Computing unique values in an empty catalog results in an error

[andersy005]

Description

Computing unique values in an empty catalog result in an error. We should add a check for an empty dataframe to ensure we return an empty data frame early.

What I Did

In [4]: import intake

In [5]: cat = intake.open_esm_datastore("./tests/sample-collections/catalog-dict-records.json")

In [6]: cat_sub = cat.search(variable='foo')
/Users/abanihi/devel/intake/intake-esm/intake_esm/_types.py:303: UserWarning: Query: {'variable': ['foo']} returned zero results.
  results = search(self.df, _query, self.columns_with_iterables)

In [7]: cat_sub
Out[7]: <aws-cesm1-le catalog with 0 dataset(s) from 0 asset(s)>

In [8]: cat_sub.esmcat.unique()

Traceback:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-37a1bfa1a437> in <module>
----> 1 cat_sub.esmcat.unique()

~/devel/intake/intake-esm/intake_esm/_types.py in unique(self)
    276 
    277     def unique(self) -> pd.Series:
--> 278         return pd.Series(self._unique())
    279 
    280     def nunique(self) -> pd.Series:

~/devel/intake/intake-esm/intake_esm/_types.py in _unique(self)
    273             return list(tlz.unique(values))
    274 
--> 275         return self.df[self.df.columns].apply(_find_unique, result_type='reduce').to_dict()
    276 
    277     def unique(self) -> pd.Series:

~/.mambaforge/envs/intake-esm-dev/lib/python3.9/site-packages/pandas/core/frame.py in apply(self, func, axis, raw, result_type, args, **kwargs)
   8739             kwargs=kwargs,
   8740         )
-> 8741         return op.apply()
   8742 
   8743     def applymap(

~/.mambaforge/envs/intake-esm-dev/lib/python3.9/site-packages/pandas/core/apply.py in apply(self)
    680         # one axis empty
    681         elif not all(self.obj.shape):
--> 682             return self.apply_empty_result()
    683 
    684         # raw

~/.mambaforge/envs/intake-esm-dev/lib/python3.9/site-packages/pandas/core/apply.py in apply_empty_result(self)
    750                 r = np.nan
    751 
--> 752             return self.obj._constructor_sliced(r, index=self.agg_axis)
    753         else:
    754             return self.obj.copy()

~/.mambaforge/envs/intake-esm-dev/lib/python3.9/site-packages/pandas/core/series.py in __init__(self, data, index, dtype, name, copy, fastpath)
    428                 index = ibase.default_index(len(data))
    429             elif is_list_like(data):
--> 430                 com.require_length_match(data, index)
    431 
    432             # create/copy the manager

~/.mambaforge/envs/intake-esm-dev/lib/python3.9/site-packages/pandas/core/common.py in require_length_match(data, index)
    529     """
    530     if len(data) != len(index):
--> 531         raise ValueError(
    532             "Length of values "
    533             f"({len(data)}) "

ValueError: Length of values (0) does not match length of index (5)

Relevant lines:

intake-esm/intake_esm/_types.py

Line 275 in ad175fa

return self.df[self.df.columns].apply(_find_unique, result_type='reduce').to_dict()