Fix: StringArray non-extensible due to inconsisent assertion

doc/source/whatsnew/v1.1.0.rst

@@ -1186,6 +1186,7 @@ ExtensionArray
 - Fixed bug that caused :meth:`Series.__repr__()` to crash for extension types whose elements are multidimensional arrays (:issue:`33770`).
 - Fixed bug where :meth:`Series.update` would raise a ``ValueError`` for ``ExtensionArray`` dtypes with missing values (:issue:`33980`)
 - Fixed bug where :meth:`StringArray.memory_usage` was not implemented (:issue:`33963`)
+- Fixed bug where extending :meth:`StringArray._from_sequence` would fail (:issue:`34309`)
 - Fixed bug where :meth:`DataFrameGroupBy` would ignore the ``min_count`` argument for aggregations on nullable Boolean dtypes (:issue:`34051`)
 - Fixed bug where the constructor of :class:`DataFrame` with ``dtype='string'`` would fail (:issue:`27953`, :issue:`33623`)
 - Bug where :class:`DataFrame` column set to scalar extension type was considered an object type rather than the extension type (:issue:`34832`)

pandas/core/arrays/string_.py

@@ -196,7 +196,7 @@ def _validate(self):
     @classmethod
     def _from_sequence(cls, scalars, dtype=None, copy=False):
         if dtype:
-            assert dtype == "string"
+            assert isinstance(dtype, StringDtype)
 
         # convert non-na-likes to str, and nan-likes to StringDtype.na_value
         result = lib.ensure_string_array(

pandas/tests/arrays/string_/test_string.py

@@ -1,10 +1,14 @@
 import operator
+from typing import Type
 
 import numpy as np
 import pytest
 
 import pandas.util._test_decorators as td
 
+from pandas.core.dtypes.base import register_extension_dtype
+from pandas.core.dtypes.common import is_string_dtype
+
 import pandas as pd
 import pandas._testing as tm
 
@@ -338,6 +342,29 @@ def test_memory_usage():
     assert 0 < series.nbytes <= series.memory_usage() < series.memory_usage(deep=True)
 
 
+def test_string_dtype_subclassing():
+    @register_extension_dtype
+    class MyExtensionDtype(pd.StringDtype):
+        name = "my_extension"
+
+        def __repr__(self) -> str:
+            return "MyExtensionDtype"
+
+        @classmethod
+        def construct_array_type(cls) -> "Type[MyExtensionStringArray]":
+            return MyExtensionStringArray
+
+    class MyExtensionStringArray(pd.core.arrays.StringArray):
+        def __init__(self, values, copy=False):
+            super().__init__(values, copy)
+            self._dtype = MyExtensionDtype()
+
+    series = pd.Series(["test", "test2"], dtype="my_extension")
+    assert series.dtype == "my_extension"
+    assert series.values == ["test", "test2"]
+    assert is_string_dtype(series)
+
+
 @pytest.mark.parametrize("dtype", [np.float16, np.float32, np.float64])
 def test_astype_from_float_dtype(dtype):
     # https://github.com/pandas-dev/pandas/issues/36451