[LostJourneyman]

Quick note, the more pythonic way of checking par in this case would be just to:

  if par:
    do_stuff()

Python does some really cool, if slightly unintuitive, checks for "truthiness" where 0, [], {}, false, '' and None (could be other cases too, brain's a little foggy today) all evaluate as false. This controls for:

  if par = None:
    do_stuff()

not catching empty lists, empty dicts, etc.

[andi999]

How do you define the function in that case?

[LostJourneyman]

So, a more complete example might help:

  def myFunc(par=None):
    if par:
      # do whatever is needed when the parameter exists
    pass
    #finish processing

This allows you to pass in [], {}, None, '', or omit par all together, which will all behave as if par=None, or pass in data and use it appropriately. You can also use

  if not par:
    pass

if you want to do the inverse.