How to use the cantools.database.Database function in cantools

# KCD to DBC.
        argv = [
            'cantools',
            'convert',
            'test_command_line_convert.kcd',
            'test_command_line_convert.dbc'
        ]

        if os.path.exists('test_command_line_convert.dbc'):
            os.remove('test_command_line_convert.dbc')

        with patch('sys.argv', argv):
            cantools._main()

        db = cantools.database.Database()
        db.add_dbc_file('test_command_line_convert.dbc')
        self.assertEqual(db.version, '1.0')

def test_empty_ns_dbc(self):
        """Test loading a DBC-file with empty NS_.

        """

        db = cantools.database.Database()

        filename = os.path.join('tests', 'files', 'empty_ns.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.nodes), 0)

def test_convert(self):
        # DBC to KCD.
        argv = [
            'cantools',
            'convert',
            'tests/files/dbc/motohawk.dbc',
            'test_command_line_convert.kcd'
        ]

        if os.path.exists('test_command_line_convert.kcd'):
            os.remove('test_command_line_convert.kcd')

        with patch('sys.argv', argv):
            cantools._main()

        db = cantools.database.Database()
        db.add_kcd_file('test_command_line_convert.kcd')
        self.assertEqual(db.version, '1.0')

        # KCD to DBC.
        argv = [
            'cantools',
            'convert',
            'test_command_line_convert.kcd',
            'test_command_line_convert.dbc'
        ]

        if os.path.exists('test_command_line_convert.dbc'):
            os.remove('test_command_line_convert.dbc')

        with patch('sys.argv', argv):
            cantools._main()

def test_empty_ns_dbc(self):
        """Test loading a DBC-file with empty NS_.

        """

        db = cantools.database.Database()

        filename = os.path.join('tests', 'files', 'empty_ns.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.nodes), 0)

def test_add_two_dbc_files(self):
        """Test adding two DBC-files to the same database.

        """

        db = cantools.database.Database()

        filename = os.path.join('tests', 'files', 'add_two_dbc_files_1.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.messages), 2)
        self.assertEqual(db.get_message_by_name('M1').frame_id, 1)
        self.assertEqual(db.get_message_by_frame_id(2).name, 'M2')

        filename = os.path.join('tests', 'files', 'add_two_dbc_files_2.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.messages), 3)
        self.assertEqual(db.get_message_by_name('M1').frame_id, 2)
        self.assertEqual(db.get_message_by_frame_id(2).name, 'M1')

# KCD to DBC.
        argv = [
            'cantools',
            'convert',
            'test_command_line_convert.kcd',
            'test_command_line_convert.dbc'
        ]

        if os.path.exists('test_command_line_convert.dbc'):
            os.remove('test_command_line_convert.dbc')

        with patch('sys.argv', argv):
            cantools._main()

        db = cantools.database.Database()
        db.add_dbc_file('test_command_line_convert.dbc')
        self.assertEqual(db.version, '1.0')

def test_add_two_dbc_files(self):
        """Test adding two DBC-files to the same database.

        """

        db = cantools.database.Database()

        filename = os.path.join('tests', 'files', 'add_two_dbc_files_1.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.messages), 2)
        self.assertEqual(db.get_message_by_name('M1').frame_id, 1)
        self.assertEqual(db.get_message_by_frame_id(2).name, 'M2')

        filename = os.path.join('tests', 'files', 'add_two_dbc_files_2.dbc')
        db.add_dbc_file(filename)
        self.assertEqual(len(db.messages), 3)
        self.assertEqual(db.get_message_by_name('M1').frame_id, 2)
        self.assertEqual(db.get_message_by_frame_id(2).name, 'M1')