Example of how slicing may be implemented

apply_slice.py

def apply_slice(it, sl):
    start, stop, step = sl.indices(len(it))

    elems = []
    i = start
    if step > 0:
        while i < stop:
            elems.append(it[i])
            i += step
    elif step < 0:
        while i > stop:
            elems.append(it[i])
            i += step

    # Some type-specific conversions
    if isinstance(it, str): elems = ''.join(elems)
    #...

    return elems

#===================================================================================================

# Some string, and some example slices
st = "The quick brown fox jumps over the lazy dog"
slices = [
          slice(0,10,2),
          slice(None, None, None),
          slice(None, None, 1),
          slice(None, None, -1),
          slice(len(st), -len(st)-1, -1),
          slice(len(st), -len(st)-1),
]
# Apply each, and compare the outputs
for sl in slices:
    sliced1 = st[sl]
    sliced2 = apply_slice(st, sl)
    try:
        assert sliced1 == sliced2
        print("%-25s  %r" % (sl, sliced1))
    except AssertionError:
        print("---")
        print(sl)
        print(sliced1)
        print(sliced2)
        print("---")

'''
Output:

slice(0, 10, 2)            'Teqik'
slice(None, None, None)    'The quick brown fox jumps over the lazy dog'
slice(None, None, 1)       'The quick brown fox jumps over the lazy dog'
slice(None, None, -1)      'god yzal eht revo spmuj xof nworb kciuq ehT'
slice(43, -44, -1)         'god yzal eht revo spmuj xof nworb kciuq ehT'
slice(43, -44, None)       ''
'''