Comment by dragonwriter

> That doesn't negate the fact that it's a concern, though, does it?

Yes, the fact that most people learn very early the correct way to have a constant value of a mutable type used when an explicit argument is not given and that using a mutable value directly as a default argument value uses a mutable value shared between invocations (which is occasionally desirable) means that the way those two things are done in Python isn't a substantial problem.

(And, no, I don't think a constant mutable list is actually all that commonly needed as a default argument in most languages where mutable and immutable iterables share a common interface; if you are actually mutating the argument, it is probably not an optional argument, if you aren't mutating it, an immutable value -- like a python tuple -- works fine.)