我正在使用xarray非常酷的pydap后端（http://xarray.pydata.org/en/stable/io.html#opendap）来读取通过IRI的OPenDAP存储的数据：
import xarray as xr
remote_data = xr.open_dataarray('http://iridl.ldeo.columbia.edu/SOURCES/.Models/.SubX/.RSMAS/.CCSM4/.hindcast/.zg/dods')
print(remote_data)
#<xarray.DataArray 'zg' (P: 2, S: 6569, M: 3, L: 45, Y: 181, X: 360)>
#[115569730800 values with dtype=float32]
#Coordinates:
# * L (L) timedelta64[ns] 0 days 12:00:00 1 days 12:00:00 ...
# * Y (Y) float32 -90.0 -89.0 -88.0 -87.0 -86.0 -85.0 -84.0 -83.0 ...
# * S (S) datetime64[ns] 1999-01-07 1999-01-08 1999-01-09 1999-01-10 ...
# * M (M) float32 1.0 2.0 3.0
# * X (X) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 ...
# * P (P) int32 500 200
#Attributes:
# level_type: pressure level
# standard_name: geopotential_height
# long_name: Geopotential Height
# units: m
作为参考，它是次季节预测数据，其中L是提前期（45天预测），S是初始化日期，M是整体。
我想做一个合奏的意思，我只对500 hPa水平感兴趣。但是，它会崩溃并提供RuntimeError: NetCDF: Access failure：
da = remote_data.sel(P=500)
da_ensmean = da.mean(dim='M')
RuntimeError Traceback (most recent call last)
<ipython-input-46-eca488e9def5> in <module>()
1 remote_data = xr.open_dataarray('http://iridl.ldeo.columbia.edu/SOURCES/.Models' '/.SubX/.RSMAS/.CCSM4/.hindcast/.zg/dods')
2 da = remote_data.sel(P=500)
----> 3 da_ensmean = da.mean(dim='M')
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/common.py in wrapped_func(self, dim, axis, skipna, keep_attrs, **kwargs)
20 keep_attrs=False, **kwargs):
21 return self.reduce(func, dim, axis, keep_attrs=keep_attrs,
---> 22 skipna=skipna, allow_lazy=True, **kwargs)
23 else:
24 def wrapped_func(self, dim=None, axis=None, keep_attrs=False,
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/dataarray.py in reduce(self, func, dim, axis, keep_attrs, **kwargs)
1359 summarized data and the indicated dimension(s) removed.
1360 """
-> 1361 var = self.variable.reduce(func, dim, axis, keep_attrs, **kwargs)
1362 return self._replace_maybe_drop_dims(var)
1363
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/variable.py in reduce(self, func, dim, axis, keep_attrs, allow_lazy, **kwargs)
1264 if dim is not None:
1265 axis = self.get_axis_num(dim)
-> 1266 data = func(self.data if allow_lazy else self.values,
1267 axis=axis, **kwargs)
1268
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/variable.py in data(self)
293 return self._data
294 else:
--> 295 return self.values
296
297 @data.setter
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/variable.py in values(self)
385 def values(self):
386 """The variable's data as a numpy.ndarray"""
--> 387 return _as_array_or_item(self._data)
388
389 @values.setter
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/variable.py in _as_array_or_item(data)
209 TODO: remove this (replace with np.asarray) once these issues are fixed
210 """
--> 211 data = np.asarray(data)
212 if data.ndim == 0:
213 if data.dtype.kind == 'M':
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/numpy/core/numeric.py in asarray(a, dtype, order)
490
491 """
--> 492 return array(a, dtype, copy=False, order=order)
493
494
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/indexing.py in __array__(self, dtype)
622
623 def __array__(self, dtype=None):
--> 624 self._ensure_cached()
625 return np.asarray(self.array, dtype=dtype)
626
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/indexing.py in _ensure_cached(self)
619 def _ensure_cached(self):
620 if not isinstance(self.array, NumpyIndexingAdapter):
--> 621 self.array = NumpyIndexingAdapter(np.asarray(self.array))
622
623 def __array__(self, dtype=None):
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/numpy/core/numeric.py in asarray(a, dtype, order)
490
491 """
--> 492 return array(a, dtype, copy=False, order=order)
493
494
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/indexing.py in __array__(self, dtype)
600
601 def __array__(self, dtype=None):
