Range

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

This approach starts by checking if the two strands are of equal length. 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.

range in Python is a built-in function that returns a sequence of numbers. range produces an infinite sequence, but it can be limited by providing a <stop> argument. The range function can also take a <start> argument and a <step> argument. The inputs are built up like this: range(<start>, <stop>, <step>). When only a <stop> argument is provided, <start> defaults to 0 and <step> defaults to 1.

We use range to iterate over the indexes of the strand_a string. We do that by passing the length of the string to range by using the built-in function len. Iterating over range gives us an index number we can use to access a character in the string.

We can then compare the character at the index in strand_a to the character at the same index in strand_b. If the two values are not equal, we increment the <count> variable by 1.

After the loop completes, we return the <count> variable.