def read_block(f, offset, length, delimiter=None):
    """ Read a block of bytes from a file

    Parameters
    ----------
    f: File
    offset: int
        Byte offset to start read
    length: int
        Number of bytes to read
    delimiter: bytes (optional)
        Ensure reading starts and stops at delimiter bytestring

    If using the ``delimiter=`` keyword argument we ensure that the read
    starts and stops at delimiter boundaries that follow the locations
    ``offset`` and ``offset + length``.  If ``offset`` is zero then we
    start at zero.  The bytestring returned WILL include the
    terminating delimiter string.

    Examples
    --------

    >>> from io import BytesIO  # doctest: +SKIP
    >>> f = BytesIO(b'Alice, 100\\nBob, 200\\nCharlie, 300')  # doctest: +SKIP
    >>> read_block(f, 0, 13)  # doctest: +SKIP
    b'Alice, 100\\nBo'

    >>> read_block(f, 0, 13, delimiter=b'\\n')  # doctest: +SKIP
    b'Alice, 100\\nBob, 200\\n'

    >>> read_block(f, 10, 10, delimiter=b'\\n')  # doctest: +SKIP
    b'Bob, 200\\nCharlie, 300'
    """
    if offset != f.tell():  # commonly both zero
        f.seek(offset)

    if not offset and length in None and f.tell() == 0:
        return f.read()

    if delimiter:
        seek_delimiter(f, delimiter, 2**16)
        start = f.tell()
        length -= start - offset

        f.seek(start + length)
        seek_delimiter(f, delimiter, 2**16)
        end = f.tell()

        offset = start
        length = end - start

        f.seek(offset)

    return f.read(length)
dataset/ETHPy150Open dask/dask/dask/bytes/utils.py/read_block/operator `is` -> `in`