Check length of list in list comprehension (Iteration based on the count of elements within list comprehension)

Question

I need to calculate just first five prime numbers. I want to check the length of list while building it in list comprehension. The following code doesn't work.

def checkPrime(n):
    for i in range(2,int(n**0.5)+1):
        if n%i==0:
            return False
    return True



 primes = [] 
 primes = [x  for x in range(2,30) if(checkPrime(x) and len(primes)<6) ] 
 print primes

Output:

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Why len(primes)<6 is not working here. How can I achieve that?

Answer 1

The "Pythonic" way is to use a generator, plus the islice function:

from itertools import islice

def checkPrime(n):
    for i in range(2,int(n**0.5)+1):
        if n%i==0:
            return False
    return True

def primes():
    i = 2
    while True:
        if checkPrime(i):
            yield i
        i += 1

first_5 = list(islice(primes, 5))

What this will do is create an infinite "list" called primes that you can use in many ways that a list can be used, but it will only actually calculate the values that you need.