bug[next]: respect DEFAULT_BACKEND and no_backend mechanism (#1380)

fixes #1376.

Thanks @DropD for the testcase.

src/gt4py/next/embedded/operators.py

@@ -17,7 +17,7 @@
 
 from gt4py import eve
 from gt4py._core import definitions as core_defs
-from gt4py.next import common, constructors, utils
+from gt4py.next import common, constructors, errors, utils
 from gt4py.next.embedded import common as embedded_common, context as embedded_context
 
 
@@ -77,17 +77,20 @@ def scan_loop(hpos):
 def field_operator_call(op: EmbeddedOperator, args: Any, kwargs: Any):
     if "out" in kwargs:
         # called from program or direct field_operator as program
-        offset_provider = kwargs.pop("offset_provider", None)
-
         new_context_kwargs = {}
         if embedded_context.within_context():
             # called from program
-            assert offset_provider is None
+            assert "offset_provider" not in kwargs
         else:
             # field_operator as program
+            if "offset_provider" not in kwargs:
+                raise errors.MissingArgumentError(None, "offset_provider", True)
+            offset_provider = kwargs.pop("offset_provider", None)
+
             new_context_kwargs["offset_provider"] = offset_provider
 
         out = kwargs.pop("out")
+
         domain = kwargs.pop("domain", None)
 
         flattened_out: tuple[common.Field, ...] = utils.flatten_nested_tuple((out,))
@@ -105,7 +108,10 @@ def field_operator_call(op: EmbeddedOperator, args: Any, kwargs: Any):
                 domain=out_domain,
             )
     else:
-        # called from other field_operator
+        # called from other field_operator or missing `out` argument
+        if "offset_provider" in kwargs:
+            # assuming we wanted to call the field_operator as program, otherwise `offset_provider` would not be there
+            raise errors.MissingArgumentError(None, "out", True)
         return op(*args, **kwargs)
 
 

src/gt4py/next/errors/__init__.py

@@ -21,6 +21,7 @@
 from .exceptions import (
     DSLError,
     InvalidParameterAnnotationError,
+    MissingArgumentError,
     MissingAttributeError,
     MissingParameterAnnotationError,
     UndefinedSymbolError,
@@ -33,6 +34,7 @@
     "InvalidParameterAnnotationError",
     "MissingAttributeError",
     "MissingParameterAnnotationError",
+    "MissingArgumentError",
     "UndefinedSymbolError",
     "UnsupportedPythonFeatureError",
     "set_verbose_exceptions",

src/gt4py/next/errors/exceptions.py

@@ -81,6 +81,18 @@ def __init__(self, location: Optional[SourceLocation], attr_name: str) -> None:
         self.attr_name = attr_name
 
 
+class MissingArgumentError(DSLError):
+    arg_name: str
+    is_kwarg: bool
+
+    def __init__(self, location: Optional[SourceLocation], arg_name: str, is_kwarg: bool) -> None:
+        super().__init__(
+            location, f"Expected {'keyword-' if is_kwarg else ''}argument '{arg_name}'."
+        )
+        self.attr_name = arg_name
+        self.is_kwarg = is_kwarg
+
+
 class TypeError_(DSLError):
     def __init__(self, location: Optional[SourceLocation], message: str) -> None:
         super().__init__(location, message)

src/gt4py/next/ffront/decorator.py

@@ -29,10 +29,11 @@
 
 from devtools import debug
 
+from gt4py import eve
 from gt4py._core import definitions as core_defs
 from gt4py.eve import utils as eve_utils
 from gt4py.eve.extended_typing import Any, Optional
-from gt4py.next import allocators as next_allocators, embedded as next_embedded
+from gt4py.next import allocators as next_allocators, embedded as next_embedded, errors
 from gt4py.next.common import Dimension, DimensionKind, GridType
 from gt4py.next.embedded import operators as embedded_operators
 from gt4py.next.ffront import (
@@ -61,11 +62,10 @@
     sym,
 )
 from gt4py.next.program_processors import processor_interface as ppi
-from gt4py.next.program_processors.runners import roundtrip
 from gt4py.next.type_system import type_info, type_specifications as ts, type_translation
 
