DEPR: ignoring dtype in DataFrame constructor failures

doc/source/whatsnew/v1.3.0.rst

@@ -233,7 +233,7 @@ Other enhancements
 - Add keyword ``sort`` to :func:`pivot_table` to allow non-sorting of the result (:issue:`39143`)
 - Add keyword ``dropna`` to :meth:`DataFrame.value_counts` to allow counting rows that include ``NA`` values (:issue:`41325`)
 - :meth:`Series.replace` will now cast results to ``PeriodDtype`` where possible instead of ``object`` dtype (:issue:`41526`)
-- Improved error message in ``corr` and ``cov`` methods on :class:`.Rolling`, :class:`.Expanding`, and :class:`.ExponentialMovingWindow` when ``other`` is not a :class:`DataFrame` or :class:`Series` (:issue:`41741`)
+- Improved error message in ``corr`` and ``cov`` methods on :class:`.Rolling`, :class:`.Expanding`, and :class:`.ExponentialMovingWindow` when ``other`` is not a :class:`DataFrame` or :class:`Series` (:issue:`41741`)
 
 .. ---------------------------------------------------------------------------
 
@@ -686,6 +686,7 @@ Deprecations
 - Deprecated passing arguments (apart from ``cond`` and ``other``) as positional in :meth:`DataFrame.mask` and :meth:`Series.mask` (:issue:`41485`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.clip` and :meth:`Series.clip` (other than ``"upper"`` and ``"lower"``) (:issue:`41485`)
 - Deprecated special treatment of lists with first element a Categorical in the :class:`DataFrame` constructor; pass as ``pd.DataFrame({col: categorical, ...})`` instead (:issue:`38845`)
+- Deprecated behavior of :class:`DataFrame` constructor when a ``dtype`` is passed and the data cannot be cast to that dtype. In a future version, this will raise instead of being silently ignored (:issue:`24435`)
 - Deprecated passing arguments as positional (except for ``"method"``) in :meth:`DataFrame.interpolate` and :meth:`Series.interpolate` (:issue:`41485`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.ffill`, :meth:`Series.ffill`, :meth:`DataFrame.bfill`, and :meth:`Series.bfill` (:issue:`41485`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.sort_values` (other than ``"by"``) and :meth:`Series.sort_values` (:issue:`41485`)

pandas/core/construction.py

@@ -12,6 +12,7 @@
     Sequence,
     cast,
 )
+import warnings
 
 import numpy as np
 import numpy.ma as ma
@@ -745,6 +746,17 @@ def _try_cast(
         if raise_cast_failure:
             raise
         else:
+            # we only get here with raise_cast_failure False, which means
+            #  called via the DataFrame constructor
+            # GH#24435
+            warnings.warn(
+                f"Could not cast to {dtype}, falling back to object. This "
+                "behavior is deprecated. In a future version, when a dtype is "
+                "passed to 'DataFrame', either all columns will be cast to that "
+                "dtype, or a TypeError will be raised",
+                FutureWarning,
+                stacklevel=7,
+            )
             subarr = np.array(arr, dtype=object, copy=copy)
     return subarr
 

pandas/tests/frame/test_block_internals.py

@@ -258,8 +258,11 @@ def f(dtype):
             f([("A", "datetime64[h]"), ("B", "str"), ("C", "int32")])
 
         # these work (though results may be unexpected)
-        f("int64")
-        f("float64")
+        depr_msg = "either all columns will be cast to that dtype, or a TypeError will"
+        with tm.assert_produces_warning(FutureWarning, match=depr_msg):
+            f("int64")
+        with tm.assert_produces_warning(FutureWarning, match=depr_msg):
+            f("float64")
 
