def asof(self, label):
try:<EOL><INDENT>loc = self.get_loc(label, method='<STR_LIT>')<EOL><DEDENT>except KeyError:<EOL><INDENT>return self._na_value<EOL><DEDENT>else:<EOL><INDENT>if isinstance(loc, slice):<EOL><INDENT>loc = loc.indices(len(self))[-<NUM_LIT:1>]<EOL><DEDENT>return self[loc]<EOL><DEDENT>
Return the label from the index, or, if not present, the previous one.

Assuming that the index is sorted, return the passed index label if it
is in the index, or return the previous index label if the passed one
is not in the index.

Parameters
----------
label : object
    The label up to which the method returns the latest index label.

Returns
-------
object
    The passed label if it is in the index. The previous label if the
    passed label is not in the sorted index or `NaN` if there is no
    such label.

See Also
--------
Series.asof : Return the latest value in a Series up to the
    passed index.
merge_asof : Perform an asof merge (similar to left join but it
    matches on nearest key rather than equal key).
Index.get_loc : An `asof` is a thin wrapper around `get_loc`
    with method='pad'.

Examples
--------
`Index.asof` returns the latest index label up to the passed label.

>>> idx = pd.Index(['2013-12-31', '2014-01-02', '2014-01-03'])
>>> idx.asof('2014-01-01')
'2013-12-31'

If the label is in the index, the method returns the passed label.

>>> idx.asof('2014-01-02')
'2014-01-02'

If all of the labels in the index are later than the passed label,
NaN is returned.

>>> idx.asof('1999-01-02')
nan

If the index is not sorted, an error is raised.

>>> idx_not_sorted = pd.Index(['2013-12-31', '2015-01-02',
...                            '2014-01-03'])
>>> idx_not_sorted.asof('2013-12-31')
Traceback (most recent call last):
ValueError: index must be monotonic increasing or decreasing