add future deprecation test and clean code based on review

Author: charlesdong1991

doc/source/whatsnew/v0.25.0.rst

@@ -40,9 +40,7 @@ Other API Changes
 Deprecations
 ~~~~~~~~~~~~
 
--
--
--
+- :func:`Series.str.replace`, the default regex for single character might be deprecated.
 
 
 .. _whatsnew_0250.prior_deprecations:

pandas/core/strings.py

@@ -581,7 +581,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
         # if regex is default None, and a single special character is given
         # in pat, still take it as a literal, and raise the Future warning
         if regex is None and len(pat) == 1 and re.findall(r"\W", pat):
-            warnings.warn("'%s' is interpreted as a literal in " % pat +
+            warnings.warn("'{}' is interpreted as a literal in ".format(pat) +
                           "default, not regex. It will change in the future.",
                           FutureWarning)
         f = lambda x: x.replace(pat, repl, n)

pandas/tests/test_strings.py

@@ -3441,10 +3441,13 @@ def test_replace_single_pattern(self, regex, expected_array):
         expected = Series(expected_array)
         tm.assert_series_equal(result, expected)
 
-    @pytest.mark.filterwarnings("ignore: '.' is interpreted as a literal")
     def test_replace_without_specifying_regex_parameter(self):
         values = Series(['a.c'])
         # GH: 24804
-        result = values.str.replace('.', 'b')
+        # test future deprecation warning
+        msg = "'.' is interpreted as a literal"
+        with pytest.warns(FutureWarning, match=msg):
+            result = values.str.replace('.', 'b')
         expected = Series(['abc'])
         tm.assert_series_equal(result, expected)
+