Optional type hint in Google Style Guide

Question

I have a doubt regarding Optional type hint in the Google Python Style Guide. In section 3.19.5, the following is shown as correct usage:

def func(a: Optional[Text], b: Optional[Text] = None) -> Text:

I don't understand a: Optional[Text]. Why is it not a: Optional[Text] = None?

Presumably just because the first argument doesn't have a default value. Why *would* it have `= None`? — jonrsharpe, May 05 '20 at 14:59
@jonrsharpe What value will `a` take in the function when nothing is passed? — Hari, May 05 '20 at 15:07
@Carcigenicate then why the type hint `Optional`? I seem to be missing something in my understanding. — Hari, May 05 '20 at 15:10
It won't take any value; it's a `TypeError`, the function never gets entered. — jonrsharpe, May 05 '20 at 17:03

Carcigenicate · Accepted Answer · 2020-05-05T15:10:04.287

Just because it's Optional doesn't mean that it needs a default argument, or that the argument is "optional".

Optional[Text] means "it can be a Text object, or it can be None". The None value need not be a specified default though; it can be user supplied. You may, for whatever reason, want the user to pass that argument, even if it's just None.

Part of the confusion might be the use of the term "optional" here. "Optional" in this context doesn't mean that the argument is optional. It means that it's an option type.

Optional type hint in Google Style Guide

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