You could use the UUID module for generating a random string:
import uuid
filename = str(uuid.uuid4())
This is a valid choice, given that an UUID generator is extremely unlikely to produce a duplicate identifier (a file name, in this case):
Only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%. The probability of one duplicate would be about 50% if every person on earth owns 600 million UUIDs.