String dtype: implement _get_common_dtype (pandas-dev#59682)

* String dtype: implement _get_common_dtype

* add specific tests

* try fix typing

* try fix typing

* suppress typing error

* support numpy 2.0 string

* fix typo

Author: jorisvandenbossche

pandas/core/arrays/string_.py

@@ -167,9 +167,9 @@ def __init__(
             # a consistent NaN value (and we can use `dtype.na_value is np.nan`)
             na_value = np.nan
         elif na_value is not libmissing.NA:
-            raise ValueError("'na_value' must be np.nan or pd.NA, got {na_value}")
+            raise ValueError(f"'na_value' must be np.nan or pd.NA, got {na_value}")
 
-        self.storage = storage
+        self.storage = cast(str, storage)
         self._na_value = na_value
 
     def __repr__(self) -> str:
@@ -280,6 +280,34 @@ def construct_array_type(  # type: ignore[override]
         else:
             return ArrowStringArrayNumpySemantics
 
+    def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
+        storages = set()
+        na_values = set()
+
+        for dtype in dtypes:
+            if isinstance(dtype, StringDtype):
+                storages.add(dtype.storage)
+                na_values.add(dtype.na_value)
+            elif isinstance(dtype, np.dtype) and dtype.kind in ("U", "T"):
+                continue
+            else:
+                return None
+
+        if len(storages) == 2:
+            # if both python and pyarrow storage -> priority to pyarrow
+            storage = "pyarrow"
+        else:
+            storage = next(iter(storages))  # type: ignore[assignment]
+
+        na_value: libmissing.NAType | float
+        if len(na_values) == 2:
+            # if both NaN and NA -> priority to NA
+            na_value = libmissing.NA
+        else:
+            na_value = next(iter(na_values))
+
+        return StringDtype(storage=storage, na_value=na_value)
+
     def __from_arrow__(
         self, array: pyarrow.Array | pyarrow.ChunkedArray
     ) -> BaseStringArray:

pandas/tests/arrays/categorical/test_api.py

@@ -3,8 +3,6 @@
 import numpy as np
 import pytest
 
-from pandas._config import using_string_dtype
-
 from pandas.compat import PY311
 
 from pandas import (
@@ -158,7 +156,6 @@ def test_reorder_categories_raises(self, new_categories):
         with pytest.raises(ValueError, match=msg):
             cat.reorder_categories(new_categories)
 
-    @pytest.mark.xfail(using_string_dtype(), reason="TODO(infer_string)")
     def test_add_categories(self):
         cat = Categorical(["a", "b", "c", "a"], ordered=True)
         old = cat.copy()

pandas/tests/arrays/string_/test_concat.py

@@ -0,0 +1,73 @@
+import numpy as np
+import pytest
+
+from pandas.compat import HAS_PYARROW
+
+from pandas.core.dtypes.cast import find_common_type
+
+import pandas as pd
+import pandas._testing as tm
+from pandas.util.version import Version
+
+
+@pytest.mark.parametrize(
+    "to_concat_dtypes, result_dtype",
+    [
+        # same types
+        ([("pyarrow", pd.NA), ("pyarrow", pd.NA)], ("pyarrow", pd.NA)),
+        ([("pyarrow", np.nan), ("pyarrow", np.nan)], ("pyarrow", np.nan)),
+        ([("python", pd.NA), ("python", pd.NA)], ("python", pd.NA)),
+        ([("python", np.nan), ("python", np.nan)], ("python", np.nan)),
+        # pyarrow preference
+        ([("pyarrow", pd.NA), ("python", pd.NA)], ("pyarrow", pd.NA)),
+        # NA preference
+        ([("python", pd.NA), ("python", np.nan)], ("python", pd.NA)),
+    ],
+)
+def test_concat_series(request, to_concat_dtypes, result_dtype):
+    if any(storage == "pyarrow" for storage, _ in to_concat_dtypes) and not HAS_PYARROW:
+        pytest.skip("Could not import 'pyarrow'")
+
+    ser_list = [
+        pd.Series(["a", "b", None], dtype=pd.StringDtype(storage, na_value))
+        for storage, na_value in to_concat_dtypes
+    ]
+
+    result = pd.concat(ser_list, ignore_index=True)
+    expected = pd.Series(
+        ["a", "b", None, "a", "b", None], dtype=pd.StringDtype(*result_dtype)
+    )
+    tm.assert_series_equal(result, expected)
+
+    # order doesn't matter for result
+    result = pd.concat(ser_list[::1], ignore_index=True)
+    tm.assert_series_equal(result, expected)
+
+
+def test_concat_with_object(string_dtype_arguments):
+    # _get_common_dtype cannot inspect values, so object dtype with strings still
+    # results in object dtype
+    result = pd.concat(
+        [
+            pd.Series(["a", "b", None], dtype=pd.StringDtype(*string_dtype_arguments)),
+            pd.Series(["a", "b", None], dtype=object),
+        ]
+    )
+    assert result.dtype == np.dtype("object")
+
+
+def test_concat_with_numpy(string_dtype_arguments):
+    # common type with a numpy string dtype always preserves the pandas string dtype
+    dtype = pd.StringDtype(*string_dtype_arguments)
+    assert find_common_type([dtype, np.dtype("U")]) == dtype
+    assert find_common_type([np.dtype("U"), dtype]) == dtype
+    assert find_common_type([dtype, np.dtype("U10")]) == dtype
+    assert find_common_type([np.dtype("U10"), dtype]) == dtype
+
+    # with any other numpy dtype -> object
+    assert find_common_type([dtype, np.dtype("S")]) == np.dtype("object")
+    assert find_common_type([dtype, np.dtype("int64")]) == np.dtype("object")
+
+    if Version(np.__version__) >= Version("2"):
+        assert find_common_type([dtype, np.dtypes.StringDType()]) == dtype
+        assert find_common_type([np.dtypes.StringDType(), dtype]) == dtype