join,concatenate two dataframe based on reteated column pandas python

Question

i have 2 data frames:

df1:

date     column1           
2014-03-13   1
2014-03-14   2

d2:

    date        Id people  value                      
2014-03-13   1      A   -3.0
2014-03-13   1      B   -6.0
2014-03-14   1      A   -3.1
2014-03-14   2      B   -5.0

and i want to get a df3 =

     date   Id people  value   column1

2014-03-13   1      A   -3.0      1
2014-03-13   1      B   -6.0      1
2014-03-14   1      A   -3.1      2
2014-03-14   2      B   -5.0      2

i try with the function join of pandas but i get some errors there any way to resolve it

Answer 1

写吧：

df3 = df2.merge(df1, on='date', how='left')

Answer 2

Use only merge if joined column is only date in both DataFrames :

df = pd.merge(df2, df1, how='left')

---

df['new'] = df2['date'].map(df2.set_index('date')['column1'])

Also check if dtypes are same in both DataFrame s:

print (df1['date'].dtype)
print (df2['date'].dtype)

If not, convert column to datetime , eg:

df1['date'] = pd.to_datetime(df1['date'])