CodeSOD: Plenty of IDs

Venkat joined a new team in the middle of a big refactoring effort. The code was quite a mess, glued together by long Python scripts with main functions longer than a Tolstoy novel. It was this effort to clean up the code into neat modules that highlighted some logical issues with their implementation.

 def generate_random_number(): """ Generate a 5 digit random number. """ return "".join(map(str, random.sample(range(0,10),5)))

The script this function comes from is a launcher script that provisions a VM to execute code. The VM needs a unique identifier, and we get that random number this way.

The random.sample function, in Python, samples a population without replacement. So here, we pass it a range of 0-10 (which is 0-9 inclusive), and ask for five random entries from it. Again, without replacement, so a range that could concievably contain up to 100,000 possible values is significantly constrained down to 30,000 or so- only the numbers less than 100,000 that contain no duplicate digits.

We then map the resulting array into an array of strings, then join that into a final output string.

Now, for most of us, I do suspect that 30,240 unique values is probably enough for how ever many VMs we might need to reserve at any one time; if you're not working for Amazon or Google or whatever, you're probably fine. Then again, the chances of an accidental collision are surprisingly high: after only 26 requests, there's a 1% chance of a collision. And thanks to the birthday problem, it takes 206 requests to get to a 50% chance of collision. So maybe 30,240 is not enough for anybody.

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