Detect local minima and maxima in a Python list

local-minima-and-maxima.py

def flatten(a):
    return sum(a, [])

def local_minima(a):
    if len(a) < 2:
        return list(range(len(a)))
    
    lmin = True
    curr = list()
    local_minima = list()
    
    for i, x in enumerate(a[:-1]):
        if a[i + 1] == x and lmin:
            curr.append(i)
        if a[i + 1] < x:
            lmin = True
            curr = list()
        if a[i + 1] > x:
            if lmin:
                local_minima.append(curr + [i])
            lmin = False

    if a[-1] < a[-2] or (lmin and a[-1] == a[-2]):
        local_minima.append(curr + [len(a) - 1])
    
    return flatten(local_minima)

def invert(a):
    return [-x for x in a]

def local_maxima(a):
    return local_minima(invert(a))

test.py

test = [
    ([1,2,3,2,3,2,1],   [0,3,6]),
    ([1,2,3,2,2,3,2,1], [0,3,4,7]),
    ([4,3,2,3,4],       [2]),
    ([1,2,1],           [0,2]),
    ([1,2,1,2,1],       [0,2,4]),
    ([2,1,1,1,1,2],     [1,2,3,4]),
    ([2,1,1,1,0],       [4]),
    ([1,1,1],           [0,1,2]),
    ([1,2,3],           [0]),
    ([3,2,1],           [2]),
    ([1,2,2,2,1],       [0,4]),
    ([1,2,1,2,1,2],     [0,2,4]),
    ([3,2,1,1,1],       [2,3,4]),
    ([1],               [0]),
    ([],                [])
]

for i, o in test:
    l = local_minima(i)
    print("In: {}; Expected: {}; Actual: {}; Passed: {}".format(i, o, l, l == o))