I have dataframe Polars with a lot of columns. I need to create new columnt with the result of my own function

df = pl.DataFrame({
    'Stock_name': ['reserve', 'fullfilment', 'ntl', 'ntl'],
    'doc_type': ['sales', 'moving', 'sales', 'corr'],
})


def doc_check(row):
    if (row['Stock_name'] == 'reserve') or (row['Stock_name'] == 'ntl'):
        if row['doc_type'] == 'sales':
            doc = 'sales'
        else:
            doc = row['doc_type']
    else:
        doc = row['doc_type']
    return doc


df = df.apply(doc_check).alias('doc_type_new')
df['doc_type_new'] = df.apply(doc_check)

But I received an error “TypeError: tuple indices must be integers or slices, not str”. Where is an error in my code?