You just need to fully understand when Python does evaluation. import types def function(item, stuff = lambda: []): if type(stuff) == types.Fun

You just need to fully understand when Python does evaluation. import types def function(item, stuff = lambda: []): if type(stuff) == types.FunctionType: stuff = stuff() stuff.append(item) print stuff function(1) # prints '[1]' function(2) # prints '[2]' In Scala it has a better syntax because of typed function object: trait Map[A, B] { … def getOrElse (key: A, default: ⇒ B): B … } the `default` parameter is a function, so when you do getOrElse(someKey, defaultValue) The `defaultValue` becomes a function that generates the value you put there when called.