Merge/concatenate dataframes on date where one dataframe has multiple instances of the same date

Question

python, df, pandas,merge/concatenate

left = pd.DataFrame(
    {
        "date": ["1999-11-30", "1999-12-31", "1999-11-30", "1999-12-31"],
        "stock": ["a", "a", "b", "b"]
    }
)

right = pd.DataFrame(
    {
        "date": [""1999-11-30", "1999-12-31"],
        "deflator": ["1", ".8"]
    }
)

...so anytime a date appears in left, add a column to left with the corresponding deflator from right

Answer 1

Set your index to be the date

left = pd.DataFrame(
    {
        "stock": ["a", "a", "b", "b"],
    },
    index=["1999-11-30", "1999-12-31", "1999-11-30", "1999-12-31"],
)

right = pd.DataFrame(
    {
        "deflator": ["1", ".8"],
    },
    index=["1999-11-30", "1999-12-31"],
)

Then do a join left to right

df = left.join(right, how="outer")

          stock deflator
1999-11-30  a   1
1999-11-30  b   1
1999-12-31  a   .8
1999-12-31  b   .8

Outer join allows all of left to be kept, you may also want to make the index a datetime.