Zip

def distance(strand_a, strand_b):
    if len(strand_a) != len(strand_b):
        raise ValueError("Strands must be of equal length.")
    count = 0
    for nucleotide_a, nucleotide_b in zip(strand_a, strand_b):
        if nucleotide_a != nucleotide_b:
            count += 1
    return count

This approach starts by checking if the two strands are of equal length by using len. If not, a ValueError is raised.

After that is checked, a <count> variable is initialized to 0. The count variable will be used to keep track of the number of differences between the two strands.

We use zip to iterate over the characters in strand_a and strand_b simultaneously. zip is a built in function. It takes any number of iterables and returns an iterator of tuples. Where the i-th tuple contains the i-th element from each of the argument iterables. For example, the first tuple will contain the first element from each iterable, the second tuple will contain the second element from each iterable, and so on until the shortest iterable is exhausted.

In Python, strings are iterable.

Here is an example of using zip to iterate over two strings:

>>> zipped = zip("GGACGG", "AGGACG")
>>> list(zipped)
[('G', 'A'), ('G', 'G'), ('A', 'G'), ('C', 'A'), ('G', 'C'), ('G', 'G')]

We then use the zip iterator to iterate over the tuples. We unpack the tuple into two variables, nucleotide_a and nucleotide_b. You can read more about unpacking in the concept unpacking-and-multiple-assignment.

We then compare the characters nucleotide_a and nucleotide_b. If they are not equal, we increment the count variable by 1.

After the loop is finished, we return the count variable.