downloads = {"guide": "volunteer-guide.pdf", "terms": "lending-terms.pdf"}
def select_download(identifier):
    if identifier not in downloads:
        raise ValueError("unknown download")
    return downloads[identifier]
assert select_download("guide") == "volunteer-guide.pdf"
for value in ["../private.txt", "..\\private.txt", "/etc/passwd"]:
    try:
        select_download(value)
    except ValueError:
        continue
    raise AssertionError(value)
print("known ID selected; three path inputs rejected")