[kstrauser]

You're right. And same with the mutable default argument "trap". That's cause by `def foo` being a statement, not syntax. When a module is imported, the code in it is executed. Many of the statements in that will be `def something`, which, when executed, defines a function. And because that's code that gets executed,

  def foo(bar=[]):
      bar.append('lol')
      return bar

the `bar=[]` gets executed at that time. That is, Python doesn't treat `def foo` as some magic thing that gets special cased and squirreled away for later use.