DEPR: disallow series[[slice(...)]]

[jbrockmendel]

One of the more convoluted parts of Series.__getitem__ is for handling single-entry lists that contain a slice, where we end up just returning self[key[0]]. We have two tests that hit this path, but it isn't clear to me why this should be supported.

[jorisvandenbossche]

Can you give a code example? This isn't involved with MultiIndex?

[jbrockmendel]

Can you give a code example? This isn't involved with MultiIndex?

dti = pd.date_range("2016-01-01", periods=5)
ser = pd.Series(range(5), index=dti)
key = [slice(0, 3)]

>>> ser[key]
pandas/core/internals/blocks.py:313: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.
  return self.values[slicer]
pandas/core/arrays/datetimelike.py:542: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.

  result = getitem(key)
2016-01-01    0
2016-01-02    1
2016-01-03    2
dtype: int64