Fix error when using from_awkward on an empty array (#330)

---------

Co-authored-by: Doug Davis <[email protected]>

src/dask_awkward/lib/io/io.py

@@ -89,8 +89,11 @@ def from_awkward(
 
  """
  nrows = len(source)
- chunksize = int(math.ceil(nrows / npartitions))
- locs = list(range(0, nrows, chunksize)) + [nrows]
+ if nrows == 0:
+ locs = [None, None]
+ else:
+ chunksize = int(math.ceil(nrows / npartitions))
+ locs = list(range(0, nrows, chunksize)) + [nrows]
  starts = locs[:-1]
  stops = locs[1:]
  meta = typetracer_array(source)

tests/test_io.py

@@ -394,3 +394,19 @@ def test_to_dataframe_str(
  df = ak.to_dataframe(caa)
 
  assert_eq(dd, df, check_index=False)
+
+
+def test_from_awkward_empty_array(daa) -> None:
+ # no form
+ c1 = ak.Array([])
+ assert len(c1) == 0
+ a1 = dak.from_awkward(c1, npartitions=1)
+ assert_eq(a1, c1)
+ assert len(a1) == 0
+
+ # with a form
+ c2 = ak.Array(daa.layout.form.length_zero_array(highlevel=False))
+ assert len(c2) == 0
+ a2 = dak.from_awkward(c2, npartitions=1)
+ assert len(a2) == 0
+ daa.layout.form == a2.layout.form