REGR: Fix construction of PeriodIndex from strings (#33304)

Author: dsaxton

doc/source/whatsnew/v1.1.0.rst

@@ -401,6 +401,7 @@ Datetimelike
 - :class:`Timestamp` raising confusing error message when year, month or day is missing (:issue:`31200`)
 - Bug in :class:`DatetimeIndex` constructor incorrectly accepting ``bool``-dtyped inputs (:issue:`32668`)
 - Bug in :meth:`DatetimeIndex.searchsorted` not accepting a ``list`` or :class:`Series` as its argument (:issue:`32762`)
+- Bug where :meth:`PeriodIndex` raised when passed a :class:`Series` of strings (:issue:`26109`)
 - Bug in :class:`Timestamp` arithmetic when adding or subtracting a ``np.ndarray`` with ``timedelta64`` dtype (:issue:`33296`)
 
 Timedelta

pandas/core/arrays/period.py

@@ -831,11 +831,11 @@ def period_array(
     """
     if is_datetime64_dtype(data):
         return PeriodArray._from_datetime64(data, freq)
-    if isinstance(data, (ABCPeriodIndex, ABCSeries, PeriodArray)):
+    if is_period_dtype(data):
         return PeriodArray(data, freq)
 
     # other iterable of some kind
-    if not isinstance(data, (np.ndarray, list, tuple)):
+    if not isinstance(data, (np.ndarray, list, tuple, ABCSeries)):
         data = list(data)
 
     data = np.asarray(data)

pandas/tests/arrays/test_datetimelike.py

@@ -10,7 +10,7 @@
 import pandas._testing as tm
 from pandas.core.arrays import DatetimeArray, PeriodArray, TimedeltaArray
 from pandas.core.indexes.datetimes import DatetimeIndex
-from pandas.core.indexes.period import PeriodIndex
+from pandas.core.indexes.period import Period, PeriodIndex
 from pandas.core.indexes.timedeltas import TimedeltaIndex
 
 
@@ -902,3 +902,13 @@ def test_searchsorted_datetimelike_with_listlike_invalid_dtype(values, arg):
     msg = "[Unexpected type|Cannot compare]"
     with pytest.raises(TypeError, match=msg):
         values.searchsorted(arg)
+
+
+@pytest.mark.parametrize("klass", [list, tuple, np.array, pd.Series])
+def test_period_index_construction_from_strings(klass):
+    # https://github.com/pandas-dev/pandas/issues/26109
+    strings = ["2020Q1", "2020Q2"] * 2
+    data = klass(strings)
+    result = PeriodIndex(data, freq="Q")
+    expected = PeriodIndex([Period(s) for s in strings])
+    tm.assert_index_equal(result, expected)

pandas/tests/arrays/test_period.py

@@ -37,6 +37,7 @@ def test_registered():
         ([pd.Period("2017", "D"), None], None, [17167, iNaT]),
         (pd.Series(pd.date_range("2017", periods=3)), None, [17167, 17168, 17169]),
         (pd.date_range("2017", periods=3), None, [17167, 17168, 17169]),
+        (pd.period_range("2017", periods=4, freq="Q"), None, [188, 189, 190, 191]),
     ],
 )
 def test_period_array_ok(data, freq, expected):