Type Error when making a recursive function

Question

Making a recursive function to sort the graph points. When compiling get the error:

TypeError: can only concatenate list (not "NoneType") to list

Sample xy values: [[1, 1], [2, 2], [2, 3], etc.]

def FindNext(list_xy):
    if len(list_xy) == 1:
        xo = list_xy[0][0]
        yo = list_xy[0][1]
        return [xo, yo]
    xo = list_xy[0][0]
    yo = list_xy[0][1]
    for i in range(len(list_xy)):
        xf = list_xy[i][0]
        yf = list_xy[i][1]
        dx = xo - xf
        dy = yo - yf
        d = (dx**2+dy**2)**0.5
        if d == 2**0.5 or d ==1:
            return [xo, yo] + FindNext(list_xy[1:])
FindNext(xy)

Your sample xy works for me when I run your code. Which line is the error supposedly on? — Kris
– Kris, Commented Mar 16, 2021 at 17:58
Full data xy data set: [[1, 1], [2, 2], [2, 3], [2, 4], [2, 5], [3, 6], [4, 6], [5, 6], [6, 7], [7, 7], [8, 7], [9, 8], [7, 9], [9, 9], [6, 10], [8, 10], [4, 11], [5, 11], [3, 12], [8, 12], [9, 12], [4, 13], [6, 13], [7, 13], [10, 13], [5, 14], [9, 14], [9, 15], [9, 16], [8, 17], [9, 18], [9, 19], [13, 19], [10, 20], [11, 20], [12, 20], [14, 20], [15, 21], [6, 22], [10, 22], [12, 22], [15, 22], [5, 23], [7, 23], [8, 23], [9, 23], [11, 23], [13, 23], [16, 23], [5, 24], [13, 24], [16, 24], [4, 25], [14, 25], [15, 25], [3, 26], [4, 27], [3, 28], [3, 29], [3, 30], [2, 31]] — is1220
– is1220, Commented Mar 16, 2021 at 18:44

Randy · Accepted Answer · 2021-03-16 17:57:07Z

1

Python implicitly returns None if it reaches the end of a function without hitting a return, so one of your function inputs is probably doing that. You can solve for that by adding return [] to the end of the function, though I don't know if the logic is correct for what you're calculating.

answered Mar 16, 2021 at 17:57

Randy

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