Backport PR pandas-dev#25368: BUG: Fix potential segfault after pd.Ca…

…tegorical(pd.Series(...), categories=...) (pandas-dev#25520)

doc/source/whatsnew/v0.24.2.rst

@@ -31,6 +31,8 @@ Fixed Regressions
 - Fixed regression in :class:`TimedeltaIndex` where `np.sum(index)` incorrectly returned a zero-dimensional object instead of a scalar (:issue:`25282`)
 - Fixed regression in ``IntervalDtype`` construction where passing an incorrect string with 'Interval' as a prefix could result in a ``RecursionError``. (:issue:`25338`)
 
+- Fixed regression in :class:`Categorical`, where constructing it from a categorical ``Series`` and an explicit ``categories=`` that differed from that in the ``Series`` created an invalid object which could trigger segfaults. (:issue:`25318`)
+
 .. _whatsnew_0242.enhancements:
 
 Enhancements

pandas/core/arrays/categorical.py

@@ -323,14 +323,6 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
         # we may have dtype.categories be None, and we need to
         # infer categories in a factorization step futher below
 
-        if is_categorical(values):
-            # GH23814, for perf, if values._values already an instance of
-            # Categorical, set values to codes, and run fastpath
-            if (isinstance(values, (ABCSeries, ABCIndexClass)) and
-               isinstance(values._values, type(self))):
-                values = values._values.codes.copy()
-                fastpath = True
-
         if fastpath:
             self._codes = coerce_indexer_dtype(values, dtype.categories)
             self._dtype = self._dtype.update_dtype(dtype)
@@ -382,7 +374,7 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
             dtype = CategoricalDtype(categories, dtype.ordered)
 
         elif is_categorical_dtype(values):
-            old_codes = (values.cat.codes if isinstance(values, ABCSeries)
+            old_codes = (values._values.codes if isinstance(values, ABCSeries)
                          else values.codes)
             codes = _recode_for_categories(old_codes, values.dtype.categories,
                                            dtype.categories)
@@ -2627,6 +2619,9 @@ def _recode_for_categories(codes, old_categories, new_categories):
     if len(old_categories) == 0:
         # All null anyway, so just retain the nulls
         return codes.copy()
+    elif new_categories.equals(old_categories):
+        # Same categories, so no need to actually recode
+        return codes.copy()
     indexer = coerce_indexer_dtype(new_categories.get_indexer(old_categories),
                                    new_categories)
     new_codes = take_1d(indexer, codes.copy(), fill_value=-1)

pandas/tests/arrays/categorical/test_constructors.py

@@ -212,6 +212,18 @@ def test_constructor(self):
             c = Categorical(np.array([], dtype='int64'),  # noqa
                             categories=[3, 2, 1], ordered=True)
 
+    def test_constructor_with_existing_categories(self):
+        # GH25318: constructing with pd.Series used to bogusly skip recoding
+        # categories
+        c0 = Categorical(["a", "b", "c", "a"])
+        c1 = Categorical(["a", "b", "c", "a"], categories=["b", "c"])
+
+        c2 = Categorical(c0, categories=c1.categories)
+        tm.assert_categorical_equal(c1, c2)
+
+        c3 = Categorical(Series(c0), categories=c1.categories)
+        tm.assert_categorical_equal(c1, c3)
+
     def test_constructor_not_sequence(self):
         # https://github.com/pandas-dev/pandas/issues/16022
         msg = r"^Parameter 'categories' must be list-like, was"