GH1149 Add testing for pd.Series.str behavior (#1272)

* GH1149 Add testing for pd.Series.str behavior

* GH1149 run pre-commit

* GH1149 modify test name

Author: loicdiridollou

tests/test_series.py

@@ -3975,3 +3975,11 @@ def test_timedelta_index_cumprod() -> None:
 
     if TYPE_CHECKING_INVALID_USAGE:
         assert_type(as_period_series.cumprod(), Never)
+
+
+def test_series_str_methods() -> None:
+    """Test the returns of StringMethods match the type of the input series GH1149."""
+    s_str = pd.Series(["a", "b"])
+    check(assert_type(s_str, "pd.Series[str]"), pd.Series, str)
+    check(assert_type(s_str.str.upper(), "pd.Series[str]"), pd.Series, str)
+    check(assert_type(s_str.str.lower(), "pd.Series[str]"), pd.Series, str)