basic question isinstance

[ibreeden]

Yes, recursive algorithms are always difficult to understand.
When I have a program of which I do not understand the results, I add print statements to them so I can follow what happens from step to step. Like this:

def sumtree(l):
    tot = 0
    print(f"New instance of sumtree({l})")
    for x in l:
        if not isinstance(x, list):
            tot += x
            # print(tot)
        else:
            tot += sumtree(x)
            # print('xxx')
    print(f"End of sumtree({l}), returning: {tot}")
    return tot


o = [1, [2, [3, 4], 5], 6, [7, 8]]
print(sumtree(o))

Output:
New instance of sumtree([1, [2, [3, 4], 5], 6, [7, 8]])
New instance of sumtree([2, [3, 4], 5])
New instance of sumtree([3, 4])
End of sumtree([3, 4]), returning: 7
End of sumtree([2, [3, 4], 5]), returning: 14
New instance of sumtree([7, 8])
End of sumtree([7, 8]), returning: 15
End of sumtree([1, [2, [3, 4], 5], 6, [7, 8]]), returning: 36
36

Does this make clear what happens?