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.