I have a code in python like this :

merged_df = pd.merge(df1, df2, on='GCIF_NO', how='left')


def TAB_Online(row):
    if pd.isnull(row['GCIF_NO']):
        return 0
    else:
        return 1

and then after that i build a code like this :

df1['TAB_Online'] = merged_df.apply(TAB_Online)

but it return an error like this :

KeyError: 'GCIF_NO'

after that i check whether GCIF_NO available or not on the dataframe with a code like this :

# Example check to verify column existence
if 'GCIF_NO' in df1.columns:
    print("Column 'GCIF_NO' exists in df1")
else:
    print("Column 'GCIF_NO' does not exist in df1")

and it return Column ‘GCIF_NO’ exists in df1.

so where’s my problem?