from pathlib import Path


def check_and_get_subdirectory(base_dir: Path, subdir_name: str) -> Path:
    """
    Checks existance of base_dir, NON-existance of base_dir/subdir, and
     returns the Path(base_dir / subdir).
    
    Raises: 
        FileNotFoundError if the base_dir does not exist
        OSError if the base_dir and subdir both exist.
    
    Returns: path to an unexisting subdirectory subdir in the
             base directory base_dir.
    """
    
    if not base_dir.is_dir():
        raise FileNotFoundError("Directory {} not found"
                                .format(base_dir))
        
    subdir = base_dir / subdir_name
        
    if subdir.is_dir():
        raise OSError(
            f"Directory {subdir} exists and shouldn't be overwritten."
        )
    
    return subdir