BUG: Correct behavior when reading empty dta files

Correct column selection when reading empty dta files
Correct omitted dtype information in empty dta files

closes pandas-dev#46240

Author: bashtage

doc/source/whatsnew/v2.1.0.rst

@@ -377,6 +377,7 @@ Missing
 MultiIndex
 ^^^^^^^^^^
 - Bug in :meth:`MultiIndex.set_levels` not preserving dtypes for :class:`Categorical` (:issue:`52125`)
+- Bug in displaying a :class:`MultiIndex` with a long element (:issue:`52960`)
 
 I/O
 ^^^
@@ -386,7 +387,7 @@ I/O
 - Bug in :func:`read_hdf` not properly closing store after a ``IndexError`` is raised (:issue:`52781`)
 - Bug in :func:`read_html`, style elements were read into DataFrames (:issue:`52197`)
 - Bug in :func:`read_html`, tail texts were removed together with elements containing ``display:none`` style (:issue:`51629`)
-- Bug in displaying a :class:`MultiIndex` with a long element (:issue:`52960`)
+- Bug when writing and reading empty Stata dta files where dtype information was lost (:issue:`46240`)
 
 Period
 ^^^^^^

pandas/io/stata.py

@@ -608,9 +608,10 @@ def _cast_to_stata_types(data: DataFrame) -> DataFrame:
             # Replace with NumPy-compatible column
             data[col] = data[col].astype(data[col].dtype.numpy_dtype)
         dtype = data[col].dtype
+        empty_df = data.shape[0] == 0
         for c_data in conversion_data:
             if dtype == c_data[0]:
-                if data[col].max() <= np.iinfo(c_data[1]).max:
+                if empty_df or data[col].max() <= np.iinfo(c_data[1]).max:
                     dtype = c_data[1]
                 else:
                     dtype = c_data[2]
@@ -621,14 +622,17 @@ def _cast_to_stata_types(data: DataFrame) -> DataFrame:
                 data[col] = data[col].astype(dtype)
 
         # Check values and upcast if necessary
-        if dtype == np.int8:
+
+        if dtype == np.int8 and not empty_df:
             if data[col].max() > 100 or data[col].min() < -127:
                 data[col] = data[col].astype(np.int16)
-        elif dtype == np.int16:
+        elif dtype == np.int16 and not empty_df:
             if data[col].max() > 32740 or data[col].min() < -32767:
                 data[col] = data[col].astype(np.int32)
         elif dtype == np.int64:
-            if data[col].max() <= 2147483620 and data[col].min() >= -2147483647:
+            if empty_df or (
+                data[col].max() <= 2147483620 and data[col].min() >= -2147483647
+            ):
                 data[col] = data[col].astype(np.int32)
             else:
                 data[col] = data[col].astype(np.float64)
@@ -1700,13 +1704,6 @@ def read(
         order_categoricals: bool | None = None,
     ) -> DataFrame:
         self._ensure_open()
-        # Handle empty file or chunk.  If reading incrementally raise
-        # StopIteration.  If reading the whole thing return an empty
-        # data frame.
-        if (self._nobs == 0) and (nrows is None):
-            self._can_read_value_labels = True
-            self._data_read = True
-            return DataFrame(columns=self._varlist)
 
         # Handle options
         if convert_dates is None:
@@ -1723,10 +1720,26 @@ def read(
             order_categoricals = self._order_categoricals
         if index_col is None:
             index_col = self._index_col
-
         if nrows is None:
             nrows = self._nobs
 
+        # Handle empty file or chunk.  If reading incrementally raise
+        # StopIteration.  If reading the whole thing return an empty
+        # data frame.
+        if (self._nobs == 0) and nrows == 0:
+            self._can_read_value_labels = True
+            self._data_read = True
+            data = DataFrame(columns=self._varlist)
+            # Apply dtypes correctly
+            for i, col in enumerate(data.columns):
+                dt = self._dtyplist[i]
+                if isinstance(dt, np.dtype):
+                    if dt.char != "S":
+                        data[col] = data[col].astype(dt)
+            if columns is not None:
+                data = self._do_select_columns(data, columns)
+            return data
+
         if (self._format_version >= 117) and (not self._value_labels_read):
             self._can_read_value_labels = True
             self._read_strls()

pandas/tests/io/test_stata.py

@@ -71,6 +71,41 @@ def test_read_empty_dta(self, version):
             empty_ds2 = read_stata(path)
             tm.assert_frame_equal(empty_ds, empty_ds2)
 
+    @pytest.mark.parametrize("version", [114, 117, 118, 119, None])
+    def test_read_empty_dta_with_dtypes(self, version):
+        # GH 46240
+        # Fixing above bug revealed that types are not correctly preserved when
+        # writing empty DataFrames
+        empty_df_typed = DataFrame(
+            {
+                "i8": np.array([0], dtype=np.int8),
+                "i16": np.array([0], dtype=np.int16),
+                "i32": np.array([0], dtype=np.int32),
+                "i64": np.array([0], dtype=np.int64),
+                "u8": np.array([0], dtype=np.uint8),
+                "u16": np.array([0], dtype=np.uint16),
+                "u32": np.array([0], dtype=np.uint32),
+                "u64": np.array([0], dtype=np.uint64),
+                "f32": np.array([0], dtype=np.float32),
+                "f64": np.array([0], dtype=np.float64),
+            }
+        )
+        expected = empty_df_typed.copy()
+        # No uint# support. Downcast since values in range for int#
+        expected["u8"] = expected["u8"].astype(np.int8)
+        expected["u16"] = expected["u16"].astype(np.int16)
+        expected["u32"] = expected["u32"].astype(np.int32)
+        # No int64 supported at all. Downcast since values in range for int32
+        expected["u64"] = expected["u64"].astype(np.int32)
+        expected["i64"] = expected["i64"].astype(np.int32)
+
+        # GH 7369, make sure can read a 0-obs dta file
+        with tm.ensure_clean() as path:
+            empty_df_typed.to_stata(path, write_index=False, version=version)
+            empty_reread = read_stata(path)
+            tm.assert_frame_equal(expected, empty_reread)
+            tm.assert_series_equal(expected.dtypes, empty_reread.dtypes)
+
     @pytest.mark.parametrize("version", [114, 117, 118, 119, None])
     def test_read_index_col_none(self, version):
         df = DataFrame({"a": range(5), "b": ["b1", "b2", "b3", "b4", "b5"]})
@@ -2274,3 +2309,21 @@ def test_nullable_support(dtype, version):
         tm.assert_series_equal(df.a, reread.a)
         tm.assert_series_equal(reread.b, expected_b)
         tm.assert_series_equal(reread.c, expected_c)
+
+
+def test_empty_frame():
+    # GH 46240
+    # create an empty DataFrame with int64 and float64 dtypes
+    df = DataFrame(data={"a": range(3), "b": [1.0, 2.0, 3.0]}).head(0)
+    with tm.ensure_clean() as path:
+        df.to_stata(path, write_index=False, version=117)
+        # Read entire dataframe
+        df2 = read_stata(path)
+        assert "b" in df2
+        # Dtypes don't match since no support for int32
+        dtypes = Series({"a": np.dtype("int32"), "b": np.dtype("float64")})
+        tm.assert_series_equal(df2.dtypes, dtypes)
+        # read one column of empty .dta file
+        df3 = read_stata(path, columns=["a"])
+        assert "b" not in df3
+        tm.assert_series_equal(df3.dtypes, dtypes.loc[["a"]])