ENH: Provide dict object for to_dict() pandas-dev#16122

Author: dwkenefick

pandas/core/common.py

@@ -484,13 +484,14 @@ def _dict_compat(d):
 def standardize_mapping(into):
     """
     Helper function to standardize a supplied mapping.
+
     .. versionadded:: 0.21.0
 
     Parameters
     ----------
     into : instance or subclass of collections.Mapping
         Must be a class, an initialized collections.defaultdict,
-        or an empty instance of a collections.Mapping subclass.
+        or an instance of a collections.Mapping subclass.
 
     Returns
     -------

pandas/core/frame.py

@@ -887,8 +887,7 @@ def to_dict(self, orient='dict', into=dict):
             The collections.Mapping subclass used for all Mappings
             in the return value.  Can be the actual class or an empty
             instance of the mapping type you want.  If you want a
-            collections.defaultdict, you must pass an initialized
-            instance.
+            collections.defaultdict, you must pass it initialized.
 
             .. versionadded:: 0.21.0
 

pandas/core/series.py

@@ -1084,7 +1084,7 @@ def to_dict(self, into=dict):
             The collections.Mapping subclass to use as the return
             object. Can be the actual class or an empty
             instance of the mapping type you want.  If you want a
-            collections.defaultdict, you must pass an initialized
+            collections.defaultdict, you must pass it initialized.
 
             .. versionadded:: 0.21.0
 

pandas/tests/frame/test_convert_to.py

@@ -206,6 +206,16 @@ def test_to_dict(self, mapping):
             for k2, v2 in compat.iteritems(v):
                 assert (v2 == recons_data[k2][k])
 
+    @pytest.mark.parametrize('mapping', [
+        list,
+        collections.defaultdict,
+        []])
+    def test_to_dict_errors(self, mapping):
+        # GH16122
+        df = DataFrame(np.random.randn(3, 3))
+        with pytest.raises(TypeError):
+            df.to_dict(into=mapping)
+
     @pytest.mark.parametrize('tz', ['UTC', 'GMT', 'US/Eastern'])
     def test_to_records_datetimeindex_with_tz(self, tz):
         # GH13937