k-mer counter

def kmers_finder(input):
    with open(input, 'r') as data:
        sequence = data.readline()
        k = int(data.readline())

    kmers_count = {}
    top_rng = len(sequence)-k+1

    for i in range(0, top_rng):
        kmer = sequence[i:i+k]
        if len(kmer) != 14:
            print 'FAIL!'
        if kmer not in kmers_count:
            kmers_count[kmer] = 1
        else:
            kmers_count[kmer] += 1

    return kmers_count

def freqent_kmers(kmers):
    highest_value = 0
    output = []
    for k, v in kmers.items():
        if v > highest_value:
            highest_value = v
    for k, v in kmers.items():
        if v == highest_value:
            output.append(k)

    return ' '.join(output)

print freqent_kmers(kmers_finder('dataset_2_10.txt'))