An alternative to consider might be to accept a Literal[“fast”, “slow”] or an Enum FAST or SLOW, and then decode that into shipping options inside the shipping code.

Only then are you truly putting a solid boundary between your library and the folks using your library. Everything else is just praying that you and only you have an underscore on your keyboard! :)

And of course another alternative is to accept that there is no true private in Python other than defdef*, so you allow your ShippingOption to be publicly visible while also documenting that the helper-constructors are what should really be used.

*”defdef” as in function definitions inside other function definitions — closures if you will, although I prefer to write mine as taking most if not all their parameters explicitly:

  def public(foo):
    def private(foo):
      …

    class Private:
      …  # less common

    …