Using `[]` as default value for arguments makes `pylint` complain. The
reason is that it creates an array statically at the time the function
is parsed, rather than dynamically on invocation of the function. This
means, when you append to this array, you change the global instance and
every further invocation of that function works on this modified array.
While our use-cases are safe, this is indeed a common pitfall. Lets
avoid using this and resort to `None` instead.
This silences a lot of warnings from pylint about "dangerous use of []".
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