many ways to invert a dictionary

1. Simplest/most straightforward:

d = {"A":1,"B":2,"C":3}
inv = {}
for k in d.keys():
   inv[d[k]] = k

2. Alternative: use d.items(), iterate over tuples

d = {"A":1,"B":2,"C":3}
inv = {}
for a,b in d.items():
   inv[b] = a

3. Iterate over tuples, build up a reversed list, pass it to dict()

d = {"A":1,"B":2,"C":3}
invlist = []
for a,b in d.items():
   invlist.append([b,a])
inv = dict(invlist)

4. Same thing with a list comprehension:

d = {"A":1,"B":2,"C":3}
dict([[b,a] for a,b in d.items()])

5. Or using zip():

d = {"A":1,"B":2,"C":3}
dict(zip(d.values(),d.keys()))