issue_comments

get_variables_by_attributes does seem generic enough that we can safely add it to xarray. For our version, I would make this a Dataset method and always return another Dataset (no strict flag). Pulling out the single variable of a Dataset as a DataArray should be a separate method, e.g., .item() like the NumPy method.

:+1: -- I think this would be super-useful general functionality for the xarray community that doesn't come with any downside.

@shoyer this made into netcdf4 and some people in my group would like to have this in xarray too. If you think it is worth it I can put a PR together for this.

How about the function below? (Stolen from pyaxiom.)

``` python def get_variables_by_attributes(ds, strict=False, **kwargs): """ Returns variables that match specific conditions. * Can pass in key=value parameters and variables are returned that contain all of the matches. ex.: vs = nc.get_variables_by_attributes(axis='X') * Can pass in key=callable parameter and if the callable returns True. The callable should accept a single parameter, the attribute value. None is returned as the attribute valuewhen the attribute does not exist on the variable. ex.: # Get Axis variables vs = nc.get_variables_by_attributes(axis=lambda v: v in ['X', 'Y', 'Z', 'T']) # Get variable that don't have a "axis" attribute vs = nc.get_variables_by_attributes(axis=lambda v: v is None) # Get variable that have a "grid_mapping" attribute vs = nc.get_variables_by_attributes(axis=lambda v: v is not None)

* strict : True/False
    If True will return only 1 variable if only one is found. Default is False.
"""
vs = []

has_value_flag  = False
for vname, var in ds.iteritems():
    for k, v in kwargs.items():
        if callable(v):
            has_value_flag = v(getattr(var, k, None))
            if has_value_flag is False:
                break
        elif hasattr(var, k) and getattr(var, k) == v:
            has_value_flag = True
        else:
            has_value_flag = False
            break

    if has_value_flag is True:
        vs.append(ds[vname])

if strict:
    if len(vs) == 1:
        vs = vs[0]
    else:
        msg = "Expected only one variable.  Got {!r}".format
        raise ValueError(msg(vs))
return vs

```

See it in action here.

I was thinking that the data variables that matched a specified standard_name would be a subset of the variables in the data_vars object.

I would probably make this return an xray.Dataset object instead of a plain dict, but otherwise this looks about right:

python def get_std_name_vars(ds,std_name): return ds[[k for k, v in ds.data_vars.iteritems() if 'standard_name' in v.attrs and std_name in v.standard_name]]

I don't think there's a better way to do this currently. Hypothetically, we could add this to a CF-specific API for xray, e.g., as discussed in #461.