Credit Risk Modeling

In the section of ‘Preprocessing Discrete Variables: Automating Calculations’, about the code below,
 
df_inputs_prepr[‘home_ownership:RENT_OTHER_NONE_ANY’] = sum([df_inputs_prepr[‘home_ownership:RENT’], df_inputs_prepr[‘home_ownershipTHER’],
df_inputs_prepr[‘home_ownership:NONE’],df_inputs_prepr[‘home_ownership:ANY’]]). Although 'rent'  is one of the categories, but [‘home_ownership:RENT’] is not the column of the dataset. How come we can directly use df[‘home_ownership:RENT’]?