How to Compare Values Between Two pandas DataFrames

In this tutorial, you will learn how to compare values between two DataFrames.

Step-by-Step Example

Let's say, you have data on caught fish by two fishing boats:

import pandas as pd

boat1 = {
        'fish': ['salmon', 'pufferfish', 'shark'],
        'count': [99, 33, 11]
        }
boat2 = {
        'fish': ['salmon', 'pufferfish'],
        'count': [88, 22]
        }

df1 = pd.DataFrame(boat1)
df2 = pd.DataFrame(boat2)

print(df1)
print(df2)

df1:
         fish  count
0      salmon     99
1  pufferfish     33
2       shark     11

df2:
         fish  count
0      salmon     88
1  pufferfish     22

To compare them, you first have to merge the two DataFrames:

df = pd.merge(df1, df2, how='inner', on='fish',
              suffixes=('_boat1', '_boat2'))

print(df)

         fish  count_boat1  count_boat2
0      salmon           99           88
1  pufferfish           33           22

You can then compute differences:

df['count_diff'] = df['count_boat1'] - df['count_boat2']

print(df)

         fish  count_boat1  count_boat2  count_diff
0      salmon           99           88          11
1  pufferfish           33           22          11

That's it! You just learned how to compare two DataFrames.