def is_printable(s: str) -> bool:
    """Determine if a string contains only printable characters.
    Args:
        s: The string to verify.
    Returns:
        bool: `True` if all characters in `s` are printable. `False` if any
            characters in `s` can not be printed.
    """
    # Ref: https://stackoverflow.com/a/50731077
    return not any(repr(ch).startswith(("'\\x", "'\\u")) for ch in s)

if is_printable(c):
    return c
else:
    return UNDEFINED_KEY

from bullet import Input

prompt = Input("Enter a string with UTF-8 printable characters: ")
user_input = prompt.launch()
print(user_input)