Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python
import string
import sys
from itertools import chain, dropwhile, takewhile


def read(
    *shape, f=int, it=chain.from_iterable(sys.stdin), whitespaces=set(string.whitespace)
):
    def read_word():
        w = lambda c: c in whitespaces
        nw = lambda c: c not in whitespaces
        return f("".join(takewhile(nw, dropwhile(w, it))))

    if not shape:
        return read_word()
    elif len(shape) == 1:
        return [read_word() for _ in range(shape[0])]
    elif len(shape) == 2:
        return [[read_word() for _ in range(shape[1])] for _ in range(shape[0])]


def readi(*shape):
    return read(*shape)


def readi1(*shape):
    return [i - 1 for i in readi(*shape)]


def readf(*shape):
    return read(*shape, f=float)


def reads(*shape):
    return read(*shape, f=str)


def arr(*shape, fill_value=0):
    if len(shape) == 1:
        return [fill_value] * shape[0]
    elif len(shape) == 2:
        return [[fill_value] * shape[1] for _ in range(shape[0])]


def dbg(**kwargs):
    print(
        ", ".join("{} = {}".format(k, repr(v)) for k, v in kwargs.items()),
        file=sys.stderr,
    )


from collections import defaultdict


def main():
    N, M = readi(2)
    edges = readi(M, 3)
    pred = defaultdict(list)
    for a, b, _ in edges:
        pred[b].append(a)
    # dbg(pred=pred)
    s = set()
    s2 = set()

    def rec(n):
        if n not in s2:
            s2.add(n)
            for m in pred[n]:
                s.add(m)
                rec(m)

    rec(N)
    # dbg(s=s)
    dist = arr(N + 1, fill_value=float("-inf"))
    dist[1] = 0
    for i in range(N):
        for a, b, c in edges:
            new_d = dist[a] + c
            if new_d > dist[b]:
                if i == N - 1 and b in s:
                    print("inf")
                    return
                dist[b] = new_d
    print(dist[N])


if __name__ == "__main__":
    main()


Fixed submission:
#!/usr/bin/env python
import string
import sys
from itertools import chain, dropwhile, takewhile


def read(
    *shape, f=int, it=chain.from_iterable(sys.stdin), whitespaces=set(string.whitespace)
):
    def read_word():
        w = lambda c: c in whitespaces
        nw = lambda c: c not in whitespaces
        return f("".join(takewhile(nw, dropwhile(w, it))))

    if not shape:
        return read_word()
    elif len(shape) == 1:
        return [read_word() for _ in range(shape[0])]
    elif len(shape) == 2:
        return [[read_word() for _ in range(shape[1])] for _ in range(shape[0])]


def readi(*shape):
    return read(*shape)


def readi1(*shape):
    return [i - 1 for i in readi(*shape)]


def readf(*shape):
    return read(*shape, f=float)


def reads(*shape):
    return read(*shape, f=str)


def arr(*shape, fill_value=0):
    if len(shape) == 1:
        return [fill_value] * shape[0]
    elif len(shape) == 2:
        return [[fill_value] * shape[1] for _ in range(shape[0])]


def dbg(**kwargs):
    print(
        ", ".join("{} = {}".format(k, repr(v)) for k, v in kwargs.items()),
        file=sys.stderr,
    )


from collections import defaultdict
import sys

sys.setrecursionlimit(100000)

def main():
    N, M = readi(2)
    edges = readi(M, 3)
    pred = defaultdict(list)
    for a, b, _ in edges:
        pred[b].append(a)
    # dbg(pred=pred)
    s = set()
    s2 = set()

    def rec(n):
        if n not in s2:
            s2.add(n)
            for m in pred[n]:
                s.add(m)
                rec(m)

    rec(N)
    # dbg(s=s)
    dist = arr(N + 1, fill_value=float("-inf"))
    dist[1] = 0
    for i in range(N):
        for a, b, c in edges:
            new_d = dist[a] + c
            if new_d > dist[b]:
                if i == N - 1 and b in s:
                    print("inf")
                    return
                dist[b] = new_d
    print(dist[N])


if __name__ == "__main__":
    main()