fix all-but-dask tests (#4511)

* fix all-but-dask tests

* whats new

* add dummy context

Author: mathause

doc/whats-new.rst

@@ -67,6 +67,8 @@ Internal Changes
   <https://github.com/mathause>`_.
 - Removed stray spaces that stem from black removing new lines (:pull:`4504`).
   By `Mathias Hauser <https://github.com/mathause>`_.
+- Ensure tests are not skipped in the `py38-all-but-dask` test environment
+  (:issue:`4509`). By `Mathias Hauser <https://github.com/mathause>`_.
 
 .. _whats-new.0.16.1:
 

xarray/tests/__init__.py

@@ -17,6 +17,7 @@
 from xarray.core.indexing import ExplicitlyIndexed
 from xarray.core.options import set_options
 from xarray.testing import (  # noqa: F401
+    assert_chunks_equal,
     assert_duckarray_allclose,
     assert_duckarray_equal,
 )
@@ -95,6 +96,39 @@ def LooseVersion(vstring):
 
     dask.config.set(scheduler="single-threaded")
 
+
+class CountingScheduler:
+    """Simple dask scheduler counting the number of computes.
+
+    Reference: https://stackoverflow.com/questions/53289286/"""
+
+    def __init__(self, max_computes=0):
+        self.total_computes = 0
+        self.max_computes = max_computes
+
+    def __call__(self, dsk, keys, **kwargs):
+        self.total_computes += 1
+        if self.total_computes > self.max_computes:
+            raise RuntimeError(
+                "Too many computes. Total: %d > max: %d."
+                % (self.total_computes, self.max_computes)
+            )
+        return dask.get(dsk, keys, **kwargs)
+
+
+@contextmanager
+def dummy_context():
+    yield None
+
+
+def raise_if_dask_computes(max_computes=0):
+    # return a dummy context manager so that this can be used for non-dask objects
+    if not has_dask:
+        return dummy_context()
+    scheduler = CountingScheduler(max_computes)
+    return dask.config.set(scheduler=scheduler)
+
+
 flaky = pytest.mark.flaky
 network = pytest.mark.network
 

xarray/tests/test_accessor_dt.py

@@ -6,13 +6,14 @@
 
 from . import (
     assert_array_equal,
+    assert_chunks_equal,
     assert_equal,
     assert_identical,
+    raise_if_dask_computes,
     raises_regex,
     requires_cftime,
     requires_dask,
 )
-from .test_dask import assert_chunks_equal, raise_if_dask_computes
 
 
 class TestDatetimeAccessor:

xarray/tests/test_dask.py

@@ -23,6 +23,7 @@
     assert_equal,
     assert_frame_equal,
     assert_identical,
+    raise_if_dask_computes,
     raises_regex,
     requires_pint_0_15,
     requires_scipy_or_netCDF4,
@@ -36,30 +37,6 @@
 ON_WINDOWS = sys.platform == "win32"
 
 
-class CountingScheduler:
-    """Simple dask scheduler counting the number of computes.
-
-    Reference: https://stackoverflow.com/questions/53289286/"""
-
-    def __init__(self, max_computes=0):
-        self.total_computes = 0
-        self.max_computes = max_computes
-
-    def __call__(self, dsk, keys, **kwargs):
-        self.total_computes += 1
-        if self.total_computes > self.max_computes:
-            raise RuntimeError(
-                "Too many computes. Total: %d > max: %d."
-                % (self.total_computes, self.max_computes)
-            )
-        return dask.get(dsk, keys, **kwargs)
-
-
-def raise_if_dask_computes(max_computes=0):
-    scheduler = CountingScheduler(max_computes)
-    return dask.config.set(scheduler=scheduler)
-
-
 def test_raise_if_dask_computes():
     data = da.from_array(np.random.RandomState(0).randn(4, 6), chunks=(2, 2))
     with raises_regex(RuntimeError, "Too many computes"):

xarray/tests/test_dataarray.py

@@ -32,6 +32,7 @@
     assert_equal,
     assert_identical,
     has_dask,
+    raise_if_dask_computes,
     raises_regex,
     requires_bottleneck,
     requires_dask,
@@ -42,8 +43,6 @@
     source_ndarray,
 )
 
-from .test_dask import raise_if_dask_computes
-
 
 class TestDataArray:
     @pytest.fixture(autouse=True)