Allow to manually exclude fields in dataclass_array_container

arraycontext/__init__.py

@@ -44,7 +44,7 @@
         serialize_container, deserialize_container,
         register_multivector_as_array_container)
 from .container.arithmetic import with_container_arithmetic
-from .container.dataclass import dataclass_array_container
+from .container.dataclass import dataclass_array_container, ExcludedField
 
 from .container.traversal import (
         map_array_container,
@@ -85,7 +85,7 @@
         "serialize_container", "deserialize_container",
         "register_multivector_as_array_container",
         "with_container_arithmetic",
-        "dataclass_array_container",
+        "dataclass_array_container", "ExcludedField",
 
         "map_array_container", "multimap_array_container",
         "rec_map_array_container", "rec_multimap_array_container",

arraycontext/container/dataclass.py

@@ -2,6 +2,8 @@
 
 """
 .. currentmodule:: arraycontext
+
+.. autoclass:: ExcludedField
 .. autofunction:: dataclass_array_container
 """
 
@@ -31,11 +33,34 @@
 """
 
 from dataclasses import fields
+
+# NOTE: these are internal attributes and mypy says they do not exist
+from typing import _GenericAlias        # type: ignore[attr-defined]
+try:
+    from typing import _AnnotatedAlias, get_args    # type: ignore[attr-defined]
+except ImportError:
+    from typing_extensions import (                 # type: ignore[attr-defined]
+            _AnnotatedAlias, get_args)
+
 from arraycontext.container import is_array_container_type
 
 
 # {{{ dataclass containers
 
+class ExcludedField:
+    """Can be used to annotate dataclass fields to be excluded from the container.
+
+    This can be done using :class:`typing.Annotated` as follows
+
+    .. code:: python
+
+        @dataclass
+        class MyClass:
+            x: np.ndarray
+            y: Annotated[np.ndarray, ExcludedField]
+    """
+
+
 def dataclass_array_container(cls: type) -> type:
     """A class decorator that makes the class to which it is applied an
     :class:`ArrayContainer` by registering appropriate implementations of
@@ -45,11 +70,21 @@ def dataclass_array_container(cls: type) -> type:
     Attributes that are not array containers are allowed. In order to decide
     whether an attribute is an array container, the declared attribute type
     is checked by the criteria from :func:`is_array_container_type`.
+
+    To explicitly exclude fields from the container serialization (that would
+    otherwise be recognized as array containers), use :class:`typing.Annotated`
+    and :class:`ExcludedField`.
     """
+
     from dataclasses import is_dataclass, Field
     assert is_dataclass(cls)
 
     def is_array_field(f: Field) -> bool:
+        # FIXME: is there a nicer way to recognize that we hit Annotated?
+        if isinstance(f.type, _AnnotatedAlias):
+            if any(arg is ExcludedField for arg in get_args(f.type)):
+                return False
+
         if __debug__:
             if not f.init:
                 raise ValueError(
@@ -59,8 +94,7 @@ def is_array_field(f: Field) -> bool:
                 raise TypeError(
                         f"string annotation on field '{f.name}' not supported")
 
-            from typing import _SpecialForm
-            if isinstance(f.type, _SpecialForm):
+            if isinstance(f.type, _GenericAlias):
                 raise TypeError(
                         f"typing annotation not supported on field '{f.name}': "
                         f"'{f.type!r}'")

setup.py

@@ -43,6 +43,7 @@ def main():
             "pytest>=2.3",
             "loopy>=2019.1",
             "dataclasses; python_version<'3.7'",
+            "typing_extensions; python_version<'3.9'",
             "types-dataclasses",
         ],
         package_data={"arraycontext": ["py.typed"]},

test/test_utils.py

@@ -93,6 +93,40 @@ class ArrayContainerWithInitFalse:
 # }}}
 
 
+# {{{ test_dataclass_excluded_fields
+
+def test_dataclass_excluded_fields():
+    from dataclasses import dataclass
+    from arraycontext import dataclass_array_container, ExcludedField
+
+    try:
+        from typing import Annotated
+    except ImportError:
+        from typing_extensions import Annotated
+
+    @dataclass_array_container
+    @dataclass(frozen=True)
+    class ExcludedFoo:
+        x: np.ndarray
+        y: np.ndarray
+        excluded: Annotated[np.ndarray, ExcludedField]
+
+    ary = np.array([42], dtype=object)
+    c0 = ExcludedFoo(x=ary, y=ary, excluded=ary)
+
+    from arraycontext import serialize_container
+    iterable = serialize_container(c0)
+    assert len(iterable) == 2
+
+    from arraycontext import deserialize_container
+    c1 = deserialize_container(c0, iterable)
+    assert np.linalg.norm(c0.x - c1.x) < 1.0e-15
+    assert np.linalg.norm(c0.y - c1.y) < 1.0e-15
+
+
+# }}}
+
+
 if __name__ == "__main__":
     import sys
     if len(sys.argv) > 1: