python-anti-patterns | open collection of Python anti | Learning library

Just use Optional[List[int]]. You're not losing anything by making the parameter optional - you're explicitly handling None within the body of the function - so there's no reason to not use the correct type. It's Pythonic to treat empty lists and None identically anyways.

The problem with # type: ignore, as you mentioned, is that it suppresses type checking for other parameters.

Consider that there are, broadly speaking, three ways to indicate that some external expression should be inserted into a string whose format method is called:

1. Implicitly, by position

'{}, {}, {}'.format('huey', 'dewey', 'louie') gives 'huey, dewey, louie'.

1. Explicitly, by position

'{2}, {1}, {0}'.format('huey', 'dewey', 'louie') gives 'louie, dewey, huey'.

1. Explicitly, by name

'{first}, {second}, {third}'.format(first='huey', second='dewey', third='louie') gives 'huey, dewey, louie'.

Recall that in Python, keyword arguments and variable names cannot start with a number.

This limitation is relevant to our current situation: if such keyword arguments were possible, we would not be able to resolve the ambiguity between cases 2 and 3; should {0} refer to the first element of the unnamed additional arguments, or the keyword argument 0?

Since non-string keyword arguments are not possible, there is no ambiguity, and an integer within braces always means the second case. Therefore, in your code, {123} in fact refers to the 124th element of the argument-tuple passed to format, and of course there is no such element.

For completeness, let's look at f-strings, introduced in Python 3.6:

I think this is the expected behavior!
Consider that [1,2], [3,4] is a tuple literal, equivalent to the tuple ([1,2], [3,4]). (You might be using this without even noticing, for instance when assigning multiple values with a, b, c = 10, 20, 30 disregarding the ()...).

So in your example, the loop iterates through this list as follows: