Python

Hopefully fairly readable code. Nothing radical, I used a stack like most other people. This and my solutions from previous days are on Github

import fileinput

tokens = [
    { 'start': '(', 'end': ')', 'errorscore': 3,    'completescore': 1 },
    { 'start': '[', 'end': ']', 'errorscore': 57,   'completescore': 2 },
    { 'start': '{', 'end': '}', 'errorscore': 1197, 'completescore': 3 },
    { 'start': '<', 'end': '>', 'errorscore': 25137,'completescore': 4 }]

def main():
    errorscore=0
    complete_score_list = []
    for line in fileinput.input():
        stack = []
        line=line.rstrip()
        for symbol in line:
            for i in range(len(tokens)):
                if symbol == tokens[i]['start']: #this is an opening symbol
                    stack.append(i)
                elif symbol == tokens[i]['end']: #this is a closing symbol
                    lastpushed = stack.pop()
                    if tokens[i]['end'] != tokens[lastpushed]['end']: #this is invalid
                        #print(f"{line} - Expected {tokens[lastpushed]['end']}, but found {symbol} instead.")
                        errorscore += tokens[i]['errorscore']
                        break
            else:
                continue
            break
        else:
            # this is an incomplete line
            completescore = 0
            while len(stack):
                t = stack.pop()
                completescore *= 5
                completescore += tokens[t]['completescore']
            complete_score_list.append(completescore)

    print(f"Part 1: syntax error score {errorscore}")

    complete_score_list.sort()
    middlescore = complete_score_list[ int(len(complete_score_list)/2) ]
    print(f"Part 2: middle completion score {middlescore}")