[eiopa]

ES6 got this right too.

This design decision makes me die a little every time I have to do:

  def foo(x=None):
      if not x:
          x = []
      ...

[dajonker]

That will get you into trouble as well, please use:

    if x is None:
        x = []

... etc.

[switch007]

Or even "prettier":

  x = [] if x is None else x

[knite]

I'm partial to:

x = x or []

[acveilleux]

Only works when x is truthy for all legal x values, excluding x=0.