Random prime Number in python

Question

I currently have ↓ set as my randprime(p,q) function. Is there any way to condense this, via something like a genexp or listcomp? Here's my function:

n = randint(p, q)
while not isPrime(n):
    n = randint(p, q)

Answer 1

It's better to just generate the list of primes, and then choose from that line. As is, with your code there is the slim chance that it will hit an infinite loop, either if there are no primes in the interval or if randint always picks a non-prime then the while loop will never end.

So this is probably shorter and less troublesome:

import random
primes = [i for i in range(p,q) if isPrime(i)]
n = random.choice(primes)

The other advantage of this is there is no chance of deadlock if there are no primes in the interval. As stated this can be slow depending on the range, so it would be quicker if you cached the primes ahead of time:

# initialising primes
minPrime = 0
maxPrime = 1000
cached_primes = [i for i in range(minPrime,maxPrime) if isPrime(i)]

#elsewhere in the code
import random
n = random.choice([i for i in cached_primes if p<i<q])

Again, further optimisations are possible, but are very much dependant on your actual code... and you know what they say about premature optimisations.