diff --git a/doc/source/whatsnew/v0.21.0.txt b/doc/source/whatsnew/v0.21.0.txt
index 33b7e128ef8bf..87896778bea14 100644
--- a/doc/source/whatsnew/v0.21.0.txt
+++ b/doc/source/whatsnew/v0.21.0.txt
@@ -273,6 +273,30 @@ named ``.isna()`` and ``.notna()``, these are included for classes ``Categorical
The configuration option ``pd.options.mode.use_inf_as_null`` is deprecated, and ``pd.options.mode.use_inf_as_na`` is added as a replacement.
+.. _whatsnew_210.api.multiindex_single:
+
+MultiIndex Constructor with a Single Level
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``MultiIndex`` constructors no longer squeeze a MultiIndex with all
+length-one levels down to a regular ``Index``. This affects all the
+``MultiIndex`` constructors. (:issue:`17178`)
+
+Previous behavior:
+
+.. code-block:: ipython
+
+ In [2]: pd.MultiIndex.from_tuples([('a',), ('b',)])
+ Out[2]: Index(['a', 'b'], dtype='object')
+
+Length 1 levels are no longer special-cased. They behave exactly as if you had
+length 2+ levels, so a :class:`MultiIndex` is always returned from all of the
+``MultiIndex`` constructors:
+
+.. ipython:: python
+
+ pd.MultiIndex.from_tuples([('a',), ('b',)])
+
.. _whatsnew_0210.api:
Other API Changes
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index b5b3df64d24c0..5991ec825c841 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -67,7 +67,8 @@
_dict_compat,
standardize_mapping)
from pandas.core.generic import NDFrame, _shared_docs
-from pandas.core.index import Index, MultiIndex, _ensure_index
+from pandas.core.index import (Index, MultiIndex, _ensure_index,
+ _ensure_index_from_sequences)
from pandas.core.indexing import (maybe_droplevels, convert_to_index_sliceable,
check_bool_indexer)
from pandas.core.internals import (BlockManager,
@@ -1155,9 +1156,9 @@ def from_records(cls, data, index=None, exclude=None, columns=None,
else:
try:
to_remove = [arr_columns.get_loc(field) for field in index]
-
- result_index = MultiIndex.from_arrays(
- [arrays[i] for i in to_remove], names=index)
+ index_data = [arrays[i] for i in to_remove]
+ result_index = _ensure_index_from_sequences(index_data,
+ names=index)
exclude.update(index)
except Exception:
@@ -3000,7 +3001,7 @@ def set_index(self, keys, drop=True, append=False, inplace=False,
to_remove.append(col)
arrays.append(level)
- index = MultiIndex.from_arrays(arrays, names=names)
+ index = _ensure_index_from_sequences(arrays, names)
if verify_integrity and not index.is_unique:
duplicates = index.get_duplicates()
diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 323d50166e7b6..d20a0b0a2c73d 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -1,6 +1,9 @@
-from pandas.core.indexes.base import (Index, _new_Index, # noqa
- _ensure_index, _get_na_value,
- InvalidIndexError)
+from pandas.core.indexes.base import (Index,
+ _new_Index,
+ _ensure_index,
+ _ensure_index_from_sequences,
+ _get_na_value,
+ InvalidIndexError) # noqa
from pandas.core.indexes.category import CategoricalIndex # noqa
from pandas.core.indexes.multi import MultiIndex # noqa
from pandas.core.indexes.interval import IntervalIndex # noqa
@@ -22,7 +25,8 @@
'InvalidIndexError', 'TimedeltaIndex',
'PeriodIndex', 'DatetimeIndex',
'_new_Index', 'NaT',
- '_ensure_index', '_get_na_value', '_get_combined_index',
+ '_ensure_index', '_ensure_index_from_sequences', '_get_na_value',
+ '_get_combined_index',
'_get_objs_combined_axis', '_union_indexes',
'_get_consensus_names',
'_all_indexes_same']
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 31cf1e48b8529..6a30eaefaaae7 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -4012,7 +4012,76 @@ def invalid_op(self, other=None):
Index._add_comparison_methods()
+def _ensure_index_from_sequences(sequences, names=None):
+ """Construct an index from sequences of data.
