Literal type becomes str in comprehension

[injust]

Code sample in pyright playground

from typing import Literal

type DiffLine = tuple[Literal[" ", "+", "-"], str]

diff: list[DiffLine] = []
reveal_type(diff)  # Type of "diff" is "list[tuple[Literal[' ', '+', '-'], str]]"
changes = {change for change, _ in diff}
reveal_type(changes)  # set[str]

I'm surprised that changes is typed set[str] instead of set[Literal[' ', '+', '-']].

[erictraut]

This is expected behavior. Literal values are not retained when inferring the type of a list, dict, or set expression. You can override the default inference rules by adding a type annotation on the target variable. Pyright will then use bidirectional type inference.

changes: set[Literal[" ", "+", "-"]] = {change for change, _ in diff}