Is your feature request related to a problem? Please describe.
Reading attributes from h5py-files is rather slow. So instead of retrieving it immediately I wanted to create a lazy dict-class that only retrieves the attribute values when necessary. But this is difficult to achieve since xarray keeps forcing the attrs to dicts in a lot of places.

Describe the solution you'd like

• Replace in

  xarray/xarray/core/variable.py

  Line 865 in dddac11

  self._attrs = dict(value)

  and

  xarray/xarray/core/dataset.py

  Line 798 in dddac11

  self._attrs = dict(value)

  with a asdict(value) function that checks if the input is a valid dict-like, if not convert to dict. Things that might be good to check:
  • MutableMapping
  • hasattr(dict_like, "copy")
  • isinstance(dict_like, dict) == True
• Remove unneccessary conversions to dict. For example

  xarray/xarray/core/merge.py

  Line 523 in dddac11

  return dict(variable_attrs[0])

  should not be necessary as attrs from variables/dataarrays/datasets have already been forced to dicts when they were initialized.

Describe alternatives you've considered

• One could lazify with dicts as well, for example by replacing the value with a function. This however won't look good in reprs, that's why having a convienence class is nice.
• dict(LazyDict) always forces to dict, it does not let it pass through unchanged even if isinstance(LazyDict, dict) == True.

Interesting reading:
https://stackoverflow.com/questions/16669367/setup-dictionary-lazily
https://stackoverflow.com/questions/3387691/how-to-perfectly-override-a-dict