Multi-conditional generator expression in Python?

Question

I am trying to create an approximation solution for the Traveling Salesperson Problem using the nearest neighbor method. The cities are represented by a weighted adjacency matrix in the form of nested lists like this: [[0, 4, 3, 5], [4, 0, 5, 3], [3, 5, 0, 4], [5, 3, 4, 0]]

There is also a boolean matrix which determines which nodes have already been visited.

My Question: I am trying to use a multi-conditional generator expression to find the non-zero minimum value in each list which has not already been visited. I am attempting to do this using this line: listMin = min(i for i in next if ((i>0) and (visited[next.index(i)] == False))) How can I get this to work?

Note that this generator expression functions as intended when I am simply finding non-zero values, it is the additional conditional statement that is throwing it off by giving me a ValueError: min() arg is empty sequence.

More code here:

start = matrix[0]
visited[0] = True
listMin = min(i for i in start if i>0)
cost = listMin
minIndex = start.index(listMin)

while (False in visited):
    next = matrix[minIndex]
    visited[minIndex] = True
    order.append(int(minIndex))

    listMin = min(i for i in next if ((i>0) and (visited[next.index(i)] == False)))
    minIndex = next.index(listMin)
    
    cost += listMin`

EDIT- Wanted to add that my code seems to be working for the test case after using an if statement to validate that min node has not been visited, but I am simply curious if using a generator expression in this way is possible

Answer 1

ValueError: min() arg is empty sequence.

If you allow min first argument to be empty, you should use keyword argument default to set what it should return in such case. Simple example:

v1 = min([], default=-1)  # using list
v2 = min((i for i in []), default=-1)  # using generator, note brackets usage
print(v1, v2)  # -1 -1

Just use value which never appears in your data, if you then want to check if first argument provided to min was empty.