ENH: Add columns parameter to from_dict (pandas-dev#19802)

Author: reidy-p

doc/source/whatsnew/v0.23.0.txt

@@ -295,6 +295,7 @@ Other Enhancements
 - ``IntervalIndex.astype`` now supports conversions between subtypes when passed an ``IntervalDtype`` (:issue:`19197`)
 - :class:`IntervalIndex` and its associated constructor methods (``from_arrays``, ``from_breaks``, ``from_tuples``) have gained a ``dtype`` parameter (:issue:`19262`)
 - Added :func:`SeriesGroupBy.is_monotonic_increasing` and :func:`SeriesGroupBy.is_monotonic_decreasing` (:issue:`17015`)
+- :func:`DataFrame.from_dict` now accepts a ``columns`` argument that can be used to specify the column names when ``orient='index'`` is used (:issue:`18529`)
 
 .. _whatsnew_0230.api_breaking:
 

pandas/core/frame.py

@@ -876,7 +876,7 @@ def dot(self, other):
     # IO methods (to / from other formats)
 
     @classmethod
-    def from_dict(cls, data, orient='columns', dtype=None):
+    def from_dict(cls, data, orient='columns', dtype=None, columns=None):
         """
         Construct DataFrame from dict of array-like or dicts
 
@@ -890,12 +890,17 @@ def from_dict(cls, data, orient='columns', dtype=None):
             (default). Otherwise if the keys should be rows, pass 'index'.
         dtype : dtype, default None
             Data type to force, otherwise infer
+        columns: list, default None
+            Column labels to use when orient='index'. Raises a ValueError
+            if used with orient='columns'
+
+            .. versionadded:: 0.23.0
 
         Returns
         -------
         DataFrame
         """
-        index, columns = None, None
+        index = None
         orient = orient.lower()
         if orient == 'index':
             if len(data) > 0:
@@ -904,7 +909,11 @@ def from_dict(cls, data, orient='columns', dtype=None):
                     data = _from_nested_dict(data)
                 else:
                     data, index = list(data.values()), list(data.keys())
-        elif orient != 'columns':  # pragma: no cover
+        elif orient == 'columns':
+            if columns is not None:
+                raise ValueError("cannot use columns parameter with "
+                                 "orient='columns'")
+        else:  # pragma: no cover
             raise ValueError('only recognize index or columns for orient')
 
         return cls(data, index=index, columns=columns, dtype=dtype)

pandas/tests/frame/test_constructors.py

@@ -1091,6 +1091,25 @@ def test_constructor_orient(self):
         xp = DataFrame.from_dict(a).T.reindex(list(a.keys()))
         tm.assert_frame_equal(rs, xp)
 
+    def test_from_dict_columns_parameter(self):
+        # GH 18529
+        # Test new columns parameter for from_dict that was added to make
+        # from_items(..., orient='index', columns=[...]) easier to replicate
+        result = DataFrame.from_dict(OrderedDict([('A', [1, 2]),
+                                                  ('B', [4, 5])]),
+                                     orient='index', columns=['one', 'two'])
+        expected = DataFrame([[1, 2], [4, 5]], index=['A', 'B'],
+                             columns=['one', 'two'])
+        tm.assert_frame_equal(result, expected)
+
+        msg = "cannot use columns parameter with orient='columns'"
+        with tm.assert_raises_regex(ValueError, msg):
+            DataFrame.from_dict(dict([('A', [1, 2]), ('B', [4, 5])]),
+                                orient='columns', columns=['one', 'two'])
+        with tm.assert_raises_regex(ValueError, msg):
+            DataFrame.from_dict(dict([('A', [1, 2]), ('B', [4, 5])]),
+                                columns=['one', 'two'])
+
     def test_constructor_Series_named(self):
         a = Series([1, 2, 3], index=['a', 'b', 'c'], name='x')
         df = DataFrame(a)