StringArray comparisions return BooleanArray

doc/source/user_guide/text.rst

@@ -94,7 +94,12 @@ l. For ``StringDtype``, :ref:`string accessor methods<api.series.str>`
 2. Some string methods, like :meth:`Series.str.decode` are not available
    on ``StringArray`` because ``StringArray`` only holds strings, not
    bytes.
-
+3. In comparision operations, :class:`arrays.StringArray` and ``Series`` backed
+   by a ``StringArray`` will return an object with :class:`arrays.BooleanDtype`,
+   rather than a ``bool`` dtype object, depending on whether
+   there are missing values. Missing values in a ``StringArray`` will propagate
+   in comparision operations, rather than always comparing unequal like
+   :attr:`numpy.nan`.
 
 Everything else that follows in the rest of this document applies equally to
 ``string`` and ``object`` dtype.

pandas/core/arrays/string_.py

@@ -134,6 +134,10 @@ class StringArray(PandasArray):
         The string methods are available on Series backed by
         a StringArray.
 
+    Notes
+    -----
+    StringArray returns a BooleanArray for comparison methods.
+
     Examples
     --------
     >>> pd.array(['This is', 'some text', None, 'data.'], dtype="string")
@@ -148,6 +152,13 @@ class StringArray(PandasArray):
     Traceback (most recent call last):
     ...
     ValueError: StringArray requires an object-dtype ndarray of strings.
+
+    For comparision methods, this returns a :class:`pandas.BooleanArray`
+
+    >>> pd.array(["a", None, "c"], dtype="string") == "a"
+    <BooleanArray>
+    [True, NA, False]
+    Length: 3, dtype: boolean
     """
 
     # undo the PandasArray hack
@@ -255,7 +266,12 @@ def value_counts(self, dropna=False):
     # Overrride parent because we have different return types.
     @classmethod
     def _create_arithmetic_method(cls, op):
+        # Note: this handles both arithmetic and comparison methods.
         def method(self, other):
+            from pandas.arrays import BooleanArray
+
+            assert op.__name__ in ops.ARITHMETIC_BINOPS | ops.COMPARISON_BINOPS
+
             if isinstance(other, (ABCIndexClass, ABCSeries, ABCDataFrame)):
                 return NotImplemented
 
@@ -275,15 +291,16 @@ def method(self, other):
                 other = np.asarray(other)
                 other = other[valid]
 
-            result = np.empty_like(self._ndarray, dtype="object")
-            result[mask] = StringDtype.na_value
-            result[valid] = op(self._ndarray[valid], other)
-
-            if op.__name__ in {"add", "radd", "mul", "rmul"}:
+            if op.__name__ in ops.ARITHMETIC_BINOPS:
+                result = np.empty_like(self._ndarray, dtype="object")
+                result[mask] = StringDtype.na_value
+                result[valid] = op(self._ndarray[valid], other)
                 return StringArray(result)
             else:
-                dtype = "object" if mask.any() else "bool"
-                return np.asarray(result, dtype=dtype)
+                # logical
+                result = np.zeros(len(self._ndarray), dtype="bool")
+                result[valid] = op(self._ndarray[valid], other)
+                return BooleanArray(result, mask)
 
         return compat.set_function_name(method, f"__{op.__name__}__", cls)
 

pandas/core/ops/__init__.py

@@ -5,7 +5,7 @@
 """
 import datetime
 import operator
-from typing import Tuple, Union
+from typing import Set, Tuple, Union
 
 import numpy as np
 
@@ -59,6 +59,37 @@
     rxor,
 )
 
+# -----------------------------------------------------------------------------
+# constants
+ARITHMETIC_BINOPS: Set[str] = {
+    "add",
+    "sub",
+    "mul",
+    "pow",
+    "mod",
+    "floordiv",
+    "truediv",
+    "divmod",
+    "radd",
+    "rsub",
+    "rmul",
+    "rpow",
+    "rmod",
+    "rfloordiv",
+    "rtruediv",
+    "rdivmod",
+}
+
+
+COMPARISON_BINOPS: Set[str] = {
+    "eq",
+    "ne",
+    "lt",
+    "gt",
+    "le",
+    "ge",
+}
+
 # -----------------------------------------------------------------------------
 # Ops Wrapping Utilities
 

pandas/tests/arrays/string_/test_string.py

@@ -154,6 +154,37 @@ def test_add_frame():
     tm.assert_frame_equal(result, expected)
 
 
+def test_comparison_methods_scalar(all_compare_operators):
+    op_name = all_compare_operators
+
+    a = pd.array(["a", None, "c"], dtype="string")
+    other = "a"
+    result = getattr(a, op_name)(other)
+    expected = np.array([getattr(item, op_name)(other) for item in a], dtype=object)
+    expected = pd.array(expected, dtype="boolean")
+    tm.assert_extension_array_equal(result, expected)
+
+    result = getattr(a, op_name)(pd.NA)
+    expected = pd.array([None, None, None], dtype="boolean")
+    tm.assert_extension_array_equal(result, expected)
+
+
+def test_comparison_methods_array(all_compare_operators):
+    op_name = all_compare_operators
+
+    a = pd.array(["a", None, "c"], dtype="string")
+    other = [None, None, "c"]
+    result = getattr(a, op_name)(other)
+    expected = np.empty_like(a, dtype="object")
+    expected[-1] = getattr(other[-1], op_name)(a[-1])
+    expected = pd.array(expected, dtype="boolean")
+    tm.assert_extension_array_equal(result, expected)
+
+    result = getattr(a, op_name)(pd.NA)
+    expected = pd.array([None, None, None], dtype="boolean")
+    tm.assert_extension_array_equal(result, expected)
+
+
 def test_constructor_raises():
     with pytest.raises(ValueError, match="sequence of strings"):
         pd.arrays.StringArray(np.array(["a", "b"], dtype="S1"))

pandas/tests/extension/test_string.py

@@ -91,7 +91,7 @@ class TestCasting(base.BaseCastingTests):
 class TestComparisonOps(base.BaseComparisonOpsTests):
     def _compare_other(self, s, data, op_name, other):
         result = getattr(s, op_name)(other)
-        expected = getattr(s.astype(object), op_name)(other)
+        expected = getattr(s.astype(object), op_name)(other).astype("boolean")
         self.assert_series_equal(result, expected)
 
     def test_compare_scalar(self, data, all_compare_operators):