使用 xarray 读取授权的 opendap url答案

【问题标题】：read in authorized opendap url using xarray使用 xarray 读取授权的 opendap url
【发布时间】：2021-02-13 09:04:53
【问题描述】：

我想使用 xarray 打开一个 opendap url。它需要授权，因为它在 UCAR RDA 持有：

https://rda.ucar.edu/datasets/ds084.1/#!description

一个文件的url如下'https://rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2020/20200101/gfs.0p25.2020010100.f000.grib2'

我不确定是否可以将授权作为 backend_kwarg 传递？

下面的代码会给出错误信息

import xarray as xr
url = "https://rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2020/20200101/gfs.0p25.2020010100.f000.grib2"
ds = xr.open_dataset(url)
Traceback (most recent call last):
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/file_manager.py", line 199, in _acquire_with_cache_info
    file = self._cache[self._key]
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/lru_cache.py", line 53, in __getitem__
    value = self._cache[key]
KeyError: [<class 'netCDF4._netCDF4.Dataset'>, ('https://rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2020/20200101/gfs.0p25.2020010100.f000.grib2',), 'r', (('clobber', True), ('diskless', False), ('format', 'NETCDF4'), ('persist', False))]

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/api.py", line 572, in open_dataset
    store = opener(filename_or_obj, **extra_kwargs, **backend_kwargs)
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/netCDF4_.py", line 364, in open
    return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/netCDF4_.py", line 314, in __init__
    self.format = self.ds.data_model
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/netCDF4_.py", line 373, in ds
    return self._acquire()
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/netCDF4_.py", line 367, in _acquire
    with self._manager.acquire_context(needs_lock) as root:
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/file_manager.py", line 187, in acquire_context
    file, cached = self._acquire_with_cache_info(needs_lock)
  File "/Users/ray.bell/miniconda/envs/test_env/lib/python3.8/site-packages/xarray/backends/file_manager.py", line 205, in _acquire_with_cache_info
    file = self._opener(*self._args, **kwargs)
  File "netCDF4/_netCDF4.pyx", line 2357, in netCDF4._netCDF4.Dataset.__init__
  File "netCDF4/_netCDF4.pyx", line 1925, in netCDF4._netCDF4._ensure_nc_success
OSError: [Errno -78] NetCDF: Authorization failure: b'https://rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2020/20200101/gfs.0p25.2020010100.f000.grib2'

Siphon 的 session_manager 可能会暗示 auth 的样子 https://unidata.github.io/siphon/latest/examples/Basic_Usage.html#sphx-glr-examples-basic-usage-py / https://github.com/Unidata/siphon/blob/master/siphon/http_util.py#L52

【问题讨论】：

标签： python-xarray opendap python-siphon

【解决方案1】：

感谢 Ryan May 将我指向https://publicwiki.deltares.nl/display/OET/Accessing+netCDF+data+via+OPeNDAP+on+password+protected+servers

在我的主目录中创建点文件允许我读取 url。可能不是最干净的，我想可能会导致虚拟机/集群出现问题，但可以正常工作。仍然希望有backend_kwargs 的方法。

在您的主目录中创建一个文件 .netrc，如下所示：

machine rda.ucar.edu
login USR
password PWD

还有一个文件 .dodsrc 在您的主目录中，如下所示：

HTTP.COOKIEJAR=<HOME_DIR>/.cookies
HTTP.NETRC=<HOME_DIR>/.netrc

您现在可以传递需要身份验证的网址：

import xarray as xr
url = "https://rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2020/20200101/gfs.0p25.2020010100.f000.grib2"
ds = xr.open_dataset(url)

【讨论】：

【解决方案2】：

如果您不是绝对需要使用 OPeNDAP，而只是想要某种与 xarray 接口的类似 OPeNDAP 的东西，那么您可以使用 THREDDS 的 CDMRemote 协议。在这种情况下，我们可以通过requests 来利用 Siphon 对基本 HTTP 身份验证的支持：

from siphon.catalog import TDSCatalog
from siphon.http_util import session_manager

# Set options for Siphon's HTTP session manager--in this case user/password
session_manager.set_session_options(auth=('MYUSER', 'MYPASSWORD'))
cat = TDSCatalog('https://rda.ucar.edu/thredds/catalog/files/g/ds084.1/2020/20200101/catalog.xml')

selected_dataset = cat.datasets[0]
ds = selected_dataset.remote_access(service='CDMRemote', use_xarray=True)

【讨论】：