How to use the cfgrib.messages.FileStream function in cfgrib

def test_FileStream():
    res = messages.FileStream(TEST_DATA)
    leader = res.first()
    assert len(leader) > 100
    assert sum(1 for _ in res) == leader['count']
    assert len(res.index(['paramId'])) == 1

    # __file__ is not a GRIB, but contains the "GRIB" string, so it is a very tricky corner case
    res = messages.FileStream(str(__file__))
    with pytest.raises(EOFError):
        res.first()

    res = messages.FileStream(str(__file__), errors='ignore')
    with pytest.raises(EOFError):
        res.first()

def test_FileStream():
    res = messages.FileStream(TEST_DATA)
    leader = res.first()
    assert len(leader) > 100
    assert sum(1 for _ in res) == leader['count']
    assert len(res.index(['paramId'])) == 1

    # __file__ is not a GRIB, but contains the "GRIB" string, so it is a very tricky corner case
    res = messages.FileStream(str(__file__))
    with pytest.raises(EOFError):
        res.first()

    res = messages.FileStream(str(__file__), errors='ignore')
    with pytest.raises(EOFError):
        res.first()

def test_FileIndex():
    res = messages.FileIndex.from_filestream(messages.FileStream(TEST_DATA), ['paramId'])
    assert res['paramId'] == [129, 130]
    assert len(res) == 1
    assert list(res) == ['paramId']
    assert res.first()

    with pytest.raises(ValueError):
        res.getone('paramId')

    with pytest.raises(KeyError):
        res['non-existent-key']

    subres = res.subindex(paramId=130)

    assert subres.get('paramId') == [130]
    assert subres.getone('paramId') == 130
    assert len(subres) == 1

def test_FileIndex_from_indexpath_or_filestream(tmpdir):
    grib_file = tmpdir.join('file.grib')

    with open(TEST_DATA, 'rb') as file:
        grib_file.write_binary(file.read())

    # create index file
    res = messages.FileIndex.from_indexpath_or_filestream(
        messages.FileStream(str(grib_file)), ['paramId']
    )
    assert isinstance(res, messages.FileIndex)

    # read index file
    res = messages.FileIndex.from_indexpath_or_filestream(
        messages.FileStream(str(grib_file)), ['paramId']
    )
    assert isinstance(res, messages.FileIndex)

    # do not read nor create the index file
    res = messages.FileIndex.from_indexpath_or_filestream(
        messages.FileStream(str(grib_file)), ['paramId'], indexpath=''
    )
    assert isinstance(res, messages.FileIndex)

    # can't create nor read index file

def test_FileIndex_errors():
    class MyMessage(messages.ComputedKeysMessage):
        computed_keys = {'error_key': lambda m: 1 / 0}

    stream = messages.FileStream(TEST_DATA, message_class=MyMessage)
    res = messages.FileIndex.from_filestream(stream, ['paramId', 'error_key'])
    assert res['paramId'] == [129, 130]
    assert len(res) == 2
    assert list(res) == ['paramId', 'error_key']
    assert res['error_key'] == ['undef']