I'm a little confused about this as an assignment to a variable. Is that because of the 'return' omission referred to below? If all function f did was return x, I could see how this works with a 'return' prepended.

But if this expression is a line in a larger function, and is intended to reset the value of argument x if no other value is passed in for it, does this really act as an assigment to x? Because I sort of read this expression as evaluating to some value -- the passed value for x, or a [] -- but does this assign that value to the argument x? Or must it be x = [expression]

Yes, but why? Actually, it's No, because python culture aims to use one way to do thing, the least surprising one. In this case it's:

  def f(x=None): if x is None: x = []

you forgot 'return'