String dtype: disallow specifying the 'str' dtype with storage in [..] in string alias

Author: jorisvandenbossche

pandas/core/dtypes/dtypes.py

@@ -2339,7 +2339,7 @@ def construct_from_string(cls, string: str) -> ArrowDtype:
             )
         if not string.endswith("[pyarrow]"):
             raise TypeError(f"'{string}' must end with '[pyarrow]'")
-        if string == "string[pyarrow]":
+        if string in ("string[pyarrow]", "str[pyarrow]"):
             # Ensure Registry.find skips ArrowDtype to use StringDtype instead
             raise TypeError("string[pyarrow] should be constructed by StringDtype")
         if pa_version_under10p1:

pandas/tests/dtypes/test_common.py

@@ -837,6 +837,26 @@ def test_pandas_dtype_string_dtypes(string_storage):
     assert result == pd.StringDtype(string_storage, na_value=pd.NA)
 
 
+def test_pandas_dtype_string_dtype_alias_with_storage():
+    with pytest.raises(TypeError):
+        pandas_dtype("str[python]")
+
+    with pytest.raises(TypeError):
+        pandas_dtype("str[pyarrow]")
+
+    result = pandas_dtype("string[python]")
+    assert result == pd.StringDtype("python", na_value=pd.NA)
+
+    if HAS_PYARROW:
+        result = pandas_dtype("string[pyarrow]")
+        assert result == pd.StringDtype("pyarrow", na_value=pd.NA)
+    else:
+        with pytest.raises(
+            ImportError, match="required for PyArrow backed StringArray"
+        ):
+            pandas_dtype("string[pyarrow]")
+
+
 @td.skip_if_installed("pyarrow")
 def test_construct_from_string_without_pyarrow_installed():
     # GH 57928