Pack() method incorrectly accesses fixed array of structs as single object in Python object API

[scweiser]

When a struct contains a fixed-size array of structs, the generated Pack() method in the Python object API treats self.items as a single struct instance instead of a list, causing runtime errors.

This is inconsistent with _UnPack(), the .pyi stub, and CreateOuter(), all of which correctly treat the field as a list.

flatc version: 25.12.19

Schema to reproduce:

struct Inner {
  x: float;
  y: float;
}

struct Outer {
  items: [Inner: 3];
}

table Root {
  data: Outer (native_inline);
}

root_type Root;

The following command
flatc --python --gen-object-api --python-typing --gen-onefile --filename-suffix "" bug.fbs

results in an incorrectly generated Pack:

pythondef Pack(self, builder):
    return CreateOuter(builder, self.items.x, self.items.y)  # self.items is a list

and a correct _UnPack:

pythondef _UnPack(self, outer):
    self.items = []
    for i in range(outer.ItemsLength()):
        inner_ = InnerT.InitFromObj(outer.Items(i))
        self.items.append(inner_)
Generated CreateOuter() — expects lists
pythondef CreateOuter(builder, items_x, items_y):
    builder.Prep(4, 24)
    for _idx0 in range(3, 0, -1):
        builder.Prep(4, 8)
        builder.PrependFloat32(items_y[_idx0-1])  # indexes into list
        builder.PrependFloat32(items_x[_idx0-1])  # indexes into list
    return builder.Offset()

The generated stubs expect list (as expected):

pythonclass OuterT(object):
    items: typing.List[InnerT]  # list, not a single InnerT

I would expect Pack() to iterate over self.items and extract scalar fields into lists, consistent with how CreateOuter() consumes them:

def Pack(self, builder):
    return CreateOuter(
        builder,
        [_item.x for _item in self.items],
        [_item.y for _item in self.items],
    )

Notes

The bug also affects nested cases where the element struct itself contains fixed arrays, requiring 2D list comprehensions
As a workaround we are patching the generated Pack() method as a post-processing step in our build pipeline, which fixes the issue locally, similar to below:

def Pack(self, builder):
    return CreateOuter(
        builder,
        [_item.x for _item in self.items],
        [_item.y for _item in self.items],
    )