Prim's Algorithm - Find MST of an undirected, arbitrarily connected graph

MST

def mst(graph):
    '''
    mst retuns the minimum spanning tree of `graph`, which must be a symmetric NxN array, where 
    element (i,j) is the weight of the connection from node i to node j. The diagonal should be all 
    zeros (because i and j are the same point), and unconnected nodes are `None`.
    
    The nth position of the array returned contains the parent node of node n. The order always
    starts from node 0 as the root of the tree, so order[0] is always None.
    '''
    order = [None] * len(graph)
    parents = [0] * len(graph)
    choices = graph[0]
    for _ in xrange(1, len(graph)):
        i, v = 0, 0
        for j, w in enumerate(choices):
            if w > 0 and (v == 0 or w < v):
                i, v = j, w
        order[i] = parents[i]
        for j, w in enumerate(graph[i]):
            if w is not None and w < choices[j]:
                choices[j], parents[j] = w, i 
    return order