+
+ A single sequence returns an Index. Many sequences returns a
+ MultiIndex.
+
+ Parameters
+ ----------
+ sequences : sequence of sequences
+ names : sequence of str
+
+ Returns
+ -------
+ index : Index or MultiIndex
+
+ Examples
+ --------
+ >>> _ensure_index_from_sequences([[1, 2, 3]], names=['name'])
+ Int64Index([1, 2, 3], dtype='int64', name='name')
+
+ >>> _ensure_index_from_sequences([['a', 'a'], ['a', 'b']],
+ names=['L1', 'L2'])
+ MultiIndex(levels=[['a'], ['a', 'b']],
+ labels=[[0, 0], [0, 1]],
+ names=['L1', 'L2'])
+
+ See Also
+ --------
+ _ensure_index
+ """
+ from .multi import MultiIndex
+
+ if len(sequences) == 1:
+ if names is not None:
+ names = names[0]
+ return Index(sequences[0], name=names)
+ else:
+ return MultiIndex.from_arrays(sequences, names=names)
+
+
def _ensure_index(index_like, copy=False):
+ """
+ Ensure that we have an index from some index-like object
+
+ Parameters
+ ----------
+ index : sequence
+ An Index or other sequence
+ copy : bool
+
+ Returns
+ -------
+ index : Index or MultiIndex
+
+ Examples
+ --------
+ >>> _ensure_index(['a', 'b'])
+ Index(['a', 'b'], dtype='object')
+
+ >>> _ensure_index([('a', 'a'), ('b', 'c')])
+ Index([('a', 'a'), ('b', 'c')], dtype='object')
+
+ >>> _ensure_index([['a', 'a'], ['b', 'c']])
+ MultiIndex(levels=[['a'], ['b', 'c']],
+ labels=[[0, 0], [0, 1]])
+
+ See Also
+ --------
+ _ensure_index_from_sequences
+ """
if isinstance(index_like, Index):
if copy:
index_like = index_like.copy()
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index ea45b4700172f..d7d5b6d128a2c 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -91,12 +91,6 @@ def __new__(cls, levels=None, labels=None, sortorder=None, names=None,
raise ValueError('Length of levels and labels must be the same.')
if len(levels) == 0:
raise ValueError('Must pass non-zero number of levels/labels')
- if len(levels) == 1:
- if names:
- name = names[0]
- else:
- name = None
- return Index(levels[0], name=name, copy=True).take(labels[0])
result = object.__new__(MultiIndex)
@@ -1084,10 +1078,6 @@ def from_arrays(cls, arrays, sortorder=None, names=None):
MultiIndex.from_product : Make a MultiIndex from cartesian product
of iterables
"""
- if len(arrays) == 1:
- name = None if names is None else names[0]
- return Index(arrays[0], name=name)
-
# Check if lengths of all arrays are equal or not,
# raise ValueError, if not
for i in range(1, len(arrays)):
diff --git a/pandas/core/reshape/reshape.py b/pandas/core/reshape/reshape.py
index 455da9246783c..b4abba8026b35 100644
--- a/pandas/core/reshape/reshape.py
+++ b/pandas/core/reshape/reshape.py
@@ -31,7 +31,7 @@
from pandas.core.frame import _shared_docs
from pandas.util._decorators import Appender
-from pandas.core.index import MultiIndex, _get_na_value
+from pandas.core.index import Index, MultiIndex, _get_na_value
class _Unstacker(object):
@@ -311,10 +311,14 @@ def _unstack_multiple(data, clocs):
recons_labels = decons_obs_group_ids(comp_ids, obs_ids, shape, clabels,
xnull=False)
- dummy_index = MultiIndex(levels=rlevels + [obs_ids],
- labels=rlabels + [comp_ids],
- names=rnames + ['__placeholder__'],
- verify_integrity=False)
+ if rlocs == []:
+ # Everything is in clocs, so the dummy df has a regular index
+ dummy_index = Index(obs_ids, name='__placeholder__')
+ else:
+ dummy_index = MultiIndex(levels=rlevels + [obs_ids],
+ labels=rlabels + [comp_ids],
+ names=rnames + ['__placeholder__'],
