Backport PR #1780: (fix): use dask array for missing element in dask …

…concatenation

src/anndata/_core/merge.py

@@ -939,19 +939,37 @@ def gen_outer_reindexers(els, shapes, new_index: pd.Index, *, axis=0):
     return reindexers
 
 
+def missing_element(
+    n: int,
+    els: list[SpArray | sparse.csr_matrix | sparse.csc_matrix | np.ndarray | DaskArray],
+    axis: Literal[0, 1] = 0,
+    fill_value: Any | None = None,
+) -> np.ndarray | DaskArray:
+    """Generates value to use when there is a missing element."""
+    should_return_dask = any(isinstance(el, DaskArray) for el in els)
+    try:
+        non_missing_elem = next(el for el in els if not_missing(el))
+    except StopIteration:  # pragma: no cover
+        msg = "All elements are missing when attempting to generate missing elements."
+        raise ValueError(msg)
+    # 0 sized array for in-memory prevents allocating unnecessary memory while preserving broadcasting.
+    off_axis_size = 0 if not should_return_dask else non_missing_elem.shape[axis - 1]
+    shape = (n, off_axis_size) if axis == 0 else (off_axis_size, n)
+    if should_return_dask:
+        import dask.array as da
+
+        return da.full(
+            shape, default_fill_value(els) if fill_value is None else fill_value
+        )
+    return np.zeros(shape, dtype=bool)
+
+
 def outer_concat_aligned_mapping(
     mappings, *, reindexers=None, index=None, axis=0, fill_value=None
 ):
     result = {}
     ns = [m.parent.shape[axis] for m in mappings]
 
-    def missing_element(n: int, axis: Literal[0, 1] = 0) -> np.ndarray:
-        """Generates value to use when there is a missing element."""
-        if axis == 0:
-            return np.zeros((n, 0), dtype=bool)
-        else:
-            return np.zeros((0, n), dtype=bool)
-
     for k in union_keys(mappings):
         els = [m.get(k, MissingVal) for m in mappings]
         if reindexers is None:
@@ -963,7 +981,9 @@ def missing_element(n: int, axis: Literal[0, 1] = 0) -> np.ndarray:
         # We should probably just handle missing elements for all types
         result[k] = concat_arrays(
             [
-                el if not_missing(el) else missing_element(n, axis=axis)
+                el
+                if not_missing(el)
+                else missing_element(n, axis=axis, els=els, fill_value=fill_value)
                 for el, n in zip(els, ns)
             ],
             cur_reindexers,

tests/test_concatenate.py

@@ -1533,6 +1533,34 @@ def test_concat_different_types_dask(merge_strategy, array_type):
     assert_equal(result2, target2)
 
 
+def test_concat_missing_elem_dask_join(join_type):
+    import dask.array as da
+
+    import anndata as ad
+
+    ad1 = ad.AnnData(X=np.ones((5, 5)))
+    ad2 = ad.AnnData(X=np.zeros((5, 5)), layers={"a": da.ones((5, 5))})
+    ad_in_memory_with_layers = ad2.to_memory()
+
+    result1 = ad.concat([ad1, ad2], join=join_type)
+    result2 = ad.concat([ad1, ad_in_memory_with_layers], join=join_type)
+    assert_equal(result1, result2)
+
+
+def test_impute_dask(axis_name):
+    import dask.array as da
+
+    from anndata._core.merge import _resolve_axis, missing_element
+
+    axis, _ = _resolve_axis(axis_name)
+    els = [da.ones((5, 5))]
+    missing = missing_element(6, els, axis=axis)
+    assert isinstance(missing, DaskArray)
+    in_memory = missing.compute()
+    assert np.all(np.isnan(in_memory))
+    assert in_memory.shape[axis] == 6
+
+
 def test_outer_concat_with_missing_value_for_df():
     # https://github.com/scverse/anndata/issues/901
     # TODO: Extend this test to cover all cases of missing values