 
-DEFAULT_BACKEND: Callable = roundtrip.executor
+DEFAULT_BACKEND: Callable = None
 
 
 def _get_closure_vars_recursively(closure_vars: dict[str, Any]) -> dict[str, Any]:
@@ -176,15 +176,15 @@ class Program:
 
     past_node: past.Program
     closure_vars: dict[str, Any]
-    definition: Optional[types.FunctionType] = None
-    backend: Optional[ppi.ProgramExecutor] = DEFAULT_BACKEND
-    grid_type: Optional[GridType] = None
+    definition: Optional[types.FunctionType]
+    backend: Optional[ppi.ProgramExecutor]
+    grid_type: Optional[GridType]
 
     @classmethod
     def from_function(
         cls,
         definition: types.FunctionType,
-        backend: Optional[ppi.ProgramExecutor] = DEFAULT_BACKEND,
+        backend: Optional[ppi.ProgramExecutor],
         grid_type: Optional[GridType] = None,
     ) -> Program:
         source_def = SourceDefinition.from_function(definition)
@@ -495,7 +495,7 @@ def program(*, backend: Optional[ppi.ProgramExecutor]) -> Callable[[types.Functi
 def program(
     definition=None,
     *,
-    backend=None,
+    backend=eve.NOTHING,  # `NOTHING` -> default backend, `None` -> no backend (embedded execution)
     grid_type=None,
 ) -> Program | Callable[[types.FunctionType], Program]:
     """
@@ -517,7 +517,9 @@ def program(
     """
 
     def program_inner(definition: types.FunctionType) -> Program:
-        return Program.from_function(definition, backend, grid_type)
+        return Program.from_function(
+            definition, DEFAULT_BACKEND if backend is eve.NOTHING else backend, grid_type
+        )
 
     return program_inner if definition is None else program_inner(definition)
 
@@ -549,17 +551,17 @@ class FieldOperator(GTCallable, Generic[OperatorNodeT]):
 
     foast_node: OperatorNodeT
     closure_vars: dict[str, Any]
-    definition: Optional[types.FunctionType] = None
-    backend: Optional[ppi.ProgramExecutor] = DEFAULT_BACKEND
-    grid_type: Optional[GridType] = None
+    definition: Optional[types.FunctionType]
+    backend: Optional[ppi.ProgramExecutor]
+    grid_type: Optional[GridType]
     operator_attributes: Optional[dict[str, Any]] = None
     _program_cache: dict = dataclasses.field(default_factory=dict)
 
     @classmethod
     def from_function(
         cls,
         definition: types.FunctionType,
-        backend: Optional[ppi.ProgramExecutor] = DEFAULT_BACKEND,
+        backend: Optional[ppi.ProgramExecutor],
         grid_type: Optional[GridType] = None,
         *,
         operator_node_cls: type[OperatorNodeT] = foast.FieldOperator,
@@ -686,6 +688,7 @@ def as_program(
         self._program_cache[hash_] = Program(
             past_node=past_node,
             closure_vars=closure_vars,
+            definition=None,
             backend=self.backend,
             grid_type=self.grid_type,
         )
@@ -698,7 +701,12 @@ def __call__(
     ) -> None:
         if not next_embedded.context.within_context() and self.backend is not None:
             # non embedded execution
-            offset_provider = kwargs.pop("offset_provider", None)
+            if "offset_provider" not in kwargs:
+                raise errors.MissingArgumentError(None, "offset_provider", True)
+            offset_provider = kwargs.pop("offset_provider")
+
+            if "out" not in kwargs:
+                raise errors.MissingArgumentError(None, "out", True)
             out = kwargs.pop("out")
             args, kwargs = type_info.canonicalize_arguments(self.foast_node.type, args, kwargs)
             # TODO(tehrengruber): check all offset providers are given
@@ -744,7 +752,7 @@ def field_operator(
     ...
 
 
-def field_operator(definition=None, *, backend=None, grid_type=None):
+def field_operator(definition=None, *, backend=eve.NOTHING, grid_type=None):
     """
     Generate an implementation of the field operator from a Python function object.
 
