Why does using a while loop make the program work but my original for loop doesn't work? (DNA pset6)

Question

just finished pset6 DNA but I am confused. When counting the consecutive dna sequences, if I used a for loop to iterate through the dna sequence text, my code would always produce 'No Match' as the output, but if I change it too a while loop to iterate through the dna sequence it would work. I can't figure out why. I commented out the entire for loop section below the while loop section.

Any help appreciated.

from sys import argv, exit
import csv

if len(argv) != 3:
    print("Usage: python dna.py data.csv sequence.txt")
    exit(1)

with open(argv[1], 'r') as csv_file:
    csv_reader = csv.reader(csv_file)
    for row in csv_reader:
        header = row
        header.pop(0)
        break

dictionary = {}

for item in header:
    dictionary[item] = 0

with open(argv[2], 'r') as dna_txt:
    dna_reader = dna_txt.read()

# This iteration using while loop works 
for key in dictionary:
    temp = 0 
    max_count = 0
    i = 0
    # This while loop works
    while i < len(dna_reader):
        if dna_reader[i : i + len(key)] == key:
            temp += 1
            if ((i + len(key)) < len(dna_reader)):
                i += len(key)
                continue
        else:
            if temp > max_count:
                max_count = temp
            temp = 0
        i += 1
    dictionary[key] = max_count

# This iteration does not work, only difference is for loop instead of while loop, why is that? Commented out so it doesn't interfere
'''for key in dictionary:
    temp = 0 
    max_count = 0

    # This for loop does not work

    for i in range(len(dna_reader)):
        if dna_reader[i : i + len(key)] == key:
            temp += 1
            if ((i + len(key)) < len(dna_reader)):
                i += len(key)
                continue
        else:
            if temp > max_count:
                max_count = temp
            temp = 0
    dictionary[key] = max_count'''

with open(argv[1], 'r') as file:
    table = csv.DictReader(file)
    for person in table:
        count = 0
        for dna in dictionary:
            if int(dictionary[dna]) == int(person[dna]):
                count += 1
            else:
                count = 0
            if count == len(header):
                print(person['name'])
                exit(1)

print('No match')
exit(0)

Answer 1

The only substantial difference I can see is the effect of i += len(key) . In a for loop, the i variable will be re-instantiated on the start of every loop. Therefore, this statement serves no practical effect. In case of the while loop, the same i variable is reused.

Consider this example:

for i in range(10):
    print(i)
    if i == 5:
        i += 2

The output of this will be:

0
1
2
3
4
5
6
7
8
9

Hope this helps.