from typing import Sequence, reveal_type

def test(command: Sequence[str] | str) -> str:
    reveal_type(command)  # Sequence[str]
    parts = map(str, command)  # works
    if isinstance(command, str):
        return command
    else:
        reveal_type(command)  # Sequence[str] & ~str
        parts = map(str, command)  # fails with "not assignable to `Iterable[Buffer]`"
        return ' '.join(parts)

print(test('echo hello'))
print(test(['echo', 'hello']))