pandas DataFrame concat / update (“upsert”)?

Question

I am looking for an elegant way to append all the rows from one DataFrame to another DataFrame (both DataFrames having the same index and column structure), but in cases whe

Answer 1

One solution is to conatenate df1 with new rows in df2 (i.e. where the index does not match). Then update the values with those from df2.

df = pd.concat([df1, df2[~df2.index.isin(df1.index)]])
df.update(df2)

>>> df
             A   B
2015-10-01  A1  B1
2015-10-02  a1  b1
2015-10-03  a2  b2
2015-10-04  a3  b3

EDIT: Per the suggestion of @chrisb, this can further be simplified as follows:

pd.concat([df1[~df1.index.isin(df2.index)], df2])

Thanks Chris!