+ verify_integrity=False)
if isinstance(data, Series):
dummy = data.copy()
@@ -446,7 +450,12 @@ def _slow_pivot(index, columns, values):
def unstack(obj, level, fill_value=None):
if isinstance(level, (tuple, list)):
- return _unstack_multiple(obj, level)
+ if len(level) != 1:
+ # _unstack_multiple only handles MultiIndexes,
+ # and isn't needed for a single level
+ return _unstack_multiple(obj, level)
+ else:
+ level = level[0]
if isinstance(obj, DataFrame):
if isinstance(obj.index, MultiIndex):
diff --git a/pandas/core/sparse/scipy_sparse.py b/pandas/core/sparse/scipy_sparse.py
index ea108e3e89935..d2b9583d8efe5 100644
--- a/pandas/core/sparse/scipy_sparse.py
+++ b/pandas/core/sparse/scipy_sparse.py
@@ -71,7 +71,11 @@ def robust_get_level_values(i):
labels_to_i = Series(labels_to_i)
if len(subset) > 1:
labels_to_i.index = MultiIndex.from_tuples(labels_to_i.index)
- labels_to_i.index.names = [index.names[i] for i in subset]
+ labels_to_i.index.names = [index.names[i] for i in subset]
+ else:
+ labels_to_i.index = Index(x[0] for x in labels_to_i.index)
+ labels_to_i.index.name = index.names[subset[0]]
+
labels_to_i.name = 'value'
return (labels_to_i)
diff --git a/pandas/core/strings.py b/pandas/core/strings.py
index 2f95e510bba5e..48bc2ee05dd68 100644
--- a/pandas/core/strings.py
+++ b/pandas/core/strings.py
@@ -1452,7 +1452,12 @@ def cons_row(x):
if expand:
result = list(result)
- return MultiIndex.from_tuples(result, names=name)
+ out = MultiIndex.from_tuples(result, names=name)
+ if out.nlevels == 1:
+ # We had all tuples of length-one, which are
+ # better represented as a regular Index.
+ out = out.get_level_values(0)
+ return out
else:
return Index(result, name=name)
else:
diff --git a/pandas/io/parsers.py b/pandas/io/parsers.py
index a9821be3fa5e2..8b1a921536a1d 100755
--- a/pandas/io/parsers.py
+++ b/pandas/io/parsers.py
@@ -23,7 +23,8 @@
is_scalar, is_categorical_dtype)
from pandas.core.dtypes.missing import isna
from pandas.core.dtypes.cast import astype_nansafe
-from pandas.core.index import Index, MultiIndex, RangeIndex
+from pandas.core.index import (Index, MultiIndex, RangeIndex,
+ _ensure_index_from_sequences)
from pandas.core.series import Series
from pandas.core.frame import DataFrame
from pandas.core.categorical import Categorical
@@ -1444,7 +1445,8 @@ def _agg_index(self, index, try_parse_dates=True):
arr, _ = self._infer_types(arr, col_na_values | col_na_fvalues)
arrays.append(arr)
- index = MultiIndex.from_arrays(arrays, names=self.index_names)
+ names = self.index_names
+ index = _ensure_index_from_sequences(arrays, names)
return index
@@ -1808,7 +1810,7 @@ def read(self, nrows=None):
try_parse_dates=True)
arrays.append(values)
- index = MultiIndex.from_arrays(arrays)
+ index = _ensure_index_from_sequences(arrays)
if self.usecols is not None:
names = self._filter_usecols(names)
@@ -3138,9 +3140,8 @@ def _get_empty_meta(columns, index_col, index_names, dtype=None):
if index_col is None or index_col is False:
index = Index([])
else:
- index = [Series([], dtype=dtype[index_name])
- for index_name in index_names]
- index = MultiIndex.from_arrays(index, names=index_names)
+ data = [Series([], dtype=dtype[name]) for name in index_names]
+ index = _ensure_index_from_sequences(data, names=index_names)
index_col.sort()
for i, n in enumerate(index_col):
columns.pop(n - i)
diff --git a/pandas/tests/indexes/test_base.py b/pandas/tests/indexes/test_base.py
