Python for JS Devs · Reference

Type hints — TypeScript → Python

Modern Python 3.10+ hints. Prefer lowercase built-ins and | None.

Meaning	TypeScript	Python (3.10+)
String	`string`	`str`
Integer	`number`	`int`
Float	`number`	`float`
Boolean	`boolean`	`bool`
Array	`string[]`	`list[str]`
Tuple	`[int, int]`	`tuple[int, int]`
Dict / record	`Record<string,int>`	`dict[str, int]`
Set	`Set<str>`	`set[str]`
Optional / nullable	`string \| null`	`str \| None`
Union	`str \| int`	`str \| int`
Literal union	`"a" \| "b"`	`Literal["a", "b"]`
Any	`any`	`Any` (from `typing`)
Function param	`(n: string)`	`(n: str)`
Return type	`): string`	`) -> str:`
Optional param	`role?: string`	`role: str \| None = None`
Callable	`(x: int) => str`	`Callable[[int], str]`
Variable	`let x: int = 3`	`x: int = 3`

Full function

def summarize(scores: list[int], label: str | None = None) -> dict[str, int]:
    return {"count": len(scores), "total": sum(scores)}

Remember Plain hints are not enforced at runtime (like TS — erased) — a checker (mypy/ty/pyright) verifies them in dev. The exception is Pydantic/FastAPI, which read hints at runtime to validate. Legacy code uses Optional[str] / List[str] from typing — prefer the lowercase forms above.

Run a checker with uv uvx ty check or uvx mypy . — no install needed.

← SyntaxFastAPI →