How to implement left outer join in python pandas? [duplicate]
set param indicator=True, this will add a column _merge you then filter just the rows that are left_only:
In [46]:df1 = pd.DataFrame({'key': ['A', 'B', 'C', 'D'],'value1': np.random.randn(4)})df2 = pd.DataFrame({'key': ['B', 'D', 'D', 'E'], 'value2': np.random.randn(4)})df3 = df1.merge(df2, on=['key'], how='left', indicator=True)df3Out[46]: key value1 value2 _merge0 A -0.346861 NaN left_only1 B 1.120739 0.558272 both2 C 0.023881 NaN left_only3 D -0.598771 -0.823035 both4 D -0.598771 0.369423 bothIn [48]:df3[df3['_merge'] == 'left_only']Out[48]: key value1 value2 _merge0 A -0.346861 NaN left_only2 C 0.023881 NaN left_onlyif on older version then use isin with ~ to negate the mask:
In [50]:df3[~df3['key'].isin(df2['key'])]Out[50]: key value1 value20 A -0.346861 NaN2 C 0.023881 NaN