Allow str.contains to accept a regex as well as a string (#989)

* Allow str.contains to accept a regex as well as a string

* Apply black formatting

Author: rowillia

pandas-stubs/core/strings.pyi

@@ -96,7 +96,12 @@ class StringMethods(NoNewAttributesMixin, Generic[T, _TS]):
     def get(self, i: int) -> T: ...
     def join(self, sep: str) -> T: ...
     def contains(
-        self, pat: str, case: bool = ..., flags: int = ..., na=..., regex: bool = ...
+        self,
+        pat: str | re.Pattern,
+        case: bool = ...,
+        flags: int = ...,
+        na=...,
+        regex: bool = ...,
     ) -> Series[bool]: ...
     def match(
         self, pat: str, case: bool = ..., flags: int = ..., na: Any = ...

tests/test_series.py

@@ -1452,6 +1452,11 @@ def test_string_accessors():
     check(assert_type(s.str.cat(sep="X"), str), str)
     check(assert_type(s.str.center(10), pd.Series), pd.Series)
     check(assert_type(s.str.contains("a"), "pd.Series[bool]"), pd.Series, np.bool_)
+    check(
+        assert_type(s.str.contains(re.compile(r"a")), "pd.Series[bool]"),
+        pd.Series,
+        np.bool_,
+    )
     check(assert_type(s.str.count("pp"), "pd.Series[int]"), pd.Series, np.integer)
     check(assert_type(s.str.decode("utf-8"), pd.Series), pd.Series)
     check(assert_type(s.str.encode("latin-1"), pd.Series), pd.Series)