Get a value from dataframe with different shape based on two columns

I have two dataframes colored by approximately matching marks:

df1:

df2:

The “marks” are not the same in each of them, but some are close. How can I copy the “Evaluated” value from df2 to df1 based on relevant “name” and “mark”?

My code is:

df1 = pd.DataFrame({'Name': ['Lisa', 'Lisa', 'Lisa', 'Hann', 'Hann', 'Hann'], 
                    'Marks': [25.123, 26.425, 27.456, 25.789, 26.124, 26.225], 
                    'Evaluated':['','','','','','']})

df2 = pd.DataFrame({'Name':['Lisa', 'Lisa', 'Lisa', 'Lisa', 'Hann', 'Hann', 'Hann', 'Hann'],
                    'Marks':[25.125, 26.422, 27.451, 27.465, 25.786, 25.796, 26.121, 26.227],
                    'Evaluated':[0, 0, 1, 1, 1, 1, 1, 1]})

df3 = pd.merge(df1.round(2), 
               df2.round(2), 
               how='left', 
               on=['Name', 'Marks'])

Expected result is df3

df3:

How can I do an approximate match and get the value of the last column? I tried to use df.loc and df.where but they didn’t work because tables are in different shapes. What I expect is similar function to Excel’s Vlookup function where approximation is True. My code changes the values at the end, which I would love to keep as it was in df1. Probably I could make a copy from what I had before, but I believe there is a more pythonic way to solve it, rather than merging the tables.

Thanks in advance!

Answer

You can try pandas.merge_asof

df1 = df1.sort_values(['Marks'])
df2 = df2.sort_values(['Marks'])

df3 = pd.merge_asof(df1[['Name', 'Marks']],
                    df2,
                    on='Marks',
                    direction='nearest',
                    by='Name')

print(df3)

   Name   Marks  Evaluated
0  Lisa  25.123          0
1  Hann  25.789          1
2  Hann  26.124          1
3  Hann  26.225          1
4  Lisa  26.425          0
5  Lisa  27.456          1