So here is the code:
hou.hscript("opdefaultshape -c")
hou.hscript("opdefaultshape Sop circle")
hou.hscript("opdefaultshape Dop circle")
hou.hscript("opdefaultshape Cop2 circle")
hou.hscript("opdefaultshape Chop circle")
hou.hscript("opdefaultshape Object circle")
You can just copy-paste to the scripts 123.py in the Houdini scripts preference folder.
# 123.py is a python script that houdiini reads at startup time