How do you want to access your hierarchy?
If you're always going to be checking for a full path, then as suggested, use a tuple:
eg:
>>> d["a","b1","c",1,"d"] = value
However, if you're going to be doing things like "quickly find all the items below "a -> b1", it may make more sense to store it as a nested hashtable (otherwise you must iterate through all items to find those you're intereted in).
For this, a defaultdict is probably the easiest way to store. For example:
from collections import defaultdict
def new_dict(): return defaultdict(new_dict)
d = defaultdict(new_dict)
d["a"]["b1"]["c"][1]["d"] = "test"
d["a"]["b2"]["c"][2]["d"] = "test2"
d["a"]["c"][1]["d"] = "test3"
print d["a"]["c"][1]["d"] # Prints test3
print d["a"].keys() # Prints ["c", "b1", "b2"]