vector assembler spark random forest regressor

pipeline_stages=[]
categorical_columns = ['job', 'marital', 'education', 'default', 'housing', 'loan', 'contact', 'poutcome','deposit']
for value in categorical_columns:
    string_indexer = StringIndexer(inputCol=value, outputCol=value+"indexer")
    pipeline_stages += [string_indexer]

numerical_columns = ['age', 'duration', 'campaign', 'pdays', 'previous']
combined_columns = [columns + "indexer" for columns in categorical_columns] + numerical_columns

vector_assembler = VectorAssembler(inputCols=combined_columns, outputCol="features")
pipeline_stages += [vector_assembler]