def list_flatten(l, a=None):
    #check a
    if a is None:
        #initialize with empty list
        a = []

    for i in l:
        if isinstance(i, list):
            list_flatten(i, a)
        else:
            a.append(i)
    return a


In [3]: list_flatten([1, 4, 5])
Out[3]: [1, 4, 5]

In [4]: list_flatten([1, 4, [5]])
Out[4]: [1, 4, 5]

In [5]: list_flatten([1, 4, [5, 0]])
Out[5]: [1, 4, 5, 0]

In [6]: list_flatten([1, 4, [5, 0, 44]])
Out[6]: [1, 4, 5, 0, 44]

In [7]: list_flatten([1, 4, [5, 0, 4, [4]]])
Out[7]: [1, 4, 5, 0, 4, 4]

In [8]: list_flatten([1, 4, [5, 0, 4, [4, 5, 6, 6, 6]]])
Out[8]: [1, 4, 5, 0, 4, 4, 5, 6, 6, 6]

In [9]: list_flatten([1, 4, [5, 0, 4, [4, 5, 6, 6, 6, [0]]]])
Out[9]: [1, 4, 5, 0, 4, 4, 5, 6, 6, 6, 0]