Left Merge in Pandas Python

January 20, 2025May 25, 2022

We can use how='left' tells merge() to left merge two dataframes. The following is the Pandas syntax, in which df_1 and df_2 are two dataframes to be merged.

df_1.merge(df_2, how=’left’, left_index=True, right_index=True)

Step 1: Prepare the data to be left merged

The following is the two dataframes to be left merged.

import pandas as pd
# dataframe 1
df_1 = pd.DataFrame({'Brand': ['Tesla', 'Toyota','Tesla','Ford'], 
     'Location': ['CA', 'CA','NY','MA']},index=list('abcd'))
print("df_1: \n",df_1)

# dataframe 2
df_2 = pd.DataFrame({ 
    'Name': ['Jake', 'Jacob','John','Jess','James']},index=list('abdek'))
print("df_2 \n",df_2)

df_1: 
     Brand Location
a   Tesla       CA
b  Toyota       CA
c   Tesla       NY
d    Ford       MA

df_2 
     Name
a   Jake
b  Jacob
d   John
e   Jess
k  James

Step 2: Use merge() to left merge two dataframes

Use how='left' in merge() to left merge two dataframes.

# use how='left' in merge() to left join two dataframes 
joined_dataframe=df_1.merge(df_2, how='left', left_index=True,right_index=True)
print(joined_dataframe)

    Brand Location   Name
a   Tesla       CA   Jake
b  Toyota       CA  Jacob
c   Tesla       NY    NaN
d    Ford       MA   John

Further Reading