
• Adopt X | Y union syntax and X | None in place of Optional[X].
• Enforce minimum Python version ≥ 3.10 to support the new union syntax natively at runtime.
• Retain from typing import Literal, TypeVar, Protocol, etc., which have no built-in replacements.

Example diff:

# Before
from typing import Dict, List, Optional, Tuple, Union

def read_system(spelist: List[str]) -> Dict[str, int]: ...
def parse_index_str(s: Union[str, Literal["all"]]) -> Union[None, Tuple]: ...
def foo(x: Optional[str] = None): ...

# After
from typing import Literal

def read_system(spelist: list[str]) -> dict[str, int]: ...
def parse_index_str(s: str | Literal["all"]) -> None | tuple: ...
def foo(x: str | None = None): ...

Motivation

This is a non-functional, maintenance-only change. No computational logic is affected. It future-proofs the codebase against the Python 3.14 removal, reduces boilerplate imports, and improves readability.