Python - Why use anything other than uuid4() for unique strings?

Question

I see quit a few implementations of unique string generation for things like uploaded image names, session IDs, et al, and many of them employ the usage of hashes like SHA1, or others.

I'm not questioning the legitimacy of using custom methods like this, but rather just the reason. If I want a unique string, I just say this:

>>> import uuid
>>> uuid.uuid4()
UUID('07033084-5cfd-4812-90a4-e4d24ffb6e3d')

And I'm done with it. I wasn't very trusting before I read up on uuid, so I did this:

>>> import uuid
>>> s = set()
>>> for i in range(5000000):  # That's 5 million!
>>>     s.add(uuid.uuid4())
...
...
>>> len(s)
5000000

Not one repeater (I wouldn't expect one now considering the odds are like 1.108e+50, but it's comforting to see it in action). You could even half the odds by just making your string by combining 2 uuid4()s.

So, with that said, why do people spend time on random() and other stuff for unique strings, etc? Is there an important security issue or other regarding uuid?

Answer 1

One possible reason is that you want the unique string to be human-readable. UUIDs just aren't easy to read.

Answer 2

Well, sometimes you want collisions. If someone uploads the same exact image twice, maybe you'd rather tell them it's a duplicate rather than just make another copy with a new name.

Answer 3

uuids are long, and meaningless (for instance, if you order by uuid, you get a meaningless result).

And, because it's too long, I wouldn't want to put it in a URL or expose it to the user in any shape or form.