code help

from random import sample

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
prev_chars = ["", "", ""]
stream = []

for _ in range(10):
    chars = sample(alphabet, 3)

    # This while loop *should* ensure that there are no repeated letters in the same position in consecutive streams
    #   (e.g. ['C', 'A', 'Y'] and ['C', 'R', 'X'])
    flag = True
    while flag:
        # Set flag to false.  If the while loop needs to be run once more, it will be set to true later
        flag = False

        # Iterate through the list of letters and previous letters
        for char, prev_char in zip(chars, prev_chars):
            # If the current letter is the same as the previous letter,
            if char == prev_char:
                # Generate a new list of letters
                chars = sample(alphabet, 3)

                # Set the flag to True so the while loop runs at least once more to check the most recent chars
                flag = True

                # Break out of the for loop
                break

            # If the loop doesn't break, set the flag to False
            # I suspect this is in the wrong place, but I'm not sure where to put it
            #flag = False

    stream.append(chars)
    prev_chars = chars

print(stream)