update_metadata cannot handle invalid blank attributes

[corviday]

I'm not sure if fixing this issue is actually possible.

I attempted to use the update_metadata script to remove an invalid global attribute from some netCDF files. Specifically, some files had a blank string for global: history, which results in all sorts of weird errors.

I wasn't able to remove the invalid attribute with update metadata, and got this traceback:

2018-12-20 16:17:39 INFO: Processing file: /storage/data/climate/downscale/BCCAQ2/bccaqv2_with_metadata/tasmin_day_BCCAQv2+ANUSPLIN300_BNU-ESM_historical+rcp45_r1i1p1_19500101-21001231.nc
2018-12-20 16:17:39 INFO: Global attributes:
Traceback (most recent call last):
  File "climate-explorer-data-prep/scripts/update_metadata", line 31, in <module>
    main(args)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 247, in main
    process_updates(dataset, updates)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 227, in process_updates
    apply_attribute_updates(dataset, target, update)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 202, in apply_attribute_updates
    apply_attribute_updates(dataset, target, element)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 196, in apply_attribute_updates
    modify_attribute(dataset, target, *attr_updates)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 181, in modify_attribute
    return delete_attribute(target, name)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/dp/update_metadata.py", line 145, in delete_attribute
    if hasattr(target, name):
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/nchelpers/decorators.py", line 26, in wrapper
    res = func(*args, **kwargs)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/nchelpers/__init__.py", line 353, in __getattribute__
    is_indirected, indirected_property = _indirection_info(value)
  File "/local_temp/lzeman/climate-explorer-data-prep/venv/lib64/python3.4/site-packages/nchelpers/__init__.py", line 148, in _indirection_info
    if isinstance(value, six.string_types) and value[0] == '@':
IndexError: string index out of range

As a workaround, I first set the attribute to a valid string with update_metadata. Then I ran update_metadata a second time to delete the attribute.

Not a very high priority, since there is a workaround.