issue_comments: 277498382

@shoyer, the pushed code represents my progress. The initial PR had a bug-- essentially a calculation couldn't be performed following the load. This fixes that bug and provides a test to ensure that this doesn't happen. However, I'm having trouble with h5netcdf, which I'm not very familiar with compared to netcdf. This represents my current progress, I just need some more time (or even inspiration from you) to sort out this last key issue...

I'm getting the following error:

```bash ================================================================================================================== FAILURES ================================================================================================================== _____________ OpenMFDatasetTest.test_4_open_large_num_files_h5netcdf _____________

self = <xarray.tests.test_backends.OpenMFDatasetTest testMethod=test_4_open_large_num_files_h5netcdf>

@requires_dask
@requires_h5netcdf
def test_4_open_large_num_files_h5netcdf(self):

  self.validate_open_mfdataset_large_num_files(engine=['h5netcdf'])

xarray/tests/test_backends.py:1040:

xarray/tests/test_backends.py:1018: in validate_open_mfdataset_large_num_files self.assertClose(ds.foo.sum().values, np.sum(randdata)) xarray/core/dataarray.py:400: in values return self.variable.values xarray/core/variable.py:306: in values return as_array_or_item(self._data) xarray/core/variable.py:182: in _as_array_or_item data = np.asarray(data) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/numpy/core/numeric.py:482: in asarray return array(a, dtype, copy=False, order=order) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/array/core.py:1025: in __array__ x = self.compute() ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/base.py:79: in compute return compute(self, kwargs)[0] ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/base.py:179: in compute results = get(dsk, keys, kwargs) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:537: in get_sync raise_on_exception=True, kwargs) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:500: in get_async fire_task() ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:476: in fire_task callback=queue.put) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:525: in apply_sync res = func(*args, kwds) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:268: in execute_task result = _execute_task(task, data) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:248: in _execute_task args2 = [_execute_task(a, cache) for a in args] ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:248: in <listcomp> args2 = [_execute_task(a, cache) for a in args] ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:245: in _execute_task return [_execute_task(a, cache) for a in arg] ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:245: in <listcomp> return [_execute_task(a, cache) for a in arg] ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/async.py:249: in _execute_task return func(*args2) ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/dask/array/core.py:52: in getarray c = a[b] xarray/core/indexing.py:401: in getitem return type(self)(self.array[key]) xarray/core/indexing.py:376: in getitem return type(self)(self.array, self._updated_key(key)) xarray/core/indexing.py:354: in _updated_key for size, k in zip(self.array.shape, self.key): xarray/core/indexing.py:364: in shape for size, k in zip(self.array.shape, self.key): xarray/core/utils.py:414: in shape return self.array.shape xarray/backends/netCDF4.py:37: in getattr return getattr(self.datastore.ds.variables[self.var], attr) ../../anaconda/envs/test_env_xarray35/lib/python3.5/contextlib.py:66: in exit next(self.gen) xarray/backends/h5netcdf_.py:105: in ensure_open self.close() xarray/backends/h5netcdf_.py:190: in close close_ds(self.ds) xarray/backends/h5netcdf.py:70: in _close_ds find_root(ds).close() ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/h5netcdf/core.py:458: in close self._h5file.close() ../../anaconda/envs/test_env_xarray35/lib/python3.5/site-packages/h5py/_hl/files.py:302: in close self.id.close() h5py/_objects.pyx:54: in h5py._objects.with_phil.wrapper (/Users/travis/miniconda3/conda-bld/work/h5py-2.6.0/h5py/_objects.c:2840) ??? h5py/_objects.pyx:55: in h5py._objects.with_phil.wrapper (/Users/travis/miniconda3/conda-bld/work/h5py-2.6.0/h5py/_objects.c:2798) ??? h5py/h5f.pyx:282: in h5py.h5f.FileID.close (/Users/travis/miniconda3/conda-bld/work/h5py-2.6.0/h5py/h5f.c:3905) ??? h5py/_objects.pyx:54: in h5py._objects.with_phil.wrapper (/Users/travis/miniconda3/conda-bld/work/h5py-2.6.0/h5py/_objects.c:2840) ??? h5py/_objects.pyx:55: in h5py._objects.with_phil.wrapper (/Users/travis/miniconda3/conda-bld/work/h5py-2.6.0/h5py/_objects.c:2798) ???

??? E RuntimeError: dictionary changed size during iteration

h5py/_objects.pyx:119: RuntimeError ============================================================================================ 1 failed, 1415 passed, 95 skipped in 116.54 seconds ============================================================================================= Exception ignored in: <function WeakValueDictionary.__init__.\<locals>.remove at 0x10f16e598> Traceback (most recent call last): File "/Users/pwolfram/anaconda/envs/test_env_xarray35/lib/python3.5/weakref.py", line 117, in remove TypeError: 'NoneType' object is not callable ```