Trying to find a Nth prime number (for a http://projecteuler.net/problem=7)

prime-finder.py

def get_primes(start, end, primes=[]):
    '''Get the numbers starting from start with the count based on primes
    '''
    sieve = range(start, end)
    for prime in primes:
        # Get the start value for primes striking
        start_value = prime ** 2
        if start_value > end:
            break
        if start_value < start:
            num = start / prime
            if start % prime:
                num += 1
            start_value = num * prime
        # Strike out complex
        for index in xrange(start_value, end, prime):
            sieve[index - start] = False
    # Get the numbers was not striked out
    return [number for number in sieve if number]


def get_prime_by_number(number):
    '''Get the n-th prime number'''
    primes = [2, 3]
    last_num = 0
    primes_num = 2
    end_value = step = 5
    while primes_num < number:
        # Get a new step size for prime sieve
        last_num = primes_num
        primes_num = len(primes)
        last_prime = primes[primes_num - 1]
        start_value = end_value
        end_value = last_prime ** 2
        if primes_num > last_num:
            step *= (number - primes_num) * 1. / (primes_num - last_num)
        if start_value + step < end_value:
            end_value = start_value + int(step)
        else:
            step = end_value - start_value
        # Get new primes
        primes += get_primes(start_value, end_value, primes)
    return primes[number - 1]

print get_prime_by_number(100001)