         # 10822
         # invalid error message on dt inference

pandas/tests/frame/test_constructors.py

@@ -207,7 +207,9 @@ def test_constructor_mixed(self, float_string_frame):
         assert float_string_frame["foo"].dtype == np.object_
 
     def test_constructor_cast_failure(self):
-        foo = DataFrame({"a": ["a", "b", "c"]}, dtype=np.float64)
+        msg = "either all columns will be cast to that dtype, or a TypeError will"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            foo = DataFrame({"a": ["a", "b", "c"]}, dtype=np.float64)
         assert foo["a"].dtype == object
 
         # GH 3010, constructing with odd arrays
@@ -683,7 +685,10 @@ def test_constructor_dict_cast2(self):
             "A": dict(zip(range(20), tm.makeStringIndex(20))),
             "B": dict(zip(range(15), np.random.randn(15))),
         }
-        frame = DataFrame(test_data, dtype=float)
+        msg = "either all columns will be cast to that dtype, or a TypeError will"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            frame = DataFrame(test_data, dtype=float)
+
         assert len(frame) == 20
         assert frame["A"].dtype == np.object_
         assert frame["B"].dtype == np.float64

pandas/tests/indexing/multiindex/test_getitem.py

@@ -206,27 +206,26 @@ def test_frame_getitem_nan_multiindex(nulls_fixture):
     df = DataFrame(
         [[11, n, 13], [21, n, 23], [31, n, 33], [41, n, 43]],
         columns=cols,
-        dtype="int64",
     ).set_index(["a", "b"])
+    df["c"] = df["c"].astype("int64")
 
     idx = (21, n)
     result = df.loc[:idx]
-    expected = DataFrame(
-        [[11, n, 13], [21, n, 23]], columns=cols, dtype="int64"
-    ).set_index(["a", "b"])
+    expected = DataFrame([[11, n, 13], [21, n, 23]], columns=cols).set_index(["a", "b"])
+    expected["c"] = expected["c"].astype("int64")
     tm.assert_frame_equal(result, expected)
 
     result = df.loc[idx:]
     expected = DataFrame(
-        [[21, n, 23], [31, n, 33], [41, n, 43]], columns=cols, dtype="int64"
+        [[21, n, 23], [31, n, 33], [41, n, 43]], columns=cols
     ).set_index(["a", "b"])
+    expected["c"] = expected["c"].astype("int64")
     tm.assert_frame_equal(result, expected)
 
     idx1, idx2 = (21, n), (31, n)
     result = df.loc[idx1:idx2]
-    expected = DataFrame(
-        [[21, n, 23], [31, n, 33]], columns=cols, dtype="int64"
-    ).set_index(["a", "b"])
+    expected = DataFrame([[21, n, 23], [31, n, 33]], columns=cols).set_index(["a", "b"])
+    expected["c"] = expected["c"].astype("int64")
     tm.assert_frame_equal(result, expected)
 
 

pandas/tests/indexing/test_loc.py

@@ -284,7 +284,12 @@ def test_loc_setitem_dtype(self):
         df.loc[:, cols] = df.loc[:, cols].astype("float32")
 
         expected = DataFrame(
-            {"id": ["A"], "a": [1.2], "b": [0.0], "c": [-2.5]}, dtype="float32"
+            {
+                "id": ["A"],
+                "a": np.array([1.2], dtype="float32"),
+                "b": np.array([0.0], dtype="float32"),
+                "c": np.array([-2.5], dtype="float32"),
+            }
         )  # id is inferred as object
 
         tm.assert_frame_equal(df, expected)

pandas/tests/reshape/test_get_dummies.py

@@ -272,8 +272,9 @@ def test_dataframe_dummies_subset(self, df, sparse):
                 "from_A_a": [1, 0, 1],
                 "from_A_b": [0, 1, 0],
             },
-            dtype=np.uint8,
         )
+        cols = expected.columns
+        expected[cols[1:]] = expected[cols[1:]].astype(np.uint8)
         expected[["C"]] = df[["C"]]
         if sparse:
             cols = ["from_A_a", "from_A_b"]