'X' object has no attribute 'functionName' - Pyspark / Python

I'm new to Pyspark so just learning as I go.

I'm trying to experiment with UnitTest and I am getting some error below:

def drop_duplicates(df):    df = df.dropDuplicates(df)    return df

import unittestclass TestNotebook(unittest.TestCase):    def test_drop_duplicates(self):        data = (            ['1', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '2', '1'],            ['1', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '3', '1'],            ['1', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '2', '2'],            ['2', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '2', '1']        )        columns = ["ID", "TimeFrom", "TimeTo", "Serial", "Code"]        df = spark.createDataFrame(data, columns)        expected_data = [            ('1', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '2', '1'),            ('1', '2020-01-01 00:00:00', '2020-01-01 01:00:00', '2', '2')        ]        self.assertEqual(drop_duplicates(df), expected_data)res = unittest.main(argv=[''], verbosity=2, exit=False)

(The Assert might fail but I'll know when I move past this error) but for now I just get the following error:

  File "/tmp/ipykernel_15937/2907449366.py", line 2, in drop_duplicates    df = df.dropDuplicates(df)         ^^^^^^^^^^^^^^^^^^^^^  File "/usr/local/spark/python/pyspark/sql/dataframe.py", line 4207, in dropDuplicates    raise PySparkTypeError(pyspark.errors.exceptions.base.PySparkTypeError: [NOT_LIST_OR_TUPLE] Argument `subset` should be a list or tuple, got DataFrame.

Is there something I am missing? I'm reading the docs on this method but can't seem to figure out.