From 9235548df71c1b03e76da40641dc790becf443bc Mon Sep 17 00:00:00 2001
From: Chris Roat <1053153+chrisroat@users.noreply.github.com>
Date: Thu, 27 Jan 2022 13:46:58 -0800
Subject: [PATCH] Handle empty containers in zarr chunk checks (#5526)

---
 doc/whats-new.rst             |  2 +-
 xarray/backends/zarr.py       |  9 +++++----
 xarray/tests/test_backends.py | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 8896dd62379..50500e3d75f 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -63,7 +63,7 @@ Bug fixes
   By `Michael Delgado <https://github.com/delgadom>`_.
 - `dt.season <https://xarray.pydata.org/en/stable/generated/xarray.DataArray.dt.season.html>`_  can now handle NaN and NaT.  (:pull:`5876`).
   By `Pierre Loicq <https://github.com/pierreloicq>`_.
-
+- Determination of zarr chunks handles empty lists for encoding chunks or variable chunks that occurs in certain cirumstances (:pull:`5526`). By `Chris Roat <https://github.com/chrisroat>`_.
 
 Documentation
 ~~~~~~~~~~~~~
diff --git a/xarray/backends/zarr.py b/xarray/backends/zarr.py
index 8bd343869ff..efb22bef1d4 100644
--- a/xarray/backends/zarr.py
+++ b/xarray/backends/zarr.py
@@ -84,7 +84,8 @@ def __getitem__(self, key):
 
 def _determine_zarr_chunks(enc_chunks, var_chunks, ndim, name, safe_chunks):
     """
-    Given encoding chunks (possibly None) and variable chunks (possibly None)
+    Given encoding chunks (possibly None or []) and variable chunks
+    (possibly None or []).
     """
 
     # zarr chunk spec:
@@ -93,7 +94,7 @@ def _determine_zarr_chunks(enc_chunks, var_chunks, ndim, name, safe_chunks):
 
     # if there are no chunks in encoding and the variable data is a numpy
     # array, then we let zarr use its own heuristics to pick the chunks
-    if var_chunks is None and enc_chunks is None:
+    if not var_chunks and not enc_chunks:
         return None
 
     # if there are no chunks in encoding but there are dask chunks, we try to
@@ -102,7 +103,7 @@ def _determine_zarr_chunks(enc_chunks, var_chunks, ndim, name, safe_chunks):
     # http://zarr.readthedocs.io/en/latest/spec/v1.html#chunks
     # while dask chunks can be variable sized
     # http://dask.pydata.org/en/latest/array-design.html#chunks
-    if var_chunks and enc_chunks is None:
+    if var_chunks and not enc_chunks:
         if any(len(set(chunks[:-1])) > 1 for chunks in var_chunks):
             raise ValueError(
                 "Zarr requires uniform chunk sizes except for final chunk. "
@@ -145,7 +146,7 @@ def _determine_zarr_chunks(enc_chunks, var_chunks, ndim, name, safe_chunks):
 
     # if there are chunks in encoding and the variable data is a numpy array,
     # we use the specified chunks
-    if var_chunks is None:
+    if not var_chunks:
         return enc_chunks_tuple
 
     # the hard case
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index 356335f47e6..bea90e1fca9 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -2383,6 +2383,20 @@ def test_open_zarr_use_cftime(self):
             ds_b = xr.open_zarr(store_target, use_cftime=True)
             assert xr.coding.times.contains_cftime_datetimes(ds_b.time)
 
+    def test_write_read_select_write(self):
+        # Test for https://github.com/pydata/xarray/issues/4084
+        ds = create_test_data()
+
+        # NOTE: using self.roundtrip, which uses open_dataset, will not trigger the bug.
+        with self.create_zarr_target() as initial_store:
+            ds.to_zarr(initial_store, mode="w")
+            ds1 = xr.open_zarr(initial_store)
+
+        # Combination of where+squeeze triggers error on write.
+        ds_sel = ds1.where(ds1.coords["dim3"] == "a", drop=True).squeeze("dim3")
+        with self.create_zarr_target() as final_store:
+            ds_sel.to_zarr(final_store, mode="w")
+
 
 @requires_zarr
 class TestZarrDictStore(ZarrBase):