BUG: Series.str.extract with StringArray returning object dtype (#41441)

Author: simonjayhawkins

doc/source/whatsnew/v1.3.0.rst

@@ -751,6 +751,7 @@ Strings
 
 - Bug in the conversion from ``pyarrow.ChunkedArray`` to :class:`~arrays.StringArray` when the original had zero chunks (:issue:`41040`)
 - Bug in :meth:`Series.replace` and :meth:`DataFrame.replace` ignoring replacements with ``regex=True`` for ``StringDType`` data (:issue:`41333`, :issue:`35977`)
+- Bug in :meth:`Series.str.extract` with :class:`~arrays.StringArray` returning object dtype for empty :class:`DataFrame` (:issue:`41441`)
 
 Interval
 ^^^^^^^^

pandas/core/strings/accessor.py

@@ -3113,17 +3113,16 @@ def _str_extract_noexpand(arr, pat, flags=0):
             # error: Incompatible types in assignment (expression has type
             # "DataFrame", variable has type "ndarray")
             result = DataFrame(  # type: ignore[assignment]
-                columns=columns, dtype=object
+                columns=columns, dtype=result_dtype
             )
         else:
-            dtype = _result_dtype(arr)
             # error: Incompatible types in assignment (expression has type
             # "DataFrame", variable has type "ndarray")
             result = DataFrame(  # type:ignore[assignment]
                 [groups_or_na(val) for val in arr],
                 columns=columns,
                 index=arr.index,
-                dtype=dtype,
+                dtype=result_dtype,
             )
     return result, name
 
@@ -3140,19 +3139,19 @@ def _str_extract_frame(arr, pat, flags=0):
     regex = re.compile(pat, flags=flags)
     groups_or_na = _groups_or_na_fun(regex)
     columns = _get_group_names(regex)
+    result_dtype = _result_dtype(arr)
 
     if len(arr) == 0:
-        return DataFrame(columns=columns, dtype=object)
+        return DataFrame(columns=columns, dtype=result_dtype)
     try:
         result_index = arr.index
     except AttributeError:
         result_index = None
-    dtype = _result_dtype(arr)
     return DataFrame(
         [groups_or_na(val) for val in arr],
         columns=columns,
         index=result_index,
-        dtype=dtype,
+        dtype=result_dtype,
     )
 
 

pandas/tests/strings/test_strings.py

@@ -175,17 +175,19 @@ def test_empty_str_methods(any_string_dtype):
     tm.assert_series_equal(empty_str, empty.str.repeat(3))
     tm.assert_series_equal(empty_bool, empty.str.match("^a"))
     tm.assert_frame_equal(
-        DataFrame(columns=[0], dtype=str), empty.str.extract("()", expand=True)
+        DataFrame(columns=[0], dtype=any_string_dtype),
+        empty.str.extract("()", expand=True),
     )
     tm.assert_frame_equal(
-        DataFrame(columns=[0, 1], dtype=str), empty.str.extract("()()", expand=True)
+        DataFrame(columns=[0, 1], dtype=any_string_dtype),
+        empty.str.extract("()()", expand=True),
     )
     tm.assert_series_equal(empty_str, empty.str.extract("()", expand=False))
     tm.assert_frame_equal(
-        DataFrame(columns=[0, 1], dtype=str),
+        DataFrame(columns=[0, 1], dtype=any_string_dtype),
         empty.str.extract("()()", expand=False),
     )
-    tm.assert_frame_equal(DataFrame(dtype=str), empty.str.get_dummies())
+    tm.assert_frame_equal(DataFrame(), empty.str.get_dummies())
     tm.assert_series_equal(empty_str, empty_str.str.join(""))
     tm.assert_series_equal(empty_int, empty.str.len())
     tm.assert_series_equal(empty_object, empty_str.str.findall("a"))