Fix unpacking with dynamic offsets

`_ContextLambda` is `ExprMixin` which overwrites `__eq__`. The
`__contains__` function for tuples delegates to `__eq__` and this
interaction means that `_has_offset` and `_keep_pos` will be
unconditionally false and true when using a dynamic offset.

Author: tsheinen

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:

test/_Py/fields/test_py_offset.py

@@ -0,0 +1,39 @@
+import pytest
+
+from caterpillar.py import (
+    struct,
+    pack,
+    unpack,
+    uint8,
+    this,
+)
+
+def test_fixed_offset_unpack():
+    atom = uint8 @ 4
+    data = b"\x00\x00\x00\x00\x42"
+    assert unpack(atom, data, as_field=True) == 0x42
+
+def test_dynamic_offset_unpack():
+    @struct
+    class Format:
+        offset: uint8
+        data: uint8 @ this.offset
+
+    binary = b"\x04\x00\x00\x00\xAB"
+    assert unpack(Format, binary) == Format(offset=4, data=0xAB)
+
+# offset packs do not seem to be working correctly
+# These tests show the current behavior
+
+def test_fixed_offset_pack():
+    atom = uint8 @ 4
+    assert pack(0x42, atom, as_field=True) == b"\x42"
+
+def test_dynamic_offset_pack():
+    @struct
+    class Format:
+        offset: uint8
+        data: uint8 @ this.offset
+
+    obj = Format(offset=4, data=0xAB)
+    assert pack(obj) == b"\x04\xab"