Fixed construction / factorization of empty PA and IA (#24599)

Author: TomAugspurger

pandas/core/arrays/interval.py

@@ -217,6 +217,11 @@ def _from_sequence(cls, scalars, dtype=None, copy=False):
 
     @classmethod
     def _from_factorized(cls, values, original):
+        if len(values) == 0:
+            # An empty array returns object-dtype here. We can't create
+            # a new IA from an (empty) object-dtype array, so turn it into the
+            # correct dtype.
+            values = values.astype(original.dtype.subtype)
         return cls(values, closed=original.closed)
 
     _interval_shared_docs['from_breaks'] = """

pandas/core/arrays/period.py

@@ -189,6 +189,13 @@ def _from_sequence(cls, scalars, dtype=None, copy=False):
             freq = dtype.freq
         else:
             freq = None
+
+        if isinstance(scalars, cls):
+            validate_dtype_freq(scalars.dtype, freq)
+            if copy:
+                scalars = scalars.copy()
+            return scalars
+
         periods = np.asarray(scalars, dtype=object)
         if copy:
             periods = periods.copy()

pandas/tests/arrays/test_period.py

@@ -225,8 +225,7 @@ def test_sub_period():
 def test_where_different_freq_raises(other):
     ser = pd.Series(period_array(['2000', '2001', '2002'], freq='D'))
     cond = np.array([True, False, True])
-    with pytest.raises(IncompatibleFrequency,
-                       match="Input has different freq=H"):
+    with pytest.raises(IncompatibleFrequency, match="freq"):
         ser.where(cond, other)
 
 

pandas/tests/extension/arrow/test_bool.py

@@ -44,6 +44,11 @@ class TestConstructors(BaseArrowTests, base.BaseConstructorsTests):
     def test_from_dtype(self, data):
         pytest.skip("GH-22666")
 
+    # seems like some bug in isna on empty BoolArray returning floats.
+    @pytest.mark.xfail(reason='bad is-na for empty data')
+    def test_from_sequence_from_cls(self, data):
+        super(TestConstructors, self).test_from_sequence_from_cls(data)
+
 
 class TestReduce(base.BaseNoReduceTests):
     def test_reduce_series_boolean(self):

pandas/tests/extension/base/constructors.py

@@ -9,6 +9,14 @@
 
 class BaseConstructorsTests(BaseExtensionTests):
 
+    def test_from_sequence_from_cls(self, data):
+        result = type(data)._from_sequence(data, dtype=data.dtype)
+        self.assert_extension_array_equal(result, data)
+
+        data = data[:0]
+        result = type(data)._from_sequence(data, dtype=data.dtype)
+        self.assert_extension_array_equal(result, data)
+
     def test_array_from_scalars(self, data):
         scalars = [data[0], data[1], data[2]]
         result = data._from_sequence(scalars)

pandas/tests/extension/base/methods.py

@@ -105,6 +105,14 @@ def test_factorize_equivalence(self, data_for_grouping, na_sentinel):
         tm.assert_numpy_array_equal(l1, l2)
         self.assert_extension_array_equal(u1, u2)
 
+    def test_factorize_empty(self, data):
+        labels, uniques = pd.factorize(data[:0])
+        expected_labels = np.array([], dtype=np.intp)
+        expected_uniques = type(data)._from_sequence([], dtype=data[:0].dtype)
+
+        tm.assert_numpy_array_equal(labels, expected_labels)
+        self.assert_extension_array_equal(uniques, expected_uniques)
+
     def test_fillna_copy_frame(self, data_missing):
         arr = data_missing.take([1, 1])
         df = pd.DataFrame({"A": arr})

pandas/tests/extension/json/array.py

@@ -179,6 +179,9 @@ def _concat_same_type(cls, to_concat):
 
     def _values_for_factorize(self):
         frozen = self._values_for_argsort()
+        if len(frozen) == 0:
+            # _factorize_array expects 1-d array, this is a len-0 2-d array.
+            frozen = frozen.ravel()
         return frozen, ()
 
     def _values_for_argsort(self):