Sum

The benefit of using sum is that we can use a generator expression to create a list of booleans. We can then pass that generator to sum and it will iterate through and add up all the booleans. Where True is treated as 1 and False is treated as 0. Then that total is returned.

This can make the code a bit more concise.

Here is an example using sum with zip:

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

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.

This approach uses the zip function to iterate over two strings. You can read more about how to solve this exercise with zip in the zip approach.

What differs in this approach is that we use a generator expression to create booleans. The generator expression returns an iterator over the tuples returned by zip. Within the iteration, the tuples are unpacked into two variables, nucleotide_a and nucleotide_b. We can then compare nucleotide_a and nucleotide_b. If they are not equal, True is produced. If they are equal, False is produced. The generator expression is then passed to the sum function.

sum will then iterate over the generator expression and add up all the booleans. Where True is treated as 1 and False is treated as 0. You can read more about this behavior in Boolean as numbers. Finally the totaled booleans are returned.

This approach is also doable with range but it is a bit more verbose:

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