Fix selecting and deleting fields from zero-chunked ext-array

Author: hombit

src/nested_pandas/series/ext_array.py

@@ -1003,17 +1003,9 @@ def view_fields(self, fields: str | list[str]) -> Self:  # type: ignore[name-def
         if not set(fields).issubset(self.field_names):
             raise ValueError(f"Some fields are not found, given: {fields}, available: {self.field_names}")
 
-        chunks = []
-        for chunk in self.struct_array.iterchunks():
-            chunk = cast(pa.StructArray, chunk)
-            struct_dict = {}
-            for field in fields:
-                struct_dict[field] = chunk.field(field)
-            struct_array = pa.StructArray.from_arrays(struct_dict.values(), struct_dict.keys())
-            chunks.append(struct_array)
-        pa_array = pa.chunked_array(chunks)
-
-        return type(self)(pa_array, validate=False)
+        new = self.copy()
+        new.pa_table = new.pa_table.select(fields)
+        return new
 
     def set_flat_field(self, field: str, value: ArrayLike, *, keep_dtype: bool = False) -> None:
         """Set the field from flat-array of values
@@ -1171,16 +1163,9 @@ def pop_fields(self, fields: Iterable[str]):
         if not fields.issubset(self.field_names):
             raise ValueError(f"Some fields are not found, given: {fields}, available: {self.field_names}")
 
-        if len(self.field_names) - len(fields) == 0:
+        fields_to_keep = [field for field in self.field_names if field not in fields]
+
+        if len(fields_to_keep) == 0:
             raise ValueError("Cannot delete all fields")
 
-        chunks = []
-        for chunk in self.struct_array.iterchunks():
-            chunk = cast(pa.StructArray, chunk)
-            struct_dict = {}
-            for pa_field in chunk.type:
-                if pa_field.name not in fields:
-                    struct_dict[pa_field.name] = chunk.field(pa_field.name)
-            struct_array = pa.StructArray.from_arrays(struct_dict.values(), struct_dict.keys())
-            chunks.append(struct_array)
-        self.struct_array = pa.chunked_array(chunks)
+        self.pa_table = self.pa_table.select(fields_to_keep)

tests/nested_pandas/series/test_ext_array.py

@@ -1813,6 +1813,16 @@ def test_pop_fields_raises_for_some_invalid_fields():
         ext_array.pop_fields(["a", "c"])
 
 
+def test_pop_fields_zero_chunks():
+    """Tests that we can pop fields even when the underlying array has no chunks."""
+    ext_array = NestedExtensionArray(
+        pa.chunked_array([], type=pa.struct({"a": pa.list_(pa.int64()), "b": pa.list_(pa.int64())}))
+    )
+    assert ext_array.num_chunks == 0, "Test setup is invalid"
+    ext_array.pop_fields(["a"])
+    assert ext_array._pyarrow_dtype == pa.struct({"b": pa.list_(pa.int64())})
+
+
 def test_delete_last_field_raises():
     """Tests that we raise an error when trying to delete the last field left."""
     struct_array = pa.StructArray.from_arrays(