[BUG] Indexing expressions on series produce results different from Pandas'

[magnatelee]

Describe the bug
Pandas interprets the sub-expression idx of an expression sr[idx] as an absolute position in the series sr when idx's dtype is different from that of sr's index. However, cuDF always treats idx as a value to look up in sr's index, which can lead to different behaviors when indices have non-integral dtypes:

cudf/python/cudf/cudf/core/series.py

Lines 1183 to 1187 in 34cccfe

	def __getitem__(self, arg):
	if isinstance(arg, slice):
	return self.iloc[arg]
	else:
	return self.loc[arg]

Steps/Code to reproduce bug

In Pandas, the indexing takes both an integer and a string as the index:

>>> import pandas as pd
>>> x = pd.Series([1,2,3], index=pd.Index(["a", "b", "c"]))
>>> x["b"]
2
>>> x[1]
2

For the same example, cuDF raises a KeyError on the second access:

>>> import cudf
>>> x = cudf.Series([1,2,3], index=cudf.Index(["a", "b", "c"]))
>>> x["b"]
2
>>> x[1]
Traceback (most recent call last):
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/indexing.py", line 138, in _loc_to_iloc
    arg, closest=False
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/column/string.py", line 4938, in find_first_value
    return self._find_first_and_last(value)[0]
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/column/string.py", line 4933, in _find_first_and_last
    first = column.as_column(found_indices).find_first_value(1)
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/column/numerical.py", line 477, in find_first_value
    raise ValueError("value not found")
ValueError: value not found

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/indexing.py", line 118, in __getitem__
    arg = self._loc_to_iloc(arg)
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/indexing.py", line 142, in _loc_to_iloc
    raise KeyError("label scalar is out of bound")
KeyError: 'label scalar is out of bound'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/series.py", line 921, in __getitem__
    return self.loc[arg]
  File "/home/nfs/wonchanl/anaconda3/envs/rapids-tpcx-bb/lib/python3.7/site-packages/cudf/core/indexing.py", line 120, in __getitem__
    raise KeyError(arg)
KeyError: 1
>>>