Conventional way of dealing with optional list arguments is the following:
def func(list_of_vals = None):
if list_of_vals is None:
list_of_vals = []
...
I wounder if the following (shorter) version has any pitfalls? why nobody do that? is it considered more obscure?
list_of_vals = list_of_vals or []
The pattern if arg is None: is usual, it is familiar to Python developers and has no weird edge cases. My recommendation is just stick with the convention.
Your proposal using or logically deviates when bool(list_of_vals) == False but list_of_vals is not None, so I would recommend against doing that.
Another possible option is to "duck type" using an empty tuple:
def a(vals=()):
...
Because tuples are immutable, this has none of the pitfalls of the mutable default list. There are many use-cases where you only need to the input container to be indexable and iterable, so vals can happily remain as a tuple.