--> 602 return np.asarray(self.array, dtype=dtype)
603
604 def __getitem__(self, key):
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/numpy/core/numeric.py in asarray(a, dtype, order)
490
491 """
--> 492 return array(a, dtype, copy=False, order=order)
493
494
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/indexing.py in __array__(self, dtype)
506 def __array__(self, dtype=None):
507 array = as_indexable(self.array)
--> 508 return np.asarray(array[self.key], dtype=None)
509
510 def transpose(self, order):
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/coding/variables.py in __getitem__(self, key)
64
65 def __getitem__(self, key):
---> 66 return self.func(self.array[key])
67
68 def __repr__(self):
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/coding/variables.py in _apply_mask(data, encoded_fill_values, decoded_fill_value, dtype)
133 for fv in encoded_fill_values:
134 condition |= data == fv
--> 135 data = np.asarray(data, dtype=dtype)
136 return np.where(condition, decoded_fill_value, data)
137
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/numpy/core/numeric.py in asarray(a, dtype, order)
490
491 """
--> 492 return array(a, dtype, copy=False, order=order)
493
494
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/core/indexing.py in __array__(self, dtype)
506 def __array__(self, dtype=None):
507 array = as_indexable(self.array)
--> 508 return np.asarray(array[self.key], dtype=None)
509
510 def transpose(self, order):
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/backends/netCDF4_.py in __getitem__(self, key)
63 with self.datastore.ensure_open(autoclose=True):
64 try:
---> 65 array = getitem(self.get_array(), key.tuple)
66 except IndexError:
67 # Catch IndexError in netCDF4 and return a more informative
~/anaconda/envs/SubXNAO/lib/python3.6/site-packages/xarray/backends/common.py in robust_getitem(array, key, catch, max_retries, initial_delay)
114 for n in range(max_retries + 1):
115 try:
--> 116 return array[key]
117 except catch:
118 if n == max_retries:
netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Variable.__getitem__()
netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Variable._get()
netCDF4/_netCDF4.pyx in netCDF4._netCDF4._ensure_nc_success()
RuntimeError: NetCDF: Access failure
分解计算将删除RuntimeError。猜猜这对于所有开始时间的计算来说都太过分了。在S：
da = remote_data.isel(P=0,S=0)
da_ensmean = da.mean(dim='M')
print(da_ensmean)
<xarray.DataArray 'zg' (L: 45, Y: 181, X: 360)>
array([[[5231.1445, 5231.1445, ..., 5231.1445, 5231.1445],
[5231.1445, 5231.1445, ..., 5231.1445, 5231.1445],
...,
[5056.2383, 5056.2383, ..., 5056.2383, 5056.2383],
[5056.2383, 5056.2383, ..., 5056.2383, 5056.2383]],
[[5211.346 , 5211.346 , ..., 5211.346 , 5211.346 ],
[5211.346 , 5211.346 , ..., 5211.346 , 5211.346 ],
...,
[5082.062 , 5082.062 , ..., 5082.062 , 5082.062 ],
[5082.062 , 5082.062 , ..., 5082.062 , 5082.062 ]],
...,
[[5108.8247, 5108.8247, ..., 5108.8247, 5108.8247],
[5108.8247, 5108.8247, ..., 5108.8247, 5108.8247],
...,
[5154.2173, 5154.2173, ..., 5154.2173, 5154.2173],
[5154.2173, 5154.2173, ..., 5154.2173, 5154.2173]],
[[5106.4893, 5106.4893, ..., 5106.4893, 5106.4893],
[5106.4893, 5106.4893, ..., 5106.4893, 5106.4893],
...,
[5226.0063, 5226.0063, ..., 5226.0063, 5226.0063],
[5226.0063, 5226.0063, ..., 5226.0063, 5226.0063]]], dtype=float32)
Coordinates:
* L (L) timedelta64[ns] 0 days 12:00:00 1 days 12:00:00 ...
* Y (Y) float32 -90.0 -89.0 -88.0 -87.0 -86.0 -85.0 -84.0 -83.0 ...
S datetime64[ns] 1999-01-07
* X (X) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 ...
P int32 500
答案 0 :(得分：2)
这是一个很好的用于使用dask进行分块的用例，例如，
import xarray as xr
url = 'http://iridl.ldeo.columbia.edu/SOURCES/.Models/.SubX/.RSMAS/.CCSM4/.hindcast/.zg/dods'
remote_data = xr.open_dataarray(url, chunks={'S': 1, 'L': 1})
da = remote_data.sel(P=500)
da_ensmean = da.mean(dim='M')
此版本将使用许多较小的块并行访问数据服务器。下载231 GB的数据仍然很慢，但您的请求将有更好的成功几率。