Not that brevity is the be all and end all, but here’s what I got:
results = {}
print("Reading data file.")
with open("data", mode="r", encoding="UTF-8") as data:
for line in data:
sample = line.strip().split(',')
key = ':'.join(sorted(sample[0].split(':')))
value = float(sample[1])
prev_val = results.setdefault(key, value)
results[key] = max(prev_val, value)
print("Writing data to file..")
with open("output", mode='w', encoding="UTF-8") as output:
for key, value in results.items():
data = f"{key}, {value}\n"
output.write(data)
print("Script exit")
dict.setdefault is really handy when you find yourself writing if key not in dict: put_the_item_in_the_dict
And I PEP-8’d the names 