How to make difference between two pandas dataframes?

January 21, 2022

I have two pandas dataframes:

      import pandas as pd

      df_1 = pd.DataFrame({'ID': [1, 2, 4, 7, 30], 
                           'Instrument': ['temp', 'temp_sensor', 'temp_sensor',
                                'sensor', 'sensor'],
                           'Value': [1000, 0, 1000, 0, 1000]})

      print(df_1)

                 ID    Instrument       Value
                  1      temp           1000
                  2    temp_sensor         0
                  4    temp_sensor       1000
                  7      sensor            0
                 30      sensor          1000

      df_2 = pd.DataFrame({'ID': [1, 30], 
                           'Instrument': ['temp', 'sensor'],
                           'Value': [1000, 1000]})

      print(df_2)

                    ID    Instrument    Value
                     1      temp        1000
                    30     sensor       1000

I need to exclude from df_1 the lines that also exist in df_2. So I made the code:

      combined = df_1.append(df_2)
      combined[~combined.index.duplicated(keep=False)]

The (wrong) output is:

                  ID    Instrument      Value
                   4    temp_sensor     1000
                   7    sensor             0
                  30    sensor          1000

I would like the output to be:

                   ID      Instrument       Value
                  2    temp_sensor         0
                  4    temp_sensor       1000
                  7      sensor            0

I relied on what was explained in: How to remove a pandas dataframe from another dataframe

>Solution :

Use DataFrame.merge by all columns names with left join and parameter indicator=True and filter rows with left_only values:

s = df_1.merge(df_2, on=list(df_1.columns), how='left', indicator=True)['_merge']
df = df_1.loc[s == 'left_only']
print(df)
   ID   Instrument  Value
1   2  temp_sensor      0
2   4  temp_sensor   1000
3   7       sensor      0