BUG: Fix localize_pydatetime using meta datetimes as Timestamp (#25734)

doc/source/whatsnew/v0.25.0.rst

@@ -136,7 +136,7 @@ Categorical
 Datetimelike
 ^^^^^^^^^^^^
 
--
+- Bug with :class:`Timestamp` and :class:`CustomBusinessDay` arithmatic throwing an exception with datetime subclasses (:issue:`25734`)
 -
 -
 

pandas/_libs/tslibs/conversion.pyx

@@ -380,9 +380,11 @@ cdef _TSObject convert_datetime_to_tsobject(datetime ts, object tz,
         obj.value -= int(offset.total_seconds() * 1e9)
 
     if not PyDateTime_CheckExact(ts):
-        # datetime instance but not datetime type --> Timestamp
-        obj.value += ts.nanosecond
-        obj.dts.ps = ts.nanosecond * 1000
+        try:
+            obj.value += ts.nanosecond
+            obj.dts.ps = ts.nanosecond * 1000
+        except AttributeError:
+            pass
 
     if nanos:
         obj.value += nanos
@@ -608,8 +610,10 @@ cpdef inline datetime localize_pydatetime(datetime dt, object tz):
     if tz is None:
         return dt
     elif not PyDateTime_CheckExact(dt):
-        # i.e. is a Timestamp
-        return dt.tz_localize(tz)
+        try:
+            return dt.tz_localize(tz)
+        except AttributeError:
+            pass
     elif is_utc(tz):
         return _localize_pydatetime(dt, tz)
     try:

pandas/tests/arithmetic/test_datetime64.py

@@ -5,6 +5,7 @@
 from datetime import datetime, timedelta
 from itertools import product, starmap
 import operator
+import sys
 import warnings
 
 import numpy as np
@@ -15,6 +16,7 @@
 from pandas._libs.tslibs.offsets import shift_months
 from pandas.compat.numpy import np_datetime64_compat
 from pandas.errors import NullFrequencyError, PerformanceWarning
+from pandas.tseries.offsets import CustomBusinessDay
 
 import pandas as pd
 from pandas import (
@@ -2350,3 +2352,34 @@ def test_shift_months(years, months):
            for x in dti]
     expected = DatetimeIndex(raw)
     tm.assert_index_equal(actual, expected)
+
+
+def test_add_with_monkeypatched_datetime(monkeypatch):
+    # GH 25734
+
+    class MetaDatetime(type):
+        @classmethod
+        def __instancecheck__(self, obj):
+            return isinstance(obj, datetime)
+
+    class FakeDatetime(MetaDatetime("NewBase", (datetime,), {})):
+        pass
+
+    with monkeypatch.context() as m:
+        # monkeypatch datetime everywhere
+        for mod_name, module in list(sys.modules.items()):
+            if (mod_name == __name__ or
+               module.__name__ in ('datetime',)):
+                continue
+            for attribute_name in dir(module):
+                try:
+                    attribute_value = getattr(module, attribute_name)
+                except (ImportError, AttributeError, TypeError):
+                    continue
+                if id(datetime) == id(attribute_value):
+                    m.setattr(module, attribute_name, FakeDatetime)
+
+        dt = FakeDatetime(2000, 1, 1, tzinfo=pytz.UTC)
+        result = Timestamp(dt) + CustomBusinessDay()
+        expected = Timestamp("2000-01-03", tzinfo=pytz.UTC)
+        assert result == expected

pandas/tests/tslibs/test_conversion.py

@@ -3,11 +3,12 @@
 import numpy as np
 import pytest
 from pytz import UTC
+from datetime import datetime
 
 from pandas._libs.tslib import iNaT
 from pandas._libs.tslibs import conversion, timezones
 
-from pandas import date_range
+from pandas import date_range, Timestamp
 import pandas.util.testing as tm
 
 
@@ -66,3 +67,22 @@ def test_length_zero_copy(dtype, copy):
     arr = np.array([], dtype=dtype)
     result = conversion.ensure_datetime64ns(arr, copy=copy)
     assert result.base is (None if copy else arr)
+
+
+class FakeDatetime(datetime):
+    pass
+
+
+@pytest.mark.parametrize("dt, expected", [
+    pytest.param(Timestamp("2000-01-01"),
+                 Timestamp("2000-01-01", tz=UTC), id="timestamp"),
+    pytest.param(datetime(2000, 1, 1),
+                 datetime(2000, 1, 1, tzinfo=UTC),
+                 id="datetime"),
+    pytest.param(FakeDatetime(2000, 1, 1),
+                 FakeDatetime(2000, 1, 1, tzinfo=UTC),
+                 id="fakedatetime")])
+def test_localize_pydatetime_dt_types(dt, expected):
+    # GH 25734
+    result = conversion.localize_pydatetime(dt, UTC)
+    assert result == expected