[backport 2.3.x] BUG/TST (string dtype): fix and update tests for Sta…

…ta IO (pandas-dev#60130) (pandas-dev#60155)

BUG/TST (string dtype): fix and update tests for Stata IO (pandas-dev#60130)

(cherry picked from commit e7d54a5)

pandas/io/stata.py

@@ -605,7 +605,11 @@ def _cast_to_stata_types(data: DataFrame) -> DataFrame:
             if getattr(data[col].dtype, "numpy_dtype", None) is not None:
                 data[col] = data[col].astype(data[col].dtype.numpy_dtype)
             elif is_string_dtype(data[col].dtype):
+                # TODO could avoid converting string dtype to object here,
+                # but handle string dtype in _encode_strings
                 data[col] = data[col].astype("object")
+                # generate_table checks for None values
+                data.loc[data[col].isna(), col] = None
 
         dtype = data[col].dtype
         empty_df = data.shape[0] == 0
@@ -2671,6 +2675,7 @@ def _encode_strings(self) -> None:
                 continue
             column = self.data[col]
             dtype = column.dtype
+            # TODO could also handle string dtype here specifically
             if dtype.type is np.object_:
                 inferred_dtype = infer_dtype(column, skipna=True)
                 if not ((inferred_dtype == "string") or len(column) == 0):

pandas/tests/io/test_stata.py

@@ -11,8 +11,6 @@
 import numpy as np
 import pytest
 
-from pandas._config import using_string_dtype
-
 import pandas.util._test_decorators as td
 
 import pandas as pd
@@ -347,9 +345,8 @@ def test_write_dta6(self, datapath):
                 check_index_type=False,
             )
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     @pytest.mark.parametrize("version", [114, 117, 118, 119, None])
-    def test_read_write_dta10(self, version):
+    def test_read_write_dta10(self, version, using_infer_string):
         original = DataFrame(
             data=[["string", "object", 1, 1.1, np.datetime64("2003-12-25")]],
             columns=["string", "object", "integer", "floating", "datetime"],
@@ -362,12 +359,17 @@ def test_read_write_dta10(self, version):
         with tm.ensure_clean() as path:
             original.to_stata(path, convert_dates={"datetime": "tc"}, version=version)
             written_and_read_again = self.read_dta(path)
-            # original.index is np.int32, read index is np.int64
-            tm.assert_frame_equal(
-                written_and_read_again.set_index("index"),
-                original,
-                check_index_type=False,
-            )
+
+        expected = original.copy()
+        if using_infer_string:
+            expected["object"] = expected["object"].astype("str")
+
+        # original.index is np.int32, read index is np.int64
+        tm.assert_frame_equal(
+            written_and_read_again.set_index("index"),
+            expected,
+            check_index_type=False,
+        )
 
     def test_stata_doc_examples(self):
         with tm.ensure_clean() as path:
@@ -1153,7 +1155,6 @@ def test_categorical_ordering(self, file, datapath):
             assert parsed[col].cat.ordered
             assert not parsed_unordered[col].cat.ordered
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)", strict=False)
     @pytest.mark.filterwarnings("ignore::UserWarning")
     @pytest.mark.parametrize(
         "file",
@@ -1215,6 +1216,10 @@ def _convert_categorical(from_frame: DataFrame) -> DataFrame:
                 if cat.categories.dtype == object:
                     categories = pd.Index._with_infer(cat.categories._values)
                     cat = cat.set_categories(categories)
+                elif cat.categories.dtype == "string" and len(cat.categories) == 0:
+                    # if the read categories are empty, it comes back as object dtype
+                    categories = cat.categories.astype(object)
+                    cat = cat.set_categories(categories)
                 from_frame[col] = cat
         return from_frame
 