index 07e98c326bcaa..aa32e75ba0d58 100644
--- a/pandas/tests/indexes/test_base.py
+++ b/pandas/tests/indexes/test_base.py
@@ -17,7 +17,7 @@
DataFrame, Float64Index, Int64Index,
CategoricalIndex, DatetimeIndex, TimedeltaIndex,
PeriodIndex, isna)
-from pandas.core.index import _get_combined_index
+from pandas.core.index import _get_combined_index, _ensure_index_from_sequences
from pandas.util.testing import assert_almost_equal
from pandas.compat.numpy import np_datetime64_compat
@@ -2112,3 +2112,19 @@ def test_intersect_str_dates(self):
res = i2.intersection(i1)
assert len(res) == 0
+
+
+class TestIndexUtils(object):
+
+ @pytest.mark.parametrize('data, names, expected', [
+ ([[1, 2, 3]], None, Index([1, 2, 3])),
+ ([[1, 2, 3]], ['name'], Index([1, 2, 3], name='name')),
+ ([['a', 'a'], ['c', 'd']], None,
+ MultiIndex([['a'], ['c', 'd']], [[0, 0], [0, 1]])),
+ ([['a', 'a'], ['c', 'd']], ['L1', 'L2'],
+ MultiIndex([['a'], ['c', 'd']], [[0, 0], [0, 1]],
+ names=['L1', 'L2'])),
+ ])
+ def test_ensure_index_from_sequences(self, data, names, expected):
+ result = _ensure_index_from_sequences(data, names)
+ tm.assert_index_equal(result, expected)
diff --git a/pandas/tests/indexes/test_multi.py b/pandas/tests/indexes/test_multi.py
index c66775f4690cc..798d244468961 100644
--- a/pandas/tests/indexes/test_multi.py
+++ b/pandas/tests/indexes/test_multi.py
@@ -537,15 +537,12 @@ def test_astype(self):
self.index.astype(np.dtype(int))
def test_constructor_single_level(self):
- single_level = MultiIndex(levels=[['foo', 'bar', 'baz', 'qux']],
- labels=[[0, 1, 2, 3]], names=['first'])
- assert isinstance(single_level, Index)
- assert not isinstance(single_level, MultiIndex)
- assert single_level.name == 'first'
-
- single_level = MultiIndex(levels=[['foo', 'bar', 'baz', 'qux']],
- labels=[[0, 1, 2, 3]])
- assert single_level.name is None
+ result = MultiIndex(levels=[['foo', 'bar', 'baz', 'qux']],
+ labels=[[0, 1, 2, 3]], names=['first'])
+ assert isinstance(result, MultiIndex)
+ expected = Index(['foo', 'bar', 'baz', 'qux'], name='first')
+ tm.assert_index_equal(result.levels[0], expected)
+ assert result.names == ['first']
def test_constructor_no_levels(self):
tm.assert_raises_regex(ValueError, "non-zero number "
@@ -768,8 +765,9 @@ def test_from_arrays_empty(self):
# 1 level
result = MultiIndex.from_arrays(arrays=[[]], names=['A'])
+ assert isinstance(result, MultiIndex)
expected = Index([], name='A')
- tm.assert_index_equal(result, expected)
+ tm.assert_index_equal(result.levels[0], expected)
# N levels
for N in [2, 3]:
@@ -830,7 +828,7 @@ def test_from_product_empty(self):
# 1 level
result = MultiIndex.from_product([[]], names=['A'])
expected = pd.Index([], name='A')
- tm.assert_index_equal(result, expected)
+ tm.assert_index_equal(result.levels[0], expected)
# 2 levels
l1 = [[], ['foo', 'bar', 'baz'], []]
diff --git a/pandas/util/testing.py b/pandas/util/testing.py
index 5a17cb6d7dc47..7dac83953ad8f 100644
--- a/pandas/util/testing.py
+++ b/pandas/util/testing.py
@@ -1909,7 +1909,11 @@ def keyfunc(x):
# convert tuples to index
if nentries == 1:
+ # we have a single level of tuples, i.e. a regular Index
index = Index(tuples[0], name=names[0])
+ elif nlevels == 1:
+ name = None if names is None else names[0]
+ index = Index((x[0] for x in tuples), name=name)
else:
index = MultiIndex.from_tuples(tuples, names=names)
return index