@@ -762,7 +770,9 @@ def field_operator(definition=None, *, backend=None, grid_type=None):
     """
 
     def field_operator_inner(definition: types.FunctionType) -> FieldOperator[foast.FieldOperator]:
-        return FieldOperator.from_function(definition, backend, grid_type)
+        return FieldOperator.from_function(
+            definition, DEFAULT_BACKEND if backend is eve.NOTHING else backend, grid_type
+        )
 
     return field_operator_inner if definition is None else field_operator_inner(definition)
 
@@ -798,7 +808,7 @@ def scan_operator(
     axis: Dimension,
     forward: bool = True,
     init: core_defs.Scalar = 0.0,
-    backend=None,
+    backend=eve.NOTHING,
     grid_type: GridType = None,
 ) -> (
     FieldOperator[foast.ScanOperator]
@@ -836,8 +846,7 @@ def scan_operator(
     def scan_operator_inner(definition: types.FunctionType) -> FieldOperator:
         return FieldOperator.from_function(
             definition,
-            backend,
-            grid_type,
+            DEFAULT_BACKEND if backend is eve.NOTHING else backend,
             operator_node_cls=foast.ScanOperator,
             operator_attributes={"axis": axis, "forward": forward, "init": init},
         )

tests/next_tests/integration_tests/feature_tests/ffront_tests/ffront_test_utils.py

@@ -22,6 +22,8 @@
 import gt4py.next as gtx
 from gt4py.next.ffront import decorator
 from gt4py.next.iterator import ir as itir
+from gt4py.next.program_processors import processor_interface as ppi
+from gt4py.next.program_processors.runners import gtfn, roundtrip
 
 
 try:
@@ -36,9 +38,10 @@
 import next_tests.exclusion_matrices as definitions
 
 
+@ppi.program_executor
 def no_backend(program: itir.FencilDefinition, *args: Any, **kwargs: Any) -> None:
     """Temporary default backend to not accidentally test the wrong backend."""
-    raise ValueError("No backend selected. Backend selection is mandatory in tests.")
+    raise ValueError("No backend selected! Backend selection is mandatory in tests.")
 
 
 OPTIONAL_PROCESSORS = []

tests/next_tests/integration_tests/feature_tests/ffront_tests/test_math_builtin_execution.py

@@ -13,7 +13,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 import math
-from typing import Callable
+from typing import Callable, Optional
 
 import numpy as np
 import pytest
@@ -22,6 +22,7 @@
 from gt4py.next.ffront import dialect_ast_enums, fbuiltins, field_operator_ast as foast
 from gt4py.next.ffront.decorator import FieldOperator
 from gt4py.next.ffront.foast_passes.type_deduction import FieldOperatorTypeDeduction
+from gt4py.next.program_processors import processor_interface as ppi
 from gt4py.next.type_system import type_translation
 
 from next_tests.integration_tests import cases
@@ -39,7 +40,7 @@
 #  becomes easier.
 
 
-def make_builtin_field_operator(builtin_name: str):
+def make_builtin_field_operator(builtin_name: str, backend: Optional[ppi.ProgramExecutor]):
     # TODO(tehrengruber): creating a field operator programmatically should be
     #  easier than what we need to do here.
     # construct annotation dictionary containing the input argument and return
@@ -109,8 +110,9 @@ def make_builtin_field_operator(builtin_name: str):
     return FieldOperator(
         foast_node=typed_foast_node,
         closure_vars=closure_vars,
-        backend=None,
         definition=None,
+        backend=backend,
+        grid_type=None,
     )
 
 
@@ -129,9 +131,7 @@ def test_math_function_builtins_execution(cartesian_case, builtin_name: str, inp
     expected = ref_impl(*inputs)
     out = cartesian_case.as_field([IDim], np.zeros_like(expected))
 
-    builtin_field_op = make_builtin_field_operator(builtin_name).with_backend(
-        cartesian_case.backend
-    )
+    builtin_field_op = make_builtin_field_operator(builtin_name, cartesian_case.backend)
 
     builtin_field_op(*inps, out=out, offset_provider={})