fix(unpack+pack): parsing and building with dynamic offsets

src/caterpillar/context.py

@@ -21,7 +21,7 @@
 import warnings
 
 from typing import Annotated, Callable, Any, Generic, Protocol, get_origin
-from typing_extensions import Final, Literal, Self, Sized, overload, override, TypeVar
+from typing_extensions import Buffer, Final, Literal, Self, Sized, overload, override, TypeVar
 from types import FrameType, TracebackType
 from dataclasses import dataclass
 
@@ -105,7 +105,7 @@ def __getattribute__(self, key: Literal["_parent"]) -> _ContextLike: ...
     @overload
     def __getattribute__(self, key: Literal["_obj"]) -> _ContextLike: ...
     @overload
-    def __getattribute__(self, key: Literal["_offsets"]) -> dict[int, int]: ...
+    def __getattribute__(self, key: Literal["_offsets"]) -> dict[int, Buffer]: ...
     @overload
     def __getattribute__(self, key: Literal["_io"]) -> _StreamType: ...
     @overload
@@ -181,7 +181,7 @@ def __getitem__(self, key: Literal["_root"], /) -> _ContextLike: ...
     @overload
     def __getitem__(self, key: Literal["_io"], /) -> _StreamType: ...
     @overload
-    def __getitem__(self, key: Literal["_offsets"], /) -> dict[int, int]: ...
+    def __getitem__(self, key: Literal["_offsets"], /) -> dict[int, Buffer]: ...
     @overload
     def __getitem__(self, key: Literal["_index"], /) -> int: ...
     @overload
@@ -621,7 +621,7 @@ def __getattribute__(self, key: Literal["_obj"]) -> ContextPath[_ContextLike]: .
     @overload
     def __getattribute__(
         self, key: Literal["_offsets"]
-    ) -> ContextPath[dict[int, int]]: ...
+    ) -> ContextPath[dict[int, Buffer]]: ...
     @overload
     def __getattribute__(self, key: Literal["_io"]) -> ContextPath[_StreamType]: ...
     @overload

src/caterpillar/fields/_base.py

@@ -214,9 +214,13 @@ def offset(self) -> _ContextLambda[int] | int:
     @offset.setter
     def offset(self, value: _ContextLambda[int] | int):
         self.__offset = value
-        self._has_offset = value not in (-1, None)
+        # _ContextLambda is ExprMixin so `__eq__` is overridden
+        # and `Tuple.__contains__` isn't correct. If value is an
+        # instance of _ContextLambda we may assume it is not
+        # -1 or None
+        self._has_offset = value not in (-1, None) or isinstance(value, _ContextLambda)
         self._offset_is_lambda = callable(value)
-        self._keep_pos = value in (-1, None)
+        self._keep_pos = value in (-1, None) and not isinstance(value, _ContextLambda)
 
     @property
     def amount(self) -> _LengthT | None:

src/caterpillar/model/__init__.py

@@ -18,12 +18,6 @@
     struct,
     UnionHook,
     union,
-    unpack,
-    unpack_file,
-    pack,
-    pack_into,
-    pack_file,
-    sizeof,
     Invisible,
     StructDefMixin,
     struct_factory,
@@ -46,6 +40,14 @@
     BitfieldDefMixin,
 )
 from ._template import istemplate, template, TemplateTypeVar, derive
+from .provider import (
+    unpack,
+    unpack_file,
+    pack,
+    pack_into,
+    pack_file,
+    sizeof,
+)
 
 __all__ = [
     "Sequence",