@@ -1244,7 +1249,6 @@ def test_iterator(self, datapath):
             from_chunks = pd.concat(itr)
         tm.assert_frame_equal(parsed, from_chunks)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)", strict=False)
     @pytest.mark.filterwarnings("ignore::UserWarning")
     @pytest.mark.parametrize(
         "file",
@@ -1548,12 +1552,11 @@ def test_inf(self, infval):
             with tm.ensure_clean() as path:
                 df.to_stata(path)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_path_pathlib(self):
         df = DataFrame(
             1.1 * np.arange(120).reshape((30, 4)),
-            columns=pd.Index(list("ABCD"), dtype=object),
-            index=pd.Index([f"i-{i}" for i in range(30)], dtype=object),
+            columns=pd.Index(list("ABCD")),
+            index=pd.Index([f"i-{i}" for i in range(30)]),
         )
         df.index.name = "index"
         reader = lambda x: read_stata(x).set_index("index")
@@ -1584,13 +1587,12 @@ def test_value_labels_iterator(self, write_index):
                 value_labels = dta_iter.value_labels()
         assert value_labels == {"A": {0: "A", 1: "B", 2: "C", 3: "E"}}
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_set_index(self):
         # GH 17328
         df = DataFrame(
             1.1 * np.arange(120).reshape((30, 4)),
-            columns=pd.Index(list("ABCD"), dtype=object),
-            index=pd.Index([f"i-{i}" for i in range(30)], dtype=object),
+            columns=pd.Index(list("ABCD")),
+            index=pd.Index([f"i-{i}" for i in range(30)]),
         )
         df.index.name = "index"
         with tm.ensure_clean() as path:
@@ -1618,8 +1620,7 @@ def test_date_parsing_ignores_format_details(self, column, datapath):
         formatted = df.loc[0, column + "_fmt"]
         assert unformatted == formatted
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
-    def test_writer_117(self):
+    def test_writer_117(self, using_infer_string):
         original = DataFrame(
             data=[
                 [
@@ -1682,13 +1683,17 @@ def test_writer_117(self):
                 version=117,
             )
             written_and_read_again = self.read_dta(path)
-            # original.index is np.int32, read index is np.int64
-            tm.assert_frame_equal(
-                written_and_read_again.set_index("index"),
-                original,
-                check_index_type=False,
-            )
-            tm.assert_frame_equal(original, copy)
+
+        expected = original[:]
+        if using_infer_string:
+            # object dtype (with only strings/None) comes back as string dtype
+            expected["object"] = expected["object"].astype("str")
+
+        tm.assert_frame_equal(
+            written_and_read_again.set_index("index"),
+            expected,
+        )
+        tm.assert_frame_equal(original, copy)
 
     def test_convert_strl_name_swap(self):
         original = DataFrame(
@@ -1725,15 +1730,14 @@ def test_invalid_date_conversion(self):
             with pytest.raises(ValueError, match=msg):
                 original.to_stata(path, convert_dates={"wrong_name": "tc"})
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     @pytest.mark.parametrize("version", [114, 117, 118, 119, None])
     def test_nonfile_writing(self, version):
         # GH 21041
         bio = io.BytesIO()
         df = DataFrame(
             1.1 * np.arange(120).reshape((30, 4)),
-            columns=pd.Index(list("ABCD"), dtype=object),
-            index=pd.Index([f"i-{i}" for i in range(30)], dtype=object),
+            columns=pd.Index(list("ABCD")),
+            index=pd.Index([f"i-{i}" for i in range(30)]),
         )
         df.index.name = "index"
         with tm.ensure_clean() as path:
@@ -1744,13 +1748,12 @@ def test_nonfile_writing(self, version):
             reread = read_stata(path, index_col="index")
         tm.assert_frame_equal(df, reread)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_gzip_writing(self):
         # writing version 117 requires seek and cannot be used with gzip
         df = DataFrame(
             1.1 * np.arange(120).reshape((30, 4)),
-            columns=pd.Index(list("ABCD"), dtype=object),
-            index=pd.Index([f"i-{i}" for i in range(30)], dtype=object),
+            columns=pd.Index(list("ABCD")),
+            index=pd.Index([f"i-{i}" for i in range(30)]),
         )
         df.index.name = "index"
         with tm.ensure_clean() as path:
@@ -1777,8 +1780,7 @@ def test_unicode_dta_118(self, datapath):
 
         tm.assert_frame_equal(unicode_df, expected)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
-    def test_mixed_string_strl(self):
+    def test_mixed_string_strl(self, using_infer_string):
         # GH 23633
         output = [{"mixed": "string" * 500, "number": 0}, {"mixed": None, "number": 1}]
         output = DataFrame(output)
@@ -1796,7 +1798,10 @@ def test_mixed_string_strl(self):
                 path, write_index=False, convert_strl=["mixed"], version=117
             )
             reread = read_stata(path)
-            expected = output.fillna("")
+            expected = output.copy()
+            if using_infer_string:
+                expected["mixed"] = expected["mixed"].astype("str")
+            expected = expected.fillna("")
             tm.assert_frame_equal(reread, expected)
 
     @pytest.mark.parametrize("version", [114, 117, 118, 119, None])
@@ -1875,7 +1880,7 @@ def test_stata_119(self, datapath):
                 reader._ensure_open()
                 assert reader._nvar == 32999
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
+    @pytest.mark.filterwarnings("ignore:Downcasting behavior:FutureWarning")
     @pytest.mark.parametrize("version", [118, 119, None])
     def test_utf8_writer(self, version):
         cat = pd.Categorical(["a", "β", "ĉ"], ordered=True)
@@ -2143,14 +2148,13 @@ def test_iterator_errors(datapath, chunksize):
             pass
 
 
-@pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
 def test_iterator_value_labels():
     # GH 31544
     values = ["c_label", "b_label"] + ["a_label"] * 500
     df = DataFrame({f"col{k}": pd.Categorical(values, ordered=True) for k in range(2)})
     with tm.ensure_clean() as path:
         df.to_stata(path, write_index=False)
-        expected = pd.Index(["a_label", "b_label", "c_label"], dtype="object")
+        expected = pd.Index(["a_label", "b_label", "c_label"])
         with read_stata(path, chunksize=100) as reader:
             for j, chunk in enumerate(reader):
                 for i in range(2):