dict invert - learning question

[dave]

Hello,

here is a piece of code I wrote to check the frequency of values and 
switch them around to keys in a new dictionary.  Just to measure how 
many times a certain key occurs:

def invert(d):
	inv = {}
	for key in d:
		val = d[key]
		if val not in inv:
			inv.setdefault(val, [key])
		else:
			inv[val].append(key)
	return inv


Using the methods above (I'm just a beginner) could I have written it 
more concisely?  Any criticism/critique in the code would be greatly 
appreciated.

Many Thanks,

dave