Bug fix GH 29624: calling str.isalpha on empty series returns object dtype, not bool (pandas-dev#29680)

Author: ellequelle

doc/source/whatsnew/v1.0.0.rst

@@ -355,7 +355,7 @@ Conversion
 Strings
 ^^^^^^^
 
--
+- Calling :meth:`Series.str.isalnum` (and other "ismethods") on an empty Series would return an object dtype instead of bool (:issue:`29624`)
 -
 
 

pandas/core/strings.py

@@ -3401,59 +3401,69 @@ def rindex(self, sub, start=0, end=None):
     _doc_args["istitle"] = dict(type="titlecase", method="istitle")
     _doc_args["isnumeric"] = dict(type="numeric", method="isnumeric")
     _doc_args["isdecimal"] = dict(type="decimal", method="isdecimal")
+    # force _noarg_wrapper return type with dtype=bool (GH 29624)
     isalnum = _noarg_wrapper(
         lambda x: x.isalnum(),
         name="isalnum",
         docstring=_shared_docs["ismethods"] % _doc_args["isalnum"],
         returns_string=False,
+        dtype=bool,
     )
     isalpha = _noarg_wrapper(
         lambda x: x.isalpha(),
         name="isalpha",
         docstring=_shared_docs["ismethods"] % _doc_args["isalpha"],
         returns_string=False,
+        dtype=bool,
     )
     isdigit = _noarg_wrapper(
         lambda x: x.isdigit(),
         name="isdigit",
         docstring=_shared_docs["ismethods"] % _doc_args["isdigit"],
         returns_string=False,
+        dtype=bool,
     )
     isspace = _noarg_wrapper(
         lambda x: x.isspace(),
         name="isspace",
         docstring=_shared_docs["ismethods"] % _doc_args["isspace"],
         returns_string=False,
+        dtype=bool,
     )
     islower = _noarg_wrapper(
         lambda x: x.islower(),
         name="islower",
         docstring=_shared_docs["ismethods"] % _doc_args["islower"],
         returns_string=False,
+        dtype=bool,
     )
     isupper = _noarg_wrapper(
         lambda x: x.isupper(),
         name="isupper",
         docstring=_shared_docs["ismethods"] % _doc_args["isupper"],
         returns_string=False,
+        dtype=bool,
     )
     istitle = _noarg_wrapper(
         lambda x: x.istitle(),
         name="istitle",
         docstring=_shared_docs["ismethods"] % _doc_args["istitle"],
         returns_string=False,
+        dtype=bool,
     )
     isnumeric = _noarg_wrapper(
         lambda x: x.isnumeric(),
         name="isnumeric",
         docstring=_shared_docs["ismethods"] % _doc_args["isnumeric"],
         returns_string=False,
+        dtype=bool,
     )
     isdecimal = _noarg_wrapper(
         lambda x: x.isdecimal(),
         name="isdecimal",
         docstring=_shared_docs["ismethods"] % _doc_args["isdecimal"],
         returns_string=False,
+        dtype=bool,
     )
 
     @classmethod

pandas/tests/test_strings.py

@@ -1853,15 +1853,16 @@ def test_empty_str_methods(self):
         tm.assert_series_equal(empty_str, empty.str.get(0))
         tm.assert_series_equal(empty_str, empty_bytes.str.decode("ascii"))
         tm.assert_series_equal(empty_bytes, empty.str.encode("ascii"))
-        tm.assert_series_equal(empty_str, empty.str.isalnum())
-        tm.assert_series_equal(empty_str, empty.str.isalpha())
-        tm.assert_series_equal(empty_str, empty.str.isdigit())
-        tm.assert_series_equal(empty_str, empty.str.isspace())
-        tm.assert_series_equal(empty_str, empty.str.islower())
-        tm.assert_series_equal(empty_str, empty.str.isupper())
-        tm.assert_series_equal(empty_str, empty.str.istitle())
-        tm.assert_series_equal(empty_str, empty.str.isnumeric())
-        tm.assert_series_equal(empty_str, empty.str.isdecimal())
+        # ismethods should always return boolean (GH 29624)
+        tm.assert_series_equal(empty_bool, empty.str.isalnum())
+        tm.assert_series_equal(empty_bool, empty.str.isalpha())
+        tm.assert_series_equal(empty_bool, empty.str.isdigit())
+        tm.assert_series_equal(empty_bool, empty.str.isspace())
+        tm.assert_series_equal(empty_bool, empty.str.islower())
+        tm.assert_series_equal(empty_bool, empty.str.isupper())
+        tm.assert_series_equal(empty_bool, empty.str.istitle())
+        tm.assert_series_equal(empty_bool, empty.str.isnumeric())
+        tm.assert_series_equal(empty_bool, empty.str.isdecimal())
         tm.assert_series_equal(empty_str, empty.str.capitalize())
         tm.assert_series_equal(empty_str, empty.str.swapcase())
         tm.assert_series_equal(empty_str, empty.str.normalize("NFC"))