Often you may want to merge two pandas DataFrames by their indexes. There are three ways to do so in pandas:

1. Use join: By default, this performs a left join.

df1.join(df2)

2. Use merge. By default, this performs an inner join.

pd.merge(df1, df2, left_index=True, right_index=True)

3. Use concat. By default, this performs an outer join.

pd.concat([df1, df2], axis=1)

The following examples illustrate how to use each of these functions on the following two pandas DataFrames:

import pandas as pd

#create first DataFrame
df1 = pd.DataFrame({'rating': [90, 85, 82, 88, 94, 90, 76, 75],
                   'points': [25, 20, 14, 16, 27, 20, 12, 15]},
                   index=list('abcdefgh'))

print(df1)

   rating  points
a      90      25
b      85      20
c      82      14
d      88      16
e      94      27
f      90      20
g      76      12
h      75      15

#create second DataFrame 
df2 = pd.DataFrame({'assists': [5, 7, 7, 8, 5, 7],
                   'rebounds': [11, 8, 10, 6, 6, 9]},
                   index=list('acdgmn'))           

print(df2)

   assists  rebounds
a        5        11
c        7         8
d        7        10
g        8         6
m        5         6
n        7         9

Example 1: Merge DataFrames Using Join

The following code shows how to use join() to merge the two DataFrames:

df1.join(df2)

        rating	points	assists	rebounds
a	90	25	5.0	11.0
b	85	20	NaN	NaN
c	82	14	7.0	8.0
d	88	16	7.0	10.0
e	94	27	NaN	NaN
f	90	20	NaN	NaN
g	76	12	8.0	6.0
h	75	15	NaN	NaN

The join() function performs a left join by default, so each of the indexes in the first DataFrame are kept.

Example 2: Merge DataFrames Using Merge

The following code shows how to use merge() to merge the two DataFrames:

pd.merge(df1, df2, left_index=True, right_index=True)
	rating	points	assists	rebounds
a	90	25	5	11
c	82	14	7	8
d	88	16	7	10
g	76	12	8	6

The merge() function performs an inner join by default, so only the indexes that appear in both DataFrames are kept.

Example 3: Merge DataFrames Using Concat

The following code shows how to use concat() to merge the two DataFrames:

pd.concat([df1, df2], axis=1)

        rating	points	assists	rebounds
a	90.0	25.0	5.0	11.0
b	85.0	20.0	NaN	NaN
c	82.0	14.0	7.0	8.0
d	88.0	16.0	7.0	10.0
e	94.0	27.0	NaN	NaN
f	90.0	20.0	NaN	NaN
g	76.0	12.0	8.0	6.0
h	75.0	15.0	NaN	NaN
m	NaN	NaN	5.0	6.0
n	NaN	NaN	7.0	9.0

The concat() function performs an outer join by default, so each index value from each DataFrame is kept.

The following tutorials explain how to perform other common operations in pandas:

How to Stack Multiple Pandas DataFrames
How to Insert a Column Into a Pandas DataFrame