Insert a row to pandas dataframe

Just assign row to a particular index, using loc:

 df.loc[-1] = [2, 3, 4]  # adding a row
 df.index = df.index + 1  # shifting index
 df = df.sort_index()  # sorting by index

And you get, as desired:

    A  B  C
 0  2  3  4
 1  5  6  7
 2  7  8  9

See in Pandas documentation Indexing: Setting with enlargement.