BUG: fix initialization of Series with dict containing NaN key

closes pandas-dev#18480

Author: toobaz

doc/source/whatsnew/v0.22.0.txt

@@ -205,4 +205,5 @@ Other
 
 - Improved error message when attempting to use a Python keyword as an identifier in a numexpr query (:issue:`18221`)
 - Fixed a bug where creating a Series from an array that contains both tz-naive and tz-aware values will result in a Series whose dtype is tz-aware instead of object (:issue:`16406`)
+- Fixed construction of ``Series`` from ``dict`` containing ``NaN`` as key (:issue:`18480`)
 -

pandas/core/series.py

@@ -42,7 +42,6 @@
                                 _default_index,
                                 _asarray_tuplesafe,
                                 _values_from_object,
-                                _try_sort,
                                 _maybe_match_name,
                                 SettingWithCopyError,
                                 _maybe_box_datetimelike,
@@ -198,18 +197,26 @@ def __init__(self, data=None, index=None, dtype=None, name=None,
                     data = data.reindex(index, copy=copy)
                 data = data._data
             elif isinstance(data, dict):
-                if index is None:
-                    if isinstance(data, OrderedDict):
-                        index = Index(data)
-                    else:
-                        index = Index(_try_sort(data))
-
-                try:
-                    data = index._get_values_from_dict(data)
-                except TypeError:
-                    data = ([data.get(i, np.nan) for i in index]
-                            if data else np.nan)
-
+                if data:
+                    keys, values = zip(*compat.iteritems(data))
+                else:
+                    if index is None:
+                        index = Index([])
+                    keys = index
+                    values = np.array([np.nan] * len(index))
+                s = Series(values, index=keys, dtype=dtype)
+                if index is not None:
+                    if not index.equals(s.index):
+                        s = s.reindex(index)
+                elif not isinstance(data, OrderedDict):
+                    try:
+                        s = s.sort_index()
+                    except TypeError:
+                        pass
+                data = s._data
+                index = s.index
+                copy = False
+                dtype = None
             elif isinstance(data, SingleBlockManager):
                 if index is None:
                     index = data.index

pandas/tests/series/test_constructors.py

@@ -625,6 +625,19 @@ def test_constructor_dict(self):
         expected.iloc[1] = 1
         assert_series_equal(result, expected)
 
+    def test_constructor_dict_nan_key(self):
+        # GH 18480
+        d = {1: 'a', 2: 'b', np.nan: 'c'}
+        result = Series(d).sort_index()
+        expected = Series(['a', 'b', 'c'], index=[1, 2, np.nan])
+        assert_series_equal(result, expected)
+
+        # Different NaNs:
+        d = {1: 'a', 2: 'b', float('nan'): 'c', float('nan'): 'd'}
+        result = Series(d).sort_values()
+        expected = Series(['a', 'b', 'c', 'd'], index=[1, 2, np.nan, np.nan])
+        assert_series_equal(result, expected)
+
     def test_constructor_dict_datetime64_index(self):
         # GH 9456