dataclass: refactor evaluating string fields

arraycontext/container/dataclass.py

@@ -32,14 +32,22 @@
 """
 
 from collections.abc import Mapping, Sequence
-from dataclasses import Field, fields, is_dataclass
-from typing import Union, get_args, get_origin
+from dataclasses import fields, is_dataclass
+from typing import NamedTuple, Union, get_args, get_origin
 
 from arraycontext.container import is_array_container_type
 
 
 # {{{ dataclass containers
 
+class _Field(NamedTuple):
+    """Small lookalike for :class:`dataclasses.Field`."""
+
+    init: bool
+    name: str
+    type: type
+
+
 def is_array_type(tp: type) -> bool:
     from arraycontext import Array
     return tp is Array or is_array_container_type(tp)
@@ -73,7 +81,9 @@ def dataclass_array_container(cls: type) -> type:
 
     assert is_dataclass(cls)
 
-    def is_array_field(f: Field, field_type: type) -> bool:
+    def is_array_field(f: _Field) -> bool:
+        field_type = f.type
+
         # NOTE: unions of array containers are treated separately to handle
         # unions of only array containers, e.g. `Union[np.ndarray, Array]`, as
         # they can work seamlessly with arithmetic and traversal.
@@ -96,10 +106,8 @@ def is_array_field(f: Field, field_type: type) -> bool:
                         f"Field '{f.name}' union contains non-array container "
                         "arguments. All arguments must be array containers.")
 
-        if isinstance(field_type, str):
-            raise TypeError(
-                f"String annotation on field '{f.name}' not supported. "
-                "(this may be due to 'from __future__ import annotations')")
+        # NOTE: this should never happen due to using `inspect.get_annotations`
+        assert not isinstance(field_type, str)
 
         if __debug__:
             if not f.init:
@@ -127,36 +135,52 @@ def is_array_field(f: Field, field_type: type) -> bool:
 
         return is_array_type(field_type)
 
+    from pytools import partition
+
+    array_fields = _get_annotated_fields(cls)
+    array_fields, non_array_fields = partition(is_array_field, array_fields)
+
+    if not array_fields:
+        raise ValueError(f"'{cls}' must have fields with array container type "
+                "in order to use the 'dataclass_array_container' decorator")
+
+    return _inject_dataclass_serialization(cls, array_fields, non_array_fields)
+
+
+def _get_annotated_fields(cls: type) -> Sequence[_Field]:
+    """Get a list of fields in the class *cls* with evaluated types.
+
+    If any of the fields in *cls* have type annotations that are strings, e.g.
+    from using ``from __future__ import annotations``, this function evaluates
+    them using :func:`inspect.get_annotations`. Note that this requires the class
+    to live in a module that is importable.
+
+    :return: a list of fields.
+    """
+
     from inspect import get_annotations
 
-    array_fields: list[Field] = []
-    non_array_fields: list[Field] = []
+    result = []
     cls_ann: Mapping[str, type] | None = None
     for field in fields(cls):
         field_type_or_str = field.type
         if isinstance(field_type_or_str, str):
             if cls_ann is None:
                 cls_ann = get_annotations(cls, eval_str=True)
+
             field_type = cls_ann[field.name]
         else:
             field_type = field_type_or_str
 
-        if is_array_field(field, field_type):
-            array_fields.append(field)
-        else:
-            non_array_fields.append(field)
-
-    if not array_fields:
-        raise ValueError(f"'{cls}' must have fields with array container type "
-                "in order to use the 'dataclass_array_container' decorator")
+        result.append(_Field(init=field.init, name=field.name, type=field_type))
 
-    return _inject_dataclass_serialization(cls, array_fields, non_array_fields)
+    return result
 
 
 def _inject_dataclass_serialization(
         cls: type,
-        array_fields: Sequence[Field],
-        non_array_fields: Sequence[Field]) -> type:
+        array_fields: Sequence[_Field],
+        non_array_fields: Sequence[_Field]) -> type:
     """Implements :func:`~arraycontext.serialize_container` and
     :func:`~arraycontext.deserialize_container` for the given dataclass *cls*.