Handle zero-extent arrays during apply_over_* functions.

Now, these functions just don't do any iterations at all. We also
document that all block/chunk shapes are expected to be positive, so as
to avoid divide-by-zero errors in downstream functions.

src/delayedarray/apply_over_blocks.py

@@ -24,8 +24,14 @@ def choose_block_shape_for_iteration(x, memory: int = 10000000) -> Tuple[int, ..
         memory: Available memory in bytes, to hold a single block in memory.
 
     Returns:
-        Dimensions of the blocks.
+        Dimensions of the blocks. All values are guaranteed to be positive,
+        even if the extent of any dimension of ``x`` is zero.
     """
+    # Checking for empty dimensions and bailing out if we find any.
+    for d in x.shape:
+        if d == 0:
+            return (*(max(1, d) for d in x.shape),)
+
     num_elements = memory / x.dtype.itemsize
     chunk_dims = chunk_shape(x)
     block_size = 1
@@ -55,7 +61,6 @@ def choose_block_shape_for_iteration(x, memory: int = 10000000) -> Tuple[int, ..
             block_size = block_size_other
             block_dims[i] = 1
 
-
     return (*block_dims,)
 
 
@@ -74,8 +79,9 @@ def apply_over_blocks(x, fun: Callable, block_shape: Optional[Tuple] = None, all
             block is typically provided as a :py:class:`~numpy.ndarray`.
 
         block_shape:
-            Dimensionsof the block on the iteration dimension. If None, this is
-            chosen by :py:func:`~choose_block_shape_for_iteration`.
+            Dimensions of the block. All entries should be positive, even for
+            zero-extent dimensions of ``x``. If None, this is chosen by
+            :py:func:`~choose_block_shape_for_iteration`.
 
         allow_sparse:
             Whether to allow extraction of sparse subarrays. If true and

src/delayedarray/apply_over_dimension.py

@@ -10,12 +10,14 @@
 __copyright__ = "ltla"
 __license__ = "MIT"
 
+
 def guess_iteration_block_size(x, dimension, memory: int = 10000000) -> int:
     """
     Soft-deprecated alias for :py:func:`~choose_block_size_for_1d_iteration`.
     """
     return choose_block_size_for_1d_iteration(x, dimension, memory)
 
+
 def choose_block_size_for_1d_iteration(x, dimension: int, memory: int = 10000000) -> int:
     """
     Choose a block size for iterating over an array on a certain dimension,
@@ -29,24 +31,31 @@ def choose_block_size_for_1d_iteration(x, dimension: int, memory: int = 10000000
         memory: Available memory in bytes, to hold a single block in memory.
 
     Returns:
-        Size of the block on the iteration dimension.
+        Size of the block on the iteration dimension. This is guaranteed to be
+        positive, even if the extent of the dimension of ``x`` is zero.
     """
-    num_elements = memory / x.dtype.itemsize
     shape = x.shape
+    fulldim = shape[dimension]
 
     prod_other = 1
     for i, s in enumerate(shape):
+        if s == 0:
+            # Bailing out if there's a zero-length dimension anywhere.
+            # We set a floor of 1 to avoid divide-by-zero errors.
+            return max(1, fulldim)
         if i != dimension:
             prod_other *= s
 
+    num_elements = memory / x.dtype.itemsize
     ideal = int(num_elements / prod_other)
     if ideal == 0:
         return 1
+    if ideal >= fulldim:
+        return fulldim
 
     curdim = chunk_shape(x)[dimension]
     if ideal <= curdim:
         return ideal
-
     return int(ideal / curdim) * curdim
 
 
@@ -69,7 +78,8 @@ def apply_over_dimension(x, dimension: int, fun: Callable, block_size: Optional[
             Each block is typically provided as a :py:class:`~numpy.ndarray`.
 
         block_size:
-            Size of the block on the iteration dimension. If None, this is
+            Size of the block on the iteration dimension. This should be a
+            positive integer, even for zero-extent dimensions. If None, this is
             chosen by :py:func:`~choose_block_size_for_1d_iteration`.
 
         allow_sparse:

src/delayedarray/chunk_shape.py

@@ -12,8 +12,13 @@
 
 @singledispatch
 def chunk_shape(x: Any) -> Tuple[int, ...]:
-    """Get the dimensions of the array chunks. These define the preferred
-    blocks with which to iterate over the array in each dimension.
+    """
+    Get the dimensions of the array chunks. These define the preferred
+    intervals with which to iterate over the array in each dimension, usually
+    reflecting a particular layout on disk or in memory. The extent of each
+    chunk dimension should be positive and less than that of the array's;
+    except for zero-length dimensions, in which case the chunk's extent should
+    be greater than the array (typically 1 to avoid divide by zero errors). 
 
     Args:
         x: An array-like object.

tests/test_DelayedArray.py

@@ -150,9 +150,10 @@ def test_SparseNdarray_mean_dense(mask_rate, buffer_size):
     y = delayedarray.wrap(numpy.ma.MaskedArray([1], mask=True)) + 20
     assert y.mean() is numpy.ma.masked
 
-#    # Zero-length array is respected.
-#    y = delayedarray.wrap(numpy.ndarray((10, 0))) * 50
-#    assert numpy.isnan(y.mean())
+    # Zero-length array is respected.
+    with pytest.warns(RuntimeWarning):
+        y = delayedarray.wrap(numpy.ndarray((10, 0))) * 50
+        assert numpy.isnan(y.mean())
 
 
 @pytest.mark.parametrize("mask_rate", [0, 0.5])
@@ -179,9 +180,10 @@ def test_SparseNdarray_mean_sparse(mask_rate, buffer_size):
     y = delayedarray.wrap(ref) / 5
     assert y.mean() is numpy.ma.masked
 
