From dd22e27b9b6580884eebf9f977a4314241b78b7d Mon Sep 17 00:00:00 2001
From: Elle Hanson <elleh3113@gmail.com>
Date: Sun, 17 Nov 2019 16:27:07 -0500
Subject: [PATCH] Bug fix GH 29624: calling str.isalpha on empty series returns
 object dtype, not bool Added dtype=bool argument to make _noarg_wrapper()
 return a bool Series

---
 doc/source/whatsnew/v1.0.0.rst |  2 +-
 pandas/core/strings.py         | 10 ++++++++++
 pandas/tests/test_strings.py   | 19 ++++++++++---------
 3 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index c91ced1014dd1..58d1fef9ef5bf 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -354,7 +354,7 @@ Conversion
 Strings
 ^^^^^^^
 
--
+- Calling :meth:`Series.str.isalnum` (and other "ismethods") on an empty Series would return an object dtype instead of bool (:issue:`29624`)
 -
 
 
diff --git a/pandas/core/strings.py b/pandas/core/strings.py
index a6e0c12526d8a..55ce44d736864 100644
--- a/pandas/core/strings.py
+++ b/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
diff --git a/pandas/tests/test_strings.py b/pandas/tests/test_strings.py
index f5d28ec82d1d4..f68541b620efa 100644
--- a/pandas/tests/test_strings.py
+++ b/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"))