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()
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 didn't expect one 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?