pandas – Extend Index of a DataFrame setting all columns for new rows to NaN?

Use this (current as of pandas 1.1.3):

ix = pd.date_range(start=date(2012, 1, 1), end=date(2012, 1, 31), freq='D')
df2.reindex(ix)

Which gives:

               b
2012-01-01  0.22
2012-01-02   NaN
2012-01-03  0.30
2012-01-04   NaN
2012-01-05   NaN
[...]
2012-01-29   NaN
2012-01-30   NaN
2012-01-31   NaN

For older versions of pandas replace pd.date_range with pd.DatetimeIndex.