Reject a non-positive size or count in Spool.split

dascore/core/spool.py

@@ -332,11 +332,11 @@ def split(
         ----------
         size
             The number of patches desired in each output spool. The last
-            spool may have fewer patches.
+            spool may have fewer patches. Must be greater than zero.
         count
             The number of spools to include. If count is greater than
             the length of the spool then the output will be smaller than
-            count, with one patch per spool.
+            count, with one patch per spool. Must be greater than zero.
 
         Examples
         --------
@@ -583,12 +583,18 @@ def split(
         if not ((count is not None) ^ (size is not None)):
             msg = "Spool.split requires either spool_count or spool_size."
             raise ParameterError(msg)
+        value = count if count is not None else size
+        assert value is not None  # the check above sets exactly one of them
+        # A step of zero or less never advances start, so the loop below
+        # would yield forever.
+        if value <= 0:
+            msg = f"Spool.split requires a positive size or count, got {value}."
+            raise ParameterError(msg)
         start = 0
         if count is not None:
-            step = int(np.ceil(len(self) / count))
+            step = int(np.ceil(len(self) / value))
         else:
-            assert size is not None  # the check above sets exactly one of them
-            step = int(np.ceil(size))  # tolerate a non-integral size
+            step = int(np.ceil(value))  # tolerate a non-integral size
         while start < len(self):
             yield self[start : start + step]
             start += step

dascore/utils/misc.py

@@ -771,8 +771,9 @@ def _spool_map(spool, func, size=None, client=None, progress=True, **kwargs):
     # Now things get interesting. We need to split the spool here
     # so that patches don't get serialized.
     if size is None:
-        # split takes a patch count, so round up rather than hand it a float.
-        size = math.ceil(len(spool) / (os.cpu_count() or 1))
+        # split takes a positive patch count, so round up rather than hand
+        # it a float, and keep an empty spool from asking for zero.
+        size = max(1, math.ceil(len(spool) / (os.cpu_count() or 1)))
     spools = list(spool.split(size=size))
     # this is a hack to get the progress bar to work. Essentially, we just
     # add a secret flag to all but one spool so that progress bar is only

tests/test_core/test_spool.py

@@ -610,6 +610,13 @@ def test_uneven_size(self, random_spool):
         assert len(split[0]) == 2
         assert len(split[1]) == 1
 
+    @pytest.mark.parametrize("kwargs", [{"size": 0}, {"size": -1}, {"count": 0}])
+    def test_non_positive_raises(self, random_spool, kwargs):
+        """A size or count of zero or less would never finish yielding."""
+        msg = "requires a positive size or count"
+        with pytest.raises(ParameterError, match=msg):
+            list(random_spool.split(**kwargs))
+
     def test_non_integral_size(self, random_spool_len_10):
         """A size which isn't a whole number rounds up rather than raising."""
         split = list(random_spool_len_10.split(size=2.5))

tests/test_utils/test_misc.py

@@ -15,6 +15,7 @@
 import pytest
 from upath import UPath
 
+import dascore as dc
 from dascore.exceptions import MissingOptionalDependencyError
 from dascore.utils.misc import (
     _iter_filesystem,
@@ -669,6 +670,21 @@ def map(self, func, spools):
         assert out == [3, 3, 3]
         assert seen == ["Applying <lambda> to spool"]
 
+    def test_empty_spool_with_client(self):
+        """An empty spool asks for no work rather than a split size of zero."""
+
+        class DummyClient:
+            def map(self, func, spools):
+                return [func(spool) for spool in spools]
+
+        out = _spool_map(
+            dc.spool([]),
+            lambda patch: patch,
+            client=DummyClient(),
+            progress=False,
+        )
+        assert out == []
+
 
 class Test2DLineIntersection:
     """Tests for 2D line intersection helper."""