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I use the following code snippet to generate the random characters

char_set = string.ascii_letters + string.digits
Randomno = ''.join(random.sample(char_set*10,9))

Here, I multiply the number of symbols by $10$ so we get $620 ^ 9 = 13537086546263552000000000$ before a number repeats. But will random.sample actually generate all these possible combinations?

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As far as i know the samples are generated independently. That would be very inefficient. So random.sample draws a number in the interval $[0, 620]$ and moves on to the next index.

Because the probabilities of drawing a certain character are independent the probability of repeating a character, i.e. $..., 10, 10, ...$ is $\frac{1}{620} \frac{1}{620}$, which is very low. The chance of generating two consecutive samples that are equal, e.g. $1, 2, 3, 4, 5, 6, 7, 8, 9$ two times is $\frac{1}{620^9} \frac{1}{620^9}$ which is even lower.

So there are $620^9$ combinations which means that on average the same combinations occurs after $620^9$ trials.

However, the chance is very low but it could happen faster. As i said, its on average, because it's a random process.

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  • $\begingroup$ Randomno returns a string of characters which is 9 characters long. So I was interested in knowing the probability of the entire sequence being repeated. Say in the first run I get 123abc456. After how many runs will this sequence repeat? @hh32 $\endgroup$
    – Shre1407
    Commented Aug 31, 2017 at 13:57
  • $\begingroup$ I know that. But what i meant is that each part of that is generated independently, i.e. 1, then 2, then 3, and so on. I edited my answer $\endgroup$
    – hh32
    Commented Aug 31, 2017 at 14:01

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