Strange index errors

Hi there,

Now I am facing strange Index error with following code. There is a test case without flows like "checkio(["9876543210"])" and I could not pass this case due to strange index error. Actually, this case does not provide parameter named "flows" therefore I put a IF statement (if len(flows) > 0:), however, index error occurred under the IF block. More precisely, if e.endswith(f[0][1]): 6 lines below the if statement. I don't understand why this index error occurred even though the statement should not go through for this test case without parameter flows. Could someone give me advice to move forward?

ETHERNET = (100, 40, 10, 1, 0.1) # Ethernet bandwidth capacity in Gbps

def checkio(ring, *flows):

    if len(flows) == 0:
        flows = ring[1:]
        ring = ring[0]

    if not isinstance(ring, str):
        ring = str(ring[0])

    path_list = []

    # Get path list
    for i in range(len(ring)-1):
        path_list.append(ring[i]+ring[i+1])
    path_list.append(ring[-1]+ring[0])
    flow_dict = {}
    #print(path_list)

    if len(flows) > 0:
        for f in flows:
            # Forward
            for e in path_list:
                if e.startswith(f[0][0]):
                    s_sec = e
                if e.endswith(f[0][1]):
                    e_sec = e

            # These worked fine in my local environment but not in checkio. (IndexError: list index out of range)
            #s_loc = path_list.index([a for a in path_list if a.startswith(f[0][0])][0])
            #e_loc = path_list.index([a for a in path_list if a.endswith(f[0][1])][0])
            # Alternative coding
            s_loc = path_list.index(s_sec)
            e_loc = path_list.index(e_sec)

            if e_loc >= s_loc:
                distance = e_loc - s_loc  
            else:
                distance = len(path_list) - e_loc + s_loc - 1
            # backward case
            if distance + 1 > len(path_list) / 2:
                distance = (len(path_list) - distance - 2) * -1 #negative if backward
                s_loc -= 1
                if s_loc == -1:
                    s_loc = len(path_list) - 1 
                e_loc += 1 
                if e_loc >= len(path_list):
                    e_loc = 0

            for i in range(abs(distance) + 1):
                if distance >= 0:
                    location = s_loc + i
                    if location >= len(path_list):
                        location -= len(path_list)
                    if path_list[location] in flow_dict:
                        flow_dict[path_list[location]] += f[1]
                    else:
                        flow_dict[path_list[location]] = f[1]
                else:
                    location = s_loc - i
                    if location < 0:
                        location += len(path_list)
                    if path_list[location] in flow_dict:
                        flow_dict[path_list[location]] += f[1]
                    else:
                        flow_dict[path_list[location]] = f[1]

    #print(flow_dict)
    result_list = [0, 0, 0, 0, 0]
    for elem in flow_dict.items():
        for bw in range(1, 6):
            if ETHERNET[-bw] >= elem[1]:
                result_list[-bw] += 1
                break
        # take care of extra large numbers
        if elem[1] > ETHERNET[0]:
            result_list[0] += elem[1] // ETHERNET[0]
            if elem[1] % ETHERNET[0] != 0:
                result_list[0] += 1

    print(result_list)
    return result_list

if __name__ == '__main__':
    # These "asserts" are used only for self-checking and not necessary for auto-testing
    assert checkio("AEFCBG", ("AC", 5), ("EC", 10), ("AB", 60)) == [2, 2, 1, 0, 0]
    assert checkio("ABCDEFGH", ("AD", 4)) == [0, 0, 3, 0, 0]
    assert checkio("ABCDEFGH", ("AD", 4), ("EA", 41)) == [4, 0, 3, 0, 0]
    assert checkio(["9876543210"]) == [0, 0, 0, 0, 0]