Backport PR #51976 on branch 2.0.x (BUG: read_csv for arrow with mismatching dtypes does not work) (#51995)

BUG: read_csv for arrow with mismatching dtypes does not work (#51976)

* BUG: read_csv for arrow with mismatching dtypes does not work

* Rename var

Author: phofl

pandas/io/parsers/c_parser_wrapper.py

@@ -29,8 +29,10 @@
     is_categorical_dtype,
     pandas_dtype,
 )
-from pandas.core.dtypes.concat import union_categoricals
-from pandas.core.dtypes.dtypes import ExtensionDtype
+from pandas.core.dtypes.concat import (
+    concat_compat,
+    union_categoricals,
+)
 
 from pandas.core.indexes.api import ensure_index_from_sequences
 
@@ -378,43 +380,15 @@ def _concatenate_chunks(chunks: list[dict[int, ArrayLike]]) -> dict:
         arrs = [chunk.pop(name) for chunk in chunks]
         # Check each arr for consistent types.
         dtypes = {a.dtype for a in arrs}
-        # TODO: shouldn't we exclude all EA dtypes here?
-        numpy_dtypes = {x for x in dtypes if not is_categorical_dtype(x)}
-        if len(numpy_dtypes) > 1:
-            # error: Argument 1 to "find_common_type" has incompatible type
-            # "Set[Any]"; expected "Sequence[Union[dtype[Any], None, type,
-            # _SupportsDType, str, Union[Tuple[Any, int], Tuple[Any,
-            # Union[int, Sequence[int]]], List[Any], _DTypeDict, Tuple[Any, Any]]]]"
-            common_type = np.find_common_type(
-                numpy_dtypes,  # type: ignore[arg-type]
-                [],
-            )
-            if common_type == np.dtype(object):
-                warning_columns.append(str(name))
+        non_cat_dtypes = {x for x in dtypes if not is_categorical_dtype(x)}
 
         dtype = dtypes.pop()
         if is_categorical_dtype(dtype):
             result[name] = union_categoricals(arrs, sort_categories=False)
         else:
-            if isinstance(dtype, ExtensionDtype):
-                # TODO: concat_compat?
-                array_type = dtype.construct_array_type()
-                # error: Argument 1 to "_concat_same_type" of "ExtensionArray"
-                # has incompatible type "List[Union[ExtensionArray, ndarray]]";
-                # expected "Sequence[ExtensionArray]"
-                result[name] = array_type._concat_same_type(
-                    arrs  # type: ignore[arg-type]
-                )
-            else:
-                # error: Argument 1 to "concatenate" has incompatible
-                # type "List[Union[ExtensionArray, ndarray[Any, Any]]]"
-                # ; expected "Union[_SupportsArray[dtype[Any]],
-                # Sequence[_SupportsArray[dtype[Any]]],
-                # Sequence[Sequence[_SupportsArray[dtype[Any]]]],
-                # Sequence[Sequence[Sequence[_SupportsArray[dtype[Any]]]]]
-                # , Sequence[Sequence[Sequence[Sequence[
-                # _SupportsArray[dtype[Any]]]]]]]"
-                result[name] = np.concatenate(arrs)  # type: ignore[arg-type]
+            result[name] = concat_compat(arrs)
+            if len(non_cat_dtypes) > 1 and result[name].dtype == np.dtype(object):
+                warning_columns.append(str(name))
 
     if warning_columns:
         warning_names = ",".join(warning_columns)

pandas/tests/io/parser/test_concatenate_chunks.py

@@ -0,0 +1,36 @@
+import numpy as np
+import pytest
+
+from pandas.errors import DtypeWarning
+
+import pandas._testing as tm
+from pandas.core.arrays import ArrowExtensionArray
+
+from pandas.io.parsers.c_parser_wrapper import _concatenate_chunks
+
+
+def test_concatenate_chunks_pyarrow():
+    # GH#51876
+    pa = pytest.importorskip("pyarrow")
+    chunks = [
+        {0: ArrowExtensionArray(pa.array([1.5, 2.5]))},
+        {0: ArrowExtensionArray(pa.array([1, 2]))},
+    ]
+    result = _concatenate_chunks(chunks)
+    expected = ArrowExtensionArray(pa.array([1.5, 2.5, 1.0, 2.0]))
+    tm.assert_extension_array_equal(result[0], expected)
+
+
+def test_concatenate_chunks_pyarrow_strings():
+    # GH#51876
+    pa = pytest.importorskip("pyarrow")
+    chunks = [
+        {0: ArrowExtensionArray(pa.array([1.5, 2.5]))},
+        {0: ArrowExtensionArray(pa.array(["a", "b"]))},
+    ]
+    with tm.assert_produces_warning(DtypeWarning, match="have mixed types"):
+        result = _concatenate_chunks(chunks)
+    expected = np.concatenate(
+        [np.array([1.5, 2.5], dtype=object), np.array(["a", "b"])]
+    )
+    tm.assert_numpy_array_equal(result[0], expected)