[jvdongen]

Basically. The idiomatic solution for this in python is:

  def fn(x, my_dict=None):
     if my_dict is None:
         my_dict = {}
      ...

[brownbat]

You can use a sentinel if you want to allow None as an argument. More on that, along with a way to use mutable defaults productively:

http://effbot.org/zone/default-values.htm