day3-take2.py

import sys

total = 0

# Read the input file from the command line arguments
with open(sys.argv[1], "r") as f:
    # Read each line of the input file
    for line in f:
        # Keep track of the characters that have already been counted
        # for the current rucksack
        counted = set()

        # Split the line into two compartments
        comp1, comp2 = line[:len(line)//2], line[len(line)//2:]

        # Iterate through each character in the first compartment
        for ch in comp1:
            # If the character also appears in the second compartment,
            # and it hasn't been counted already, add its priority to the
            # running total
            if ch in comp2 and ch not in counted:
                counted.add(ch)
                # Lowercase item types have priorities 1 through 26
                if ch.islower():
                    total += ord(ch) - ord("a") + 1
                # Uppercase item types have priorities 27 through 52
                elif ch.isupper():
                    total += ord(ch) - ord("A") + 27

# Print the sum of the priorities of the item types that appear in both
# compartments of each rucksack
print(total)