I have a dataframe df which has around 40 columns. Now I want to add a new column named as comment in the dataframe, and value of that column should be coming through a function.

I have created this function –
def new_column(df):
null_col = []
for c in df.columns:
if df.select(c).first()[0] == '': null_cols.append(c)
if len(null_cols) > 1: x= (' & '.join(null_cols))
else: x = (null_cols)
return x

Calling the function here –

df_updt = df.withColumn("comment", new_column(df))

But I am getting error –
Argument col should be a Column, got str.

Is there any way this can be achieved in databricks?
I tried different version of the same function, I tried creating an udf, but udf dont take dataframe as an input parameter. I tried some other option that gemini provided, but nothing is working.

Please ignore the quotes in the function, that I had given otherwise stackoverflow was showing error.