BUG: Support timespec argument in Timestamp.isoformat() (#26131)

doc/source/whatsnew/v1.2.0.rst

@@ -607,6 +607,7 @@ Datetimelike
 - Bug in :meth:`Series.isin` with ``datetime64[ns]`` dtype and :meth:`.DatetimeIndex.isin` failing to consider timezone-aware and timezone-naive datetimes as always different (:issue:`35728`)
 - Bug in :meth:`Series.isin` with ``PeriodDtype`` dtype and :meth:`PeriodIndex.isin` failing to consider arguments with different ``PeriodDtype`` as always different (:issue:`37528`)
 - Bug in :class:`Period` constructor now correctly handles nanoseconds in the ``value`` argument (:issue:`34621` and :issue:`17053`)
+- Bug in :meth:`Timestamp.isoformat`, now handles the ``timespec`` argument from the base :class:``datetime`` class (:issue:`26131`)
 
 Timedelta
 ^^^^^^^^^

pandas/_libs/tslibs/timestamps.pyx

@@ -609,8 +609,8 @@ cdef class _Timestamp(ABCTimestamp):
     # -----------------------------------------------------------------
     # Rendering Methods
 
-    def isoformat(self, sep: str = "T") -> str:
-        base = super(_Timestamp, self).isoformat(sep=sep)
+    def isoformat(self, sep: str = "T", timespec: str = "auto") -> str:
+        base = super(_Timestamp, self).isoformat(sep=sep, timespec=timespec)
         if self.nanosecond == 0:
             return base
 
@@ -619,7 +619,7 @@ cdef class _Timestamp(ABCTimestamp):
         else:
             base1, base2 = base, ""
 
-        if self.microsecond != 0:
+        if self.microsecond != 0 and timespec in ("auto", "microseconds"):
             base1 += f"{self.nanosecond:03d}"
         else:
             base1 += f".{self.nanosecond:09d}"

pandas/tests/scalar/timestamp/test_formats.py

@@ -0,0 +1,9 @@
+from pandas import Timestamp
+
+
+def test_isoformat():
+    ts = Timestamp(
+        year=2019, month=5, day=18, hour=15, minute=17, second=8, microsecond=132263
+    )
+    assert ts.isoformat() == "2019-05-18T15:17:08.132263"
+    assert ts.isoformat(timespec="seconds") == "2019-05-18T15:17:08"