fix: don't require mock and mock_empty together; use behaviors in…

… empty array creation (#408)

src/dask_awkward/layers/layers.py

@@ -112,7 +112,11 @@ def io_func_implements_projection(func: ImplementsIOFunction) -> bool:
 
 
 def io_func_implements_mocking(func: ImplementsIOFunction) -> bool:
-    return hasattr(func, "mock") and hasattr(func, "mock_empty")
+    return hasattr(func, "mock")
+
+
+def io_func_implements_mock_empty(func: ImplementsIOFunction) -> bool:
+    return hasattr(func, "mock_empty")
 
 
 def io_func_implements_columnar(func: ImplementsIOFunction) -> bool:

src/dask_awkward/lib/io/columnar.py

@@ -64,7 +64,7 @@ def mock(self: S) -> AwkwardArray:
 
     def mock_empty(self: S, backend: BackendT = "cpu") -> AwkwardArray:
         return ak.to_backend(
-            self.form.length_zero_array(highlevel=False),
+            self.form.length_zero_array(highlevel=False, behavior=self.behavior),
             backend,
             highlevel=True,
         )

src/dask_awkward/lib/io/io.py

@@ -31,6 +31,7 @@
     BackendT,
     ImplementsMocking,
     IOFunctionWithMocking,
+    io_func_implements_mock_empty,
     io_func_implements_mocking,
 )
 from dask_awkward.lib.core import (
@@ -622,6 +623,8 @@ def from_map(
         raise ValueError("empty_on_raise and empty_backend must be used together.")
 
     if empty_on_raise and empty_backend:
+        if not io_func_implements_mock_empty(io_func):
+            raise ValueError("io_func must implement mock_empty method.")
         io_func = return_empty_on_raise(
             io_func,
             allowed_exceptions=empty_on_raise,

tests/test_io.py

@@ -398,3 +398,24 @@ def test_random_fail_from_lists():
             label="from-lists",
             empty_backend="cpu",
         )
+
+    class NoMockEmpty:
+        def __init__(self, x):
+            self.x = x
+
+        def mock(self):
+            return 5
+
+        def __call__(self, *args):
+            return self.x * args[0]
+
+    with pytest.raises(ValueError, match="must implement"):
+        array = from_map(
+            NoMockEmpty(5),
+            many,
+            meta=typetracer_array(ak.Array(many[0])),
+            divisions=divs,
+            label="from-lists",
+            empty_on_raise=(RuntimeError,),
+            empty_backend="cpu",
+        )