Don't use default arguments of lists or dictionaries.
Don't use default arguments of lists or dictionaries.

This probably doesn't matter in practice for us, but is
good practice nonetheless.  Default arguments as lists or
dictionaries are dangerous in python, since they can be
modified by the called function, and will retain that
value.  Just use None as the default value instead.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

imcleod/oz,moofrank/oz,clalancette/oz,NeilBryant/oz,cernops/oz,clalancette/oz,mgagne/oz,cernops/oz,moofrank/oz,ndonegan/oz,imcleod/oz,mgagne/oz,NeilBryant/oz,nullr0ute/oz,ndonegan/oz,nullr0ute/oz