ENH: Add empty property to Index.

Previously, attempting to evaluate an Index in a boolean context
prints  an error message listing various alternatives, one of which is
`.empty`,  which was not actually implemented on `Index`.

Author: Scott Sanderson <ssanderson@quantopian.com>

This patch had conflicts when merged, resolved by
Committer: Jeff Reback <jeff@reback.net>

closes #13207
Closes #15270 from ssanderson/add-empty-to-index and squashes the following commits:

bb0126f [Scott Sanderson] ENH: Add empty property to Index.

Author: Scott Sanderson

doc/source/api.rst

@@ -1277,6 +1277,7 @@ Attributes
    Index.nbytes
    Index.ndim
    Index.size
+   Index.empty
    Index.strides
    Index.itemsize
    Index.base

doc/source/whatsnew/v0.20.0.txt

@@ -310,11 +310,13 @@ Other Enhancements
 - ``Series/DataFrame.squeeze()`` have gained the ``axis`` parameter. (:issue:`15339`)
 - ``DataFrame.to_excel()`` has a new ``freeze_panes`` parameter to turn on Freeze Panes when exporting to Excel (:issue:`15160`)
 - HTML table output skips ``colspan`` or ``rowspan`` attribute if equal to 1. (:issue:`15403`)
+
 - ``pd.TimedeltaIndex`` now has a custom datetick formatter specifically designed for nanosecond level precision (:issue:`8711`)
 - ``pd.types.concat.union_categoricals`` gained the ``ignore_ordered`` argument to allow ignoring the ordered attribute of unioned categoricals (:issue:`13410`). See the :ref:`categorical union docs <categorical.union>` for more information.
 - ``pandas.io.json.json_normalize()`` with an empty ``list`` will return an empty ``DataFrame`` (:issue:`15534`)
 - ``pd.DataFrame.to_latex`` and ``pd.DataFrame.to_string`` now allow optional header aliases. (:issue:`15536`)
 - Re-enable the ``parse_dates`` keyword of ``read_excel`` to parse string columns as dates (:issue:`14326`)
+- Added ``.empty`` property to subclasses of ``Index``. (:issue:`15270`)
 
 .. _ISO 8601 duration: https://en.wikipedia.org/wiki/ISO_8601#Durations
 

pandas/core/base.py

@@ -774,6 +774,10 @@ def _values(self):
         """ the internal implementation """
         return self.values
 
+    @property
+    def empty(self):
+        return not self.size
+
     def max(self):
         """ The maximum value of the object """
         return nanops.nanmax(self.values)

pandas/tests/indexes/common.py

@@ -910,3 +910,9 @@ def test_nulls(self):
                     result = isnull(index)
                     self.assert_numpy_array_equal(index.isnull(), result)
                     self.assert_numpy_array_equal(index.notnull(), ~result)
+
+    def test_empty(self):
+        # GH 15270
+        index = self.create_index()
+        self.assertFalse(index.empty)
+        self.assertTrue(index[:0].empty)