Slice a page range to create a nice pagination effect.

pagination.py

def slice_pagination_range(page_range, page_number, slice_range):
    """
    Slice the given ``page_range`` to +/- ``slice_range`` values around the given ``page_number``.
    Center the slice on the given ``page_number`` (one based).
    First and last values are always in the output slice.
    If required, ellipsis are added as ``None`` values.
    """
    nb_pages = len(page_range)

    # Avoid slicing on small range
    if nb_pages <= slice_range * 2 + 3:
        yield from page_range
        return
    
    # Get slice start and stop indexes
    page_number -= 1  # zero based index 
    last_index = nb_pages - 1
    slice_start = max(1, page_number - slice_range)
    slice_stop = min(last_index - 1, page_number + slice_range)

    # Left ellipsis
    yield page_range[0]
    if slice_start > 1:
        yield None

    # Sliced range
    yield from page_range[slice_start:slice_stop + 1]

    # Rigth ellipsis
    if slice_stop < last_index - 1:
        yield None
    yield page_range[-1]


# Small range test (no slicing)
assert list(slice_pagination_range(range(1, 6), 1, 1)) == [1, 2, 3, 4, 5]
assert list(slice_pagination_range(range(1, 6), 2, 1)) == [1, 2, 3, 4, 5]
assert list(slice_pagination_range(range(1, 6), 3, 1)) == [1, 2, 3, 4, 5]
assert list(slice_pagination_range(range(1, 6), 4, 1)) == [1, 2, 3, 4, 5]
assert list(slice_pagination_range(range(1, 6), 5, 1)) == [1, 2, 3, 4, 5]

# Medium range test (one ellipsis)
assert list(slice_pagination_range(range(1, 7), 1, 1)) == [1, 2, None, 6]
assert list(slice_pagination_range(range(1, 7), 2, 1)) == [1, 2, 3, None, 6]
assert list(slice_pagination_range(range(1, 7), 3, 1)) == [1, 2, 3, 4, None, 6]
assert list(slice_pagination_range(range(1, 7), 4, 1)) == [1, None, 3, 4, 5, 6]
assert list(slice_pagination_range(range(1, 7), 5, 1)) == [1, None, 4, 5, 6]
assert list(slice_pagination_range(range(1, 7), 6, 1)) == [1, None, 5, 6]

# Big range test (two ellipsis)
assert list(slice_pagination_range(range(1, 8), 1, 1)) == [1, 2, None, 7]
assert list(slice_pagination_range(range(1, 8), 2, 1)) == [1, 2, 3, None, 7]
assert list(slice_pagination_range(range(1, 8), 3, 1)) == [1, 2, 3, 4, None, 7]
assert list(slice_pagination_range(range(1, 8), 4, 1)) == [1, None, 3, 4, 5, None, 7]
assert list(slice_pagination_range(range(1, 8), 5, 1)) == [1, None, 4, 5, 6, 7]
assert list(slice_pagination_range(range(1, 8), 6, 1)) == [1, None, 5, 6, 7]
assert list(slice_pagination_range(range(1, 8), 7, 1)) == [1, None, 6, 7]

# More realistic scenario
assert list(slice_pagination_range(range(1, 30), 1, 3)) == [1, 2, 3, 4, None, 29]
assert list(slice_pagination_range(range(1, 30), 2, 3)) == [1, 2, 3, 4, 5, None, 29]
assert list(slice_pagination_range(range(1, 30), 3, 3)) == [1, 2, 3, 4, 5, 6, None, 29]
assert list(slice_pagination_range(range(1, 30), 4, 3)) == [1, 2, 3, 4, 5, 6, 7, None, 29]
assert list(slice_pagination_range(range(1, 30), 5, 3)) == [1, 2, 3, 4, 5, 6, 7, 8, None, 29]
assert list(slice_pagination_range(range(1, 30), 6, 3)) == [1, None, 3, 4, 5, 6, 7, 8, 9, None, 29]
assert list(slice_pagination_range(range(1, 30), 12, 3)) == [1, None, 9, 10, 11, 12, 13, 14, 15, None, 29]
assert list(slice_pagination_range(range(1, 30), 24, 3)) == [1, None, 21, 22, 23, 24, 25, 26, 27, None, 29]
assert list(slice_pagination_range(range(1, 30), 25, 3)) == [1, None, 22, 23, 24, 25, 26, 27, 28, 29]
assert list(slice_pagination_range(range(1, 30), 26, 3)) == [1, None, 23, 24, 25, 26, 27, 28, 29]
assert list(slice_pagination_range(range(1, 30), 27, 3)) == [1, None, 24, 25, 26, 27, 28, 29]
assert list(slice_pagination_range(range(1, 30), 28, 3)) == [1, None, 25, 26, 27, 28, 29]
assert list(slice_pagination_range(range(1, 30), 29, 3)) == [1, None, 26, 27, 28, 29]