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# pandas.Series.asof

Series.asof(where, subset=None)

  Return the last row(s) without any NaNs before where.

  The last row (for each element in where, if list) without any NaN is taken. In case of a `DataFrame`, the last row without NaN considering only the subset of columns (if not None)

  If there is no good value, NaN is returned for a Series or a Series of NaN values for a DataFrame

  Parameters:
    where date or array-like of dates

      Date(s) before which the last row(s) are returned.

    subset str or array-like of str, default None

      For DataFrame, if not None, only use these columns to check for NaNs.

  Returns:
    scalar, Series, or DataFrame

      The return can be:

      - scalar : when self is a Series and where is a scalar

      - Series: when self is a Series and where is an array-like, or when self is a DataFrame and where is a scalar

      - DataFrame : when self is a DataFrame and where is an array-like

  See also

  `merge_asof`

    Perform an asof merge. Similar to left join.

  Notes

  Dates are assumed to be sorted. Raises if this is not the case.

  Examples

  A Series and a scalar where.

      >>> s = pd.Series([1, 2, np.nan, 4], index=[10, 20, 30, 40])
      >>> s
      10    1.0
      20    2.0
      30    NaN
      40    4.0
      dtype: float64

      >>> s.asof(20)
      2.0

  For a sequence where, a Series is returned. The first value is NaN, because the first element of where is before the first index value.

      >>> s.asof([5, 20])
      5     NaN
      20    2.0
      dtype: float64

  Missing values are not considered. The following is `2.0`, not NaN, even though NaN is at the index location for `30`.

      >>> s.asof(30)
      2.0

  Take all columns into consideration

      >>> df = pd.DataFrame({'a': [10., 20., 30., 40., 50.],
      ...                    'b': [None, None, None, None, 500]},
      ...                   index=pd.DatetimeIndex(['2018-02-27 09:01:00',
      ...                                           '2018-02-27 09:02:00',
      ...                                           '2018-02-27 09:03:00',
      ...                                           '2018-02-27 09:04:00',
      ...                                           '2018-02-27 09:05:00']))
      >>> df.asof(pd.DatetimeIndex(['2018-02-27 09:03:30',
      ...                           '2018-02-27 09:04:30']))
                            a   b
      2018-02-27 09:03:30 NaN NaN
      2018-02-27 09:04:30 NaN NaN

  Take a single column into consideration

      >>> df.asof(pd.DatetimeIndex(['2018-02-27 09:03:30',
      ...                           '2018-02-27 09:04:30']),
      ...         subset=['a'])
                              a   b
      2018-02-27 09:03:30  30.0 NaN
      2018-02-27 09:04:30  40.0 NaN