BUG: Fix Timestamp type checks to work with subclassed datetime (#25851)

doc/source/whatsnew/v0.25.0.rst

@@ -407,6 +407,7 @@ Sparse
 Other
 ^^^^^
 
+- Improved :class:`Timestamp` type checking in various datetime functions to prevent exceptions when using a subclassed `datetime` (:issue:`25851`)
 - Bug in :class:`Series` and :class:`DataFrame` repr where ``np.datetime64('NaT')`` and ``np.timedelta64('NaT')`` with ``dtype=object`` would be represented as ``NaN`` (:issue:`25445`)
 -
 -

pandas/_libs/tslib.pyx

@@ -2,7 +2,6 @@
 import cython
 
 from cpython.datetime cimport (PyDateTime_Check, PyDate_Check,
-                               PyDateTime_CheckExact,
                                PyDateTime_IMPORT,
                                timedelta, datetime, date, time)
 # import datetime C API
@@ -19,6 +18,7 @@ import pytz
 from pandas._libs.util cimport (
     is_integer_object, is_float_object, is_datetime64_object)
 
+from pandas._libs.tslibs.c_timestamp cimport _Timestamp
 
 from pandas._libs.tslibs.np_datetime cimport (
     check_dts_bounds, npy_datetimestruct, _string_to_dts, dt64_to_dtstruct,
@@ -539,8 +539,7 @@ cpdef array_to_datetime(ndarray[object] values, str errors='raise',
                                              'datetime64 unless utc=True')
                     else:
                         iresult[i] = pydatetime_to_dt64(val, &dts)
-                        if not PyDateTime_CheckExact(val):
-                            # i.e. a Timestamp object
+                        if isinstance(val, _Timestamp):
                             iresult[i] += val.nanosecond
                         check_dts_bounds(&dts)
 

pandas/_libs/tslibs/c_timestamp.pxd

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+
+from cpython.datetime cimport datetime
+
+from numpy cimport int64_t
+
+cdef class _Timestamp(datetime):
+    cdef readonly:
+        int64_t value, nanosecond
+        object freq
+        list _date_attributes
+    cpdef bint _get_start_end_field(self, str field)
+    cpdef _get_date_name_field(self, object field, object locale)
+    cdef int64_t _maybe_convert_value_to_local(self)
+    cpdef to_datetime64(self)
+    cdef _assert_tzawareness_compat(_Timestamp self, datetime other)
+    cpdef datetime to_pydatetime(_Timestamp self, bint warn=*)
+    cdef bint _compare_outside_nanorange(_Timestamp self, datetime other,
+                                         int op) except -1