API: indexing with a null key will raise a TypeError rather than a ValueError, #11356

Author: jreback

doc/source/whatsnew/v0.17.1.txt

@@ -41,6 +41,7 @@ API changes
 - Regression from 0.16.2 for output formatting of long floats/nan, restored in (:issue:`11302`)
 - Prettyprinting sets (e.g. in DataFrame cells) now uses set literal syntax (``{x, y}``) instead of
   Legacy Python syntax (``set([x, y])``) (:issue:`11215`)
+- Indexing with a null key will raise a ``TypeError``, instead of a ``ValueError`` (:issue:`11356`)
 
 .. _whatsnew_0171.deprecations:
 

pandas/core/index.py

@@ -862,9 +862,10 @@ def to_int():
             return self._invalid_indexer('label', key)
 
         if is_float(key):
-            if not self.is_floating():
-                warnings.warn("scalar indexers for index type {0} should be integers and not floating point".format(
-                    type(self).__name__), FutureWarning, stacklevel=3)
+            if isnull(key):
+                return self._invalid_indexer('label', key)
+            warnings.warn("scalar indexers for index type {0} should be integers and not floating point".format(
+                type(self).__name__), FutureWarning, stacklevel=3)
             return to_int()
 
         return key
@@ -3721,9 +3722,23 @@ def astype(self, dtype):
         return Index(self._values, name=self.name, dtype=dtype)
 
     def _convert_scalar_indexer(self, key, kind=None):
+        """
+        convert a scalar indexer
+
+        Parameters
+        ----------
+        key : label of the slice bound
+        kind : optional, type of the indexing operation (loc/ix/iloc/None)
+
+        right now we are converting
+        floats -> ints if the index supports it
+        """
+
         if kind == 'iloc':
-            return super(Float64Index, self)._convert_scalar_indexer(key,
-                                                                     kind=kind)
+            if is_integer(key):
+                return key
+            return super(Float64Index, self)._convert_scalar_indexer(key, kind=kind)
+
         return key
 
     def _convert_slice_indexer(self, key, kind=None):

pandas/core/indexing.py

@@ -1285,7 +1285,7 @@ def _has_valid_type(self, key, axis):
 
             def error():
                 if isnull(key):
-                    raise ValueError(
+                    raise TypeError(
                         "cannot use label indexing with a null key")
                 raise KeyError("the label [%s] is not in the [%s]" %
                                (key, self.obj._get_axis_name(axis)))

pandas/core/internals.py

@@ -3217,7 +3217,7 @@ def get(self, item, fastpath=True):
         else:
 
             if isnull(item):
-                raise ValueError("cannot label index with a null key")
+                raise TypeError("cannot label index with a null key")
 
             indexer = self.items.get_indexer_for([item])
             return self.reindex_indexer(new_axis=self.items[indexer],

pandas/tests/test_frame.py

@@ -5839,7 +5839,7 @@ def check(df):
 
                 def f():
                     df.loc[:,np.nan]
-                self.assertRaises(ValueError, f)
+                self.assertRaises(TypeError, f)
 
 
         df = DataFrame([[1,2,3],[4,5,6]], index=[1,np.nan])