Buggy python submission (verdict: Time Limit Exceeded):
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    n = n[::-1]
    res = [0]*2019
    res[0] += 1
    b = 0
    for i in range(len(n)):
        a = int(n[i])
        b += a*(10**i)%2019
        b %= 2019
        res[b] += 1
    ans = 0
    for i in res:
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    n = n[::-1]
    res = [0]*2019
    res[0] += 1
    b = 1
    c = 0
    for i in range(len(n)):
        a = int(n[i])
        c += a*b
        c %= 2019
        res[c] += 1
        b = b*10%2019
    ans = 0
    for i in res:
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    res = [0]*2019
    res[0] += 1
    
    for i in range(len(n)):
        a = int(n[i:])
        b = a%2019
        res[b] += 1
    ans = 0
    for i in res:
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    n = n[::-1]
    res = [0]*2019
    res[0] += 1
    b = 1
    c = 0
    for i in range(len(n)):
        a = int(n[i])
        c += a*b
        c %= 2019
        res[c] += 1
        b = b*10%2019
    ans = 0
    for i in res:
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    res = []
    
    for i in range(len(n)):
        a = int(n[i:])
        res.append(a%2019)
    res.append(0)
    c = Counter(res)
    ans = 0
    for i in c.values():
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！ not は使わず !=
#0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math

    #inf = 10**17
    #mod = 10**9 + 7

    n = input().rstrip()
    n = n[::-1]
    res = [0]*2019
    res[0] += 1
    b = 1
    c = 0
    for i in range(len(n)):
        a = int(n[i])
        c += a*b
        c %= 2019
        res[c] += 1
        b = b*10%2019
    ans = 0
    for i in res:
        if i>=2:
            ans += i*(i-1)//2
    print(ans)

if __name__ == '__main__':
    main()