What's that lambda thingo in the middle then? Pretty sure that's another function, redefined every time your function is called.

Yes, the lambda creates the function. Note that defvar does not. So there is still only one function being defined here.

Ok, I see now.

You've still got that &optional argument though. I don't see a huge amount of difference from a semantic point of view between that and the Python version though (ie. if x == None: ...).