Pandas populate new dataframe column based on matching columns in another dataframe

Question

I have a df which contains my main data which has one million rows. My main data also has 30 columns. Now I want to add another column to my df called category. The category is a column in df2 which contains around 700 rows and two other columns that will match with two columns in df.

I begin with setting an index in df2 and df that will match between the frames, however some of the index in df2 doesn't exist in df.

The remaining columns in df2 are called AUTHOR_NAME and CATEGORY.

The relevant column in df is called AUTHOR_NAME.

Some of the AUTHOR_NAME in df doesn't exist in df2 and vice versa.

The instruction I want is: when index in df matches with index in df2 and title in df matches with title in df2, add category to df, else add NaN in category.

Example data:

df2
           AUTHOR_NAME              CATEGORY
Index       
Pub1        author1                 main
Pub2        author1                 main
Pub3        author1                 main
Pub1        author2                 sub
Pub3        author2                 sub
Pub2        author4                 sub


df
            AUTHOR_NAME     ...n amount of other columns        
Index       
Pub1        author1                 
Pub2        author1     
Pub1        author2 
Pub1        author3
Pub2        author4 

expected_result
            AUTHOR_NAME             CATEGORY   ...n amount of other columns
Index
Pub1        author1                 main
Pub2        author1                 main
Pub1        author2                 sub
Pub1        author3                 NaN
Pub2        author4                 sub

If I use df2.merge(df,left_index=True,right_index=True,how='left', on=['AUTHOR_NAME']) my df becomes three times bigger than it is supposed to be.

So I thought maybe merging was the wrong way to go about this. What I am really trying to do is use df2 as a lookup table and then return type values to df depending on if certain conditions are met.

def calculate_category(df2, d):
    category_row = df2[(df2["Index"] == d["Index"]) & (df2["AUTHOR_NAME"] == d["AUTHOR_NAME"])]
    return str(category_row['CATEGORY'].iat[0])

df.apply(lambda d: calculate_category(df2, d), axis=1)

However, this throws me an error:

IndexError: ('index out of bounds', u'occurred at index 7614')

Answer 1

Consider the following dataframes df and df2

df = pd.DataFrame(dict(
        AUTHOR_NAME=list('AAABBCCCCDEEFGG'),
        title=      list('zyxwvutsrqponml')
    ))

df2 = pd.DataFrame(dict(
        AUTHOR_NAME=list('AABCCEGG'),
        title      =list('zwvtrpml'),
        CATEGORY   =list('11223344')
    ))

option 1
merge

df.merge(df2, how='left')

option 2
join

cols = ['AUTHOR_NAME', 'title']
df.join(df2.set_index(cols), on=cols)

---

both options yield