-#    # Zero-length array is respected.
-#    y = delayedarray.wrap(delayedarray.SparseNdarray((0,), None)) * 50
-#    assert numpy.isnan(y.mean())
+    # Zero-length array is respected.
+    with pytest.warns(RuntimeWarning):
+        y = delayedarray.wrap(delayedarray.SparseNdarray((0,), None, dtype=numpy.int32, index_dtype=numpy.int32)) * 50
+        assert numpy.isnan(y.mean())
 
 
 @pytest.mark.parametrize("mask_rate", [0, 0.5])
@@ -210,9 +212,10 @@ def test_SparseNdarray_var_dense(mask_rate, buffer_size):
     with pytest.warns(RuntimeWarning):
         assert y.var() is numpy.ma.masked
 
-#    # Zero-length array is respected.
-#    y = delayedarray.wrap(numpy.ndarray((10, 0))) * 50
-#    assert numpy.isnan(y.var())
+    # Zero-length array is respected.
+    with pytest.warns(RuntimeWarning):
+        y = delayedarray.wrap(numpy.ndarray((10, 0))) * 50
+        assert numpy.isnan(y.var())
 
 
 @pytest.mark.parametrize("mask_rate", [0, 0.5])
@@ -240,6 +243,7 @@ def test_SparseNdarray_var_sparse(mask_rate, buffer_size):
     with pytest.warns(RuntimeWarning):
         assert y.var() is numpy.ma.masked
 
-#    # Zero-length array is respected.
-#    y = delayedarray.wrap(delayedarray.SparseNdarray((0,), None)) * 50
-#    assert numpy.isnan(y.var())
+    # Zero-length array is respected.
+    with pytest.warns(RuntimeWarning):
+        y = delayedarray.wrap(delayedarray.SparseNdarray((0,), None, dtype=numpy.int32, index_dtype=numpy.int32)) * 50
+        assert numpy.isnan(y.var())

tests/test_SparseNdarray.py

@@ -1223,4 +1223,3 @@ def test_SparseNdarray_var(mask_rate):
     y = delayedarray.SparseNdarray((0,), None, dtype=numpy.dtype("float64"), index_dtype=numpy.dtype("int8"))
     with pytest.warns(RuntimeWarning):
         assert numpy.isnan(y.var())
-

tests/test_apply_over_blocks.py

@@ -34,6 +34,10 @@ def test_choose_block_shape_for_iteration():
     assert da.choose_block_shape_for_iteration(x, memory=0) == (1, 1)
     assert da.choose_block_shape_for_iteration(x, memory=40) == (1, 5)
 
+    # Behaves correctly with empty objects.
+    empty = np.random.rand(100, 0)
+    assert da.choose_block_shape_for_iteration(empty) == (100, 1)
+
     x = _ChunkyBoi((100, 200), (20, 25))
     assert da.choose_block_shape_for_iteration(x, memory=4000) == (20, 25)
     assert da.choose_block_shape_for_iteration(x, memory=40000) == (100, 50)
@@ -48,7 +52,7 @@ def _dense_sum(position, block):
 
 
 @pytest.mark.parametrize("mask_rate", [0, 0.2])
-def test_apply_over_dimension_dense(mask_rate):
+def test_apply_over_block_dense(mask_rate):
     x = np.ndarray([100, 200])
     counter = 0
     for i in range(x.shape[0]):
@@ -72,7 +76,7 @@ def test_apply_over_dimension_dense(mask_rate):
 
 
 @pytest.mark.parametrize("mask_rate", [0, 0.2])
-def test_apply_over_dimension_sparse(mask_rate):
+def test_apply_over_block_sparse(mask_rate):
     x = simulate_SparseNdarray((100, 200), mask_rate=mask_rate)
 
     expected = 0
@@ -105,3 +109,9 @@ def _sparse_sum(position, block):
     assert np.allclose(expected, sum(y[1] for y in output))
     assert output[0][0] == [(0, 3), (0, 7)]
     assert output[-1][0] == [(99, 100), (196, 200)]
+
+
+def test_apply_over_block_empty():
+    x = np.ndarray([100, 0])
+    output = da.apply_over_blocks(x, _dense_sum)
+    assert len(output) == 0 

tests/test_apply_over_dimension.py

@@ -34,6 +34,11 @@ def test_choose_block_size_for_1d_iteration():
     assert da.choose_block_size_for_1d_iteration(x, 0, memory=0) == 1
     assert da.choose_block_size_for_1d_iteration(x, 1, memory=0) == 1
 
+    # Behaves correctly with empty objects.
+    empty = np.random.rand(100, 0)
+    assert da.choose_block_size_for_1d_iteration(empty, 0) == 100
+    assert da.choose_block_size_for_1d_iteration(empty, 1) == 1
+
     # Making a slightly more complex situation.
     x = _ChunkyBoi((100, 200), (20, 25))
     assert da.choose_block_size_for_1d_iteration(x, 0, memory=4000) == 2
@@ -125,3 +130,12 @@ def _sparse_sum(position, block):
     assert np.allclose(expected, sum(y[1] for y in output))
     assert output[0][0] == (0, 7)
     assert output[-1][0] == (196, 200)
+
+
+def test_apply_over_dimension_empty():
+    x = np.ndarray([100, 0])
+    output = da.apply_over_dimension(x, 0, _dense_sum)
+    assert len(output) == 1
+
+    output = da.apply_over_dimension(x, 1, _dense_sum)
+    assert len(output) == 0