pandas.DataFrame.join   DataFrame.join(other, on=None, how='left', lsuffix='', rsuffix='', sort=False)[source]
 
Join columns of another DataFrame. Join columns with other DataFrame either on index or on a key column. Efficiently join multiple DataFrame objects by index at once by passing a list.  Parameters 
 
other:DataFrame, Series, or list of DataFrame


Index should be similar to one of the columns in this one. If a Series is passed, its name attribute must be set, and that will be used as the column name in the resulting joined DataFrame.  
on:str, list of str, or array-like, optional


Column or index level name(s) in the caller to join on the index in other, otherwise joins index-on-index. If multiple values given, the other DataFrame must have a MultiIndex. Can pass an array as the join key if it is not already contained in the calling DataFrame. Like an Excel VLOOKUP operation.  
how:{‘left’, ‘right’, ‘outer’, ‘inner’}, default ‘left’


How to handle the operation of the two objects.  left: use calling frame’s index (or column if on is specified) right: use other’s index. outer: form union of calling frame’s index (or column if on is specified) with other’s index, and sort it. lexicographically. inner: form intersection of calling frame’s index (or column if on is specified) with other’s index, preserving the order of the calling’s one. 
cross: creates the cartesian product from both frames, preserves the order of the left keys.  New in version 1.2.0.     
lsuffix:str, default ‘’


Suffix to use from left frame’s overlapping columns.  
rsuffix:str, default ‘’


Suffix to use from right frame’s overlapping columns.  
sort:bool, default False


Order result DataFrame lexicographically by the join key. If False, the order of the join key depends on the join type (how keyword).    Returns 
 DataFrame

A dataframe containing columns from both the caller and other.      See also  DataFrame.merge

For column(s)-on-column(s) operations.    Notes Parameters on, lsuffix, and rsuffix are not supported when passing a list of DataFrame objects. Support for specifying index levels as the on parameter was added in version 0.23.0. Examples 
>>> df = pd.DataFrame({'key': ['K0', 'K1', 'K2', 'K3', 'K4', 'K5'],
...                    'A': ['A0', 'A1', 'A2', 'A3', 'A4', 'A5']})
  
>>> df
  key   A
0  K0  A0
1  K1  A1
2  K2  A2
3  K3  A3
4  K4  A4
5  K5  A5
  
>>> other = pd.DataFrame({'key': ['K0', 'K1', 'K2'],
...                       'B': ['B0', 'B1', 'B2']})
  
>>> other
  key   B
0  K0  B0
1  K1  B1
2  K2  B2
  Join DataFrames using their indexes. 
>>> df.join(other, lsuffix='_caller', rsuffix='_other')
  key_caller   A key_other    B
0         K0  A0        K0   B0
1         K1  A1        K1   B1
2         K2  A2        K2   B2
3         K3  A3       NaN  NaN
4         K4  A4       NaN  NaN
5         K5  A5       NaN  NaN
  If we want to join using the key columns, we need to set key to be the index in both df and other. The joined DataFrame will have key as its index. 
>>> df.set_index('key').join(other.set_index('key'))
      A    B
key
K0   A0   B0
K1   A1   B1
K2   A2   B2
K3   A3  NaN
K4   A4  NaN
K5   A5  NaN
  Another option to join using the key columns is to use the on parameter. DataFrame.join always uses other’s index but we can use any column in df. This method preserves the original DataFrame’s index in the result. 
>>> df.join(other.set_index('key'), on='key')
  key   A    B
0  K0  A0   B0
1  K1  A1   B1
2  K2  A2   B2
3  K3  A3  NaN
4  K4  A4  NaN
5  K5  A5  NaN
  Using non-unique key values shows how they are matched. 
>>> df = pd.DataFrame({'key': ['K0', 'K1', 'K1', 'K3', 'K0', 'K1'],
...                    'A': ['A0', 'A1', 'A2', 'A3', 'A4', 'A5']})
  
>>> df
  key   A
0  K0  A0
1  K1  A1
2  K1  A2
3  K3  A3
4  K0  A4
5  K1  A5
  
>>> df.join(other.set_index('key'), on='key')
  key   A    B
0  K0  A0   B0
1  K1  A1   B1
2  K1  A2   B1
3  K3  A3  NaN
4  K0  A4   B0
5  